IPMP3.0, Oregon State University, Copyright 2000 TWOSPOTTED SPIDER MITES
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MANAGEMENT OF SPIDER MITES ON MINT
Research Progress Report - 1992

Prepared by Mark Morris

Note: this information is considered unpublished work and should not be used as final or finished results. It has been included in IPMP 3.0 because it may not be available from other sources, and in some cases may include information that may not reach final publication.

The Twospotted Spider Mite, Tetranychus urticae Koch (TSSM), is a key pest of peppermint in all mint production areas in Oregon (Hollingsworth and Berry 1982) and the Northwest (Morris et al. unpublished data 1991 and 1992). During the fall, propane flaming in the Willamette Valley reduces populations of spider mites and delays population increases the following season (Hollingsworth and Berry 1982). Spring flaming is also widely practiced in the Willamette Valley to manage rust of mint (Puccinia mentha) and reduces populations of spider mites as well; often to such low levels that acaricide applications are unnecessary (Morris and Fairchild, unpublished data).

In central Oregon, spider mites are more serious because of the warm dry climate. Growers rarely propane flame in the spring in central Oregon because mint rust is not an economic problem and because there is a greater chance of crop damage. However, studies have shown that spring flaming in central Oregon will significantly reduce spider mite populations throughout the growing season (Morris, et al 1991 unpublished).

Predator mites are effective regulators of spider mite populations in a number of crops (Croft and McGroarty 1977) (Helle and Sabalis 1985). The predator mite, Amblyseius fallacis Garmen, appears to be widely distributed throughout the mint growing areas of Oregon (Hollingsworth and Berry 1982) (Morris et al. 1991 unpublished). Populations of T. urticae appear to be regulated by A. fallacis under certain field conditions in central Oregon (Hollingsworth and Berry 1982, Morris et al. 1991 and 1992). However, predators are often absent, or present at such low densities, that effective biological control is not realized. This suggests that they may be disrupted by pesticides or other agronomic practices used on mint. For example, disruption of predator mites by pesticides is widely documented in the literature (Croft and McGroarty 1977) Helle and Sabalis 1985).

The goal of this project is to identify reasons why TSSM is becoming a more serious problem to mint production in the Pacific Northwest and to provide practical solutions to this problem.

The objectives of this study are to: 1) determine whether acaricide resistance is present in TSSM populations in Oregon and the Pacific Northwest; 2) investigate the overwintering behavior of A. fallacis in central Oregon, 3) investigate the population dynamics of A. fallacis and TSSM in central Oregon; 4) survey mint fields in Oregon and the Pacific Northwest for predator mite species; 5) evaluate the effects of insecticide applications on A. fallacis in central Oregon, and 6) evaluate the effectiveness of new as well as registered acaricides for managing TSSM on mint.

Resistance to Acaricides in TwoSpotted Spider Mite in Mint

This season we continued the acaricide resistance study that was started in 1991. The objectives of this study were to: 1) measure the levels and magnitude of Kelthane (dicofol) resistance in populations of TSSM collected from peppermint fields in the Pacific Northwest, and 2) to determine if TSSM populations have developed resistance to Comite (propargite) in mint.

Materials and Methods

Pesticide toxicity bioassays were run on populations of T. urticae collected in Oregon and on one population collected in Montana. For comparative purposes, toxicity bioassays were run on a population collected from a susceptible laboratory colony which has never been exposed to pesticides.

Tolerance to pesticides was determined by leaf disc bioassays using a Potter spray tower (Sabelis and helle 1985). Six concentrations each of Kelthane and Comite were tested, ensuring a wide range of responses. Water only was applied to one replication to assess control mortality. Three replications of > 25 adult female spider mites each were tested at each of the concentrations.

Mortality was assessed at 48 hours by lightly touching mites with a fine camel's-hair brush; significant movement by the spider mite was considered survival. Results were analyzed using standard low-dose probit-mortality analysis (Finney 1971).

Results and Discussion

Resistance to Kelthane MF was detected in four fields in the Pacific Northwest: one in the Willamette Valley, two in central Oregon and one in Montana. Three fields were judged to be susceptible and were not different than the laboratory susceptible strain. The data for Comite are in the process of being analyzed.

Predator Mite Survey

Materials and Methods

Peppermint and spearmint fields in Oregon, Washington, Idaho and Montana are continuing to be surveyed for the presence of predator mites. Fresh mint foliage is first examined with a 16X hand lens; plants observed to have predator mites are collected and placed in either plastic "zip lock" storage bags or brown paper bags, depending on the mint stage of growth. The samples are placed in a large plastic garbage bag which is stored in a cooler with "blue ice" and transported back to the laboratory. Predator mites are collected from the foliate with a camel's-hair brush that has been pre-moistened with 70% ethyl alcohol (ETOH). Specimens are placed into 70% ETOH until they can be cleared, mounted and identified to species.

Results and Discussion

Predator mites have been found in the majority of the peppermint fields surveyed, often at levels above 0.05/leaf. However, in several fields predator mite populations were either absent, present only at low levels or were restricted to small areas of the field. The specimens will be identified to species during the fall of 1992 and winter of 1993.

Overwintering habits of Amblyseius fallacis

To determine the effectiveness of predator mites at maintaining below economically damaging levels, it is necessary to understand their life history and behavior throughout the year. This information is useful for tailoring agricultural management practices to encourage the continued availability of healthy populations of predator mites in and around mint fields.

The objective of this study was to determine whether or not A. fallacis was able to over winter in peppermint fields in central Oregon during 1991-1992.

Materials and Methods

Sections of three peppermint fields in central Oregon that exhibited significant populations of A. fallacis were identified in the fall of 1991. Two of the fields were located in Lower Bridge and the third field was located two miles south of Madras. These field sections were sampled on a monthly basis beginning on December 16, 1991. Twenty plants were selected and examined with a 16X hand lens. The number of spider mites and predator mites that were present on the bottom four leaves were recorded.

Results and Discussion

Predator mites were detected on the lower leaves in all the fields except for Lower Bridge (field 2) on January 3, 1992, and Lower Bridge (field 7) on February 20, 1992. These data demonstrated that predator mites were able to over winter in peppermint fields in central Oregon during 1991-92. For this reason, mint growers need to consider the effects of agronomic practices on predator mites throughout the year.

Population Dynamics of T. urticae and A. fallacis in Peppermint in Central Oregon

The first objective of this study was to observe the population dynamics of A. fallacis and twospotted spider mites (TSSM) in mint fields in central Oregon during the 1992 growing season. The purpose was to learn more about the relationship between these two organisms and how effective A. fallacis would be to regulate populations in peppermint.

Because spider mite populations are often aggregated in peppermint fields (Hollingsworth and Berry 1982), they may be clumped in areas of the field known as "hot spots". The second objective was to investigate the relationship between TSSM and A. fallacis within "hot spots" of intensive activity over time (small plot analysis).

Another objective was to make careful observations on the effects of field applied pesticides on TSSM and A. fallacis.

Materials and Methods

Entire Field Sampling

One 30 acre peppermint field located 1 mile south of Madras, Oregon, was sampled on a weekly basis beginning on May 7, 1992 and continuing through August 8, 1992 (study no. 1)(for brevity, only the results of study no. 1 are reported below).

Three additional fields were studied during 1992: a 30 acre peppermint field located in Gateway, Oregon sampled between June 13, 1992 and July 22, 1992, and two 40 acre peppermint fields located in Lower Bridge, Oregon sampled on a weekly basis beginning on May 20, 1992 and continuing through August 8, 1992 (the results of these three field studies were similar to study no. 1 and the small plot studies reported below).

Thirty samples were collected using a method developed by Hollingsworth and Berry (1982). The fields were sampled using a "zig-zag pattern. Five peppermint plants were collected within a 10 foot radius at each of 30 sampling locations. The number of TSSM motiles, TSSM eggs, predator mite motiles and predator mite eggs were recorded from the top, middle and bottom leaves of each of five plants.

Small Plot Analysis

Several small plots were studied in 1992 to determine TSSM and A. fallacis population dynamics. For brevity, only the results from plot no. 4 are reported below.

Small Plot No. 4. Beginning on May 20, 1992, a section of a field located in Lower Bridge, Oregon, was divided into three plots. The West plot was 30 feet x 200 feet and had been treated with Asian (esfenvalerate 0.06 Ibs ai/A) the previous year to eliminate predator mites; the plot was then disked in the spring of 1992. The middle plot consisted of a strip of unplowed mint running the length of the pivot road; this plot was 10 feet x 200 feet and was not treated with Asian or disked in the spring. The East plot was 30 feet x 200 feet and was also disked in the spring of 1992. Sampling began on May 20,1992 and continued through July 30, 1992. Sampling consisted of collecting 10 plants/plot and placing them in plastic "zip-lock" storage bags which were put in coolers with blue ice. Samples were transported back to the laboratory where TSSM and predator mites, on all leaves, were counted with a dissecting microscope.

Results

Entire Field Analysis

Study no. 1. The results of the field study in Madras are summarized in Table 4 and Figs. 4 and 5. Populations of TSSM motiles remained well below the treatment threshold of 5.0/leaf during the course of this study. Comite (2.5 pints/acre) and Thiolux (sulfur 3.0 Ibs/acre) were applied to suppress TSSM on May 30; the same rate of Comite and Thiolux at 5.0 Ibs/acre was applied on July 20 to suppress TSSM. Populations of TSSM motiles declined after the first applications but were not observed to change after the second application. The field was harvested on August 10.

Predator mite motiles and eggs increased to a high of 0.09 and 0.06/leaf on June 18, respectively, then declined rapidly on June 24 following the crash of the TSSM population on June 18. Predator mite motiles and eggs increased until July 17, then declined on July 24. All of the predator mite motiles observed on July 24 and July 31 were juveniles. This result may have been related to the high rate of Thiolux (5.0 Ibs/acre) that was applied on July 20.

The ratio of TSSM motiles to predator mite motiles declined from a high of 250:1 on May 19 to a low of 1.55:1 on June 18 (Table 4). This ratio remained below 10:1 after June 8, except for 26:1 on June 24.

Table 4. Mean number of motile TSSM, TSSM eggs, motile predator mites, predator mite eggs and TSSM/predator mites ratios obtained from sampling an entire peppermint field located near Madras, Oregon 1992.
___________________________________________________________________________
                                                 TSSM                         Predator Mite
Sample                     Mean no.    Mean no.    Mean no.    Mean no.     Ratio
  Date                        Motiles         Eggs          Motiles        Eggs      TSSM/Pred
___________________________________________________________________________
May 7                           0.17            0.82          0.003           0.030            56.67
May 14                         0.30            1.51          0.020            0.003           15.00
May 19                         1.25            1.34          0.005            0.020          250.00
May 26                         1.74             5 49         0.020            0.030            87.00

Jun1                              1.42            8.09          0.010            0.040          142.00
Jun 8                            0.40            2.21          0.030            0.060            13.33
Jun18                           0.14            0.42          0.090            0.060              1.55
Jun 24                          0.26            0.79          0.010            0.020            26.00
Jun 30                           0.12            0.46          0.060            0.040              2.00
Jul 10                           0.31            1.17          0.040            0.040              7.75
Jul 17                           0.11            0.97          0.070            0.080              1.57   
Jul 24                           0.14            0.58          0.050            0.010              2.80
Jul 31                           0.18            0.73          0.040            0.010              4.50

Aug 8                           0.20            1.01          0.040            0.060               5.0
___________________________________________________________________________

Small Plot Analysis

Study no. 4. West Section. Populations of TSSM motiles fluctuated around the treatment threshold of 5.0 mites/leaf from May 20 until June 26 (Table 5, Figs. 6 and 7). After this date TSSM declined to levels well below the treatment threshold for the remainder of the study. TSSM eggs followed a similar trend, although at higher levels. The plot was treated three times with Comite (2.5 pints/A) and Thiolux (sulfur 4.0 Ibs/acre) to suppress populations of TSSM on May 25, June 22 and July 17. Populations of TSSM resurged after the first Comite application on May 25 but declined after June 26.

Predator mite adults and eggs increased after June 19 and the number of motiles remained above 0.10/leaf for the duration of this study. Predator mite adults continued to increase until July 30.

The ratio of TSSM motiles to predator mite motiles declined steadily from the point where no predators where detected on May 20, to 256.5:1 on June 3, 128.6:1 on June 19 to a low of 0.80:1 on July 30 (Table 5).

Table 5. Mean number of motile TSSM motiles, TSSM eggs, adult predator mites, juvenile predator mites, predator mite eggs and the ratio of TSSM motiles/predator mite motiles observed in a 30 foot by 200 foot section peppermint field located in Lower Bridge, Oregon 1992. Small plot no. 4, West section.

___________________________________________________________________________
                                         TSSM                                       Predator Mites
Sample      Mean no.    Mean no.     Mean no.    Mean no.    Mean no        Ratio
Date           Motiles 1/     Eggs           Adults         Juveniles        Eggs         TS/Pred 2/
____________________________________________________________________________
May 20          8.15         36.37              0.00             0.000            0.000          NPD 3/

Jun 03           5.13        27.32              0.01             0.000             0.010          256.50
Jun 13           4.16        22.45              0.01             0.000             0.010          138.66
Jun 19           6.43        28.29              0.02             0.002             0.020          128.60
Jun 26           4.92        31.37              0.10             0.000             0.100            27.33
Jul 03            1.15        13.35              0.23             0.030             0.230             4.11
Jul 07            0.25         5.86               0.12             0.002             0.120             1.39
Jul 13            0.91         3.82               0.12             0.000             0.120             4.78
Jul 23            0.37        1.71                0.16             0.020             0.160             1.09
Jul 30            0.05        0.68                0.11             0.040             0.110             0.80
____________________________________________________________________________
1/ TSSM Motiles = juveniles + adults
2/ Ratio = TSSM motiles/predator mite motiles: predator motiles = juveniles + adults
3/ NPD = no predators were detected

Study no. 4. Middle Section. The results of this study are shown in Table 6 and (Figs. 8 and 9). TSSM motiles remained well below the treatment threshold of 5.0/leaf during this study. No acaricides were applied during the study period. Predator mite motiles ranged from a low of 0.01 / leaf on July 7 to a high of 0.25/leaf on July 30. The ratio of TSSM to predator mites declined from a high of 33.22:1 on May 20, to 15.33:1 on June 3, and below 10:1 thereafter (Table 6 ).

Table 6. Mean number of motile TSSM motiles, TSSM eggs, adult predator mites, juvenile predator mites, predator mite eggs and the ratio of TSSM motiles/predator mite motiles observed in a 10 foot by 200 foot section peppermint field located in Lower Bridge, Oregon 1992. Small plot no. 4, Middle section .
__________________________________________________________________________
                                         TSSM                                          Predator Mites
Sample         Mean no.    Mean no.    Mean no.    Mean no.     Mean no.   Ratio
Date              Motiles 1/    Eggs            Adults        Juveniles     Eggs        TS/Pred
__________________________________________________________________________
May 20              2.99            13.30           0.080           0.010           0.080        33.22
Jun 03               0.46             1.35           0.030           0.000           0.020        15.33
Jun 13               0.82             3.39           0.100           0.000           0.030          8.20
Jun 19               0.21             1.28           0.070           0.010           0.050          2.63
Jun 26               0.23             0.99           0.100           0.000           0.080          2.30
Jul 03                0.05             0.23           0.050           0.010           0.020          0.83
Jul 07                0.02             0.06           0.002           0.000           0.002          3.33
Jul 23                0.20             0.60           0.040           0.002           0.010          4.00
Jul 30                0.06             3.08           0.190           0.050           0.340          4.17
__________________________________________________________________________
1/ TSSM Motiles = juveniles + adults
2/ Ratio = TSSM motiles/predator mite motiles: predator motiles = juveniles + adults

Study no. 4. East Section. The results of this study are shown in Table 7. Populations of TSSM motiles remained below the treatment threshold of 5.0/leaf during this study and dropped to below 1.0 motiles / leaf after June 26, and remained at low levels through July 30, at which time sampling was discontinued. Comite (propargite 2.5 pints/acre) and Thiolux (sulfur 4.0 Ibs ai/acre) was applied on May 25, June 22 and July 17 to suppress populations of TSSM. Populations of TSSM motiles and eggs resurged after the May 25 application and were not maintained below 1.0/leaf until after June 26 when predator mite populations reached their highest levels. Predator mite motiles began to increase after June 13 and remained at high levels until July 13, after which they declined following the reduction of TSSM on July 7. The ratio of TSSM motiles to predator mite motiles reached a high of 53:1 on June 3, but then declined to below 5:1 after June 19 (Table 7).

Table 7. Mean number of motile TSSM motiles, TSSM eggs, adult predator mites, juvenile predator mites, predator mite eggs and the ratio of TSSM motiles/predator mite motiles observed in a 30 foot by 200 foot section peppermint field located in Lower Bridge, Oregon 1992. Small plot no. 4, East section.
________________________________________________________________________________
                                        TSSM                                       Predator Mites
Sample        Mean no.    Mean no.    Mean no.    Mean no.     Mean no.    Ratio
Date              Motiles 1/    Eggs          Adults         Juveniles     Eggs         TS/Pred 2/
_________________________________________________________________________________
May 20              2.75         17.14           0.08                0.030            0.070         25.00
Jun 03               1.06          14.00          0.02                0.000            0.040         53.00
Jun 13               1.74          14.91          0.08                0.000            0.040         21.75
Jun 19               3.19          23.05          0.12                0.020            0.190         22.78
Jun 26               1.78           15.17         0.44                0.000            0.280           4.05
Jul 03                0.34            5.54          0.23                0.060            0.230           0.94
Jul 07                0.12            1.30          0.39                0.140            0.360           4.33
Jul 13                0.10            0.51          0.23                0.020            0.070           0.37
Jul 23                0.04            0.01          0.02                0.003            0.000           2.00
Jul 30                0.01            0.09          0.02                0.020            0.000           0.50
_________________________________________________________________________________
1/ TSSM Motiles = juveniles + adults
2/ Ratio = TSSM motiles/predator mite motiles: predator motiles = juveniles + adults

Discussion

Entire field analysis

The results of the population dynamics studies in entire fields suggest that predators are effective regulators of TSSM under some field conditions. Within entire fields where predators were detected on all sampling occasions and the ratio of TSSM motiles to predator mite motiles was consistently below 100:1, acaricide applications were required less frequently than in fields where predators were either not detected or where the ratio of TSSM motiles to predator mite motiles were frequently above 100:1. Ideally this ratio should be below 30:1 as populations of TSSM decline over time.

Applications of Comite were more effective against populations of TSSM in fields with predator mite motile populations above 0.03/leaf. Applications of sulfur at rates above 3.0 Ibs/A appeared to reduce populations of predator mite adults and eggs; this may delay the increases in predator mites that are necessary to effectively regulate TSSM populations.

Populations of TSSM were not effectively regulated in fields where predator mite populations were low, or not detected, until predator mite populations increased. At the Madras site predators appeared to regulate TSSM throughout the season (Table 4). In fields where predator mite populations were above 0.03/leaf, TSSM and predator mites followed a typical predator to prey relationship. In these situations, populations of predators declined about two weeks after the decline of TSSM.

Small Plot Analysis

TSSM populations increase in various sized "hotspots" in mint fields during the growing season. As spider mites increase in these "hotspots", it is important that predator mites colonize them in order for effective regulation to occur. Predator mite motiles tend to increase in numbers as populations of TSSM decline, which suggests a typical predator to prey relationship.

Predator mite motiles appear to work most effectively at regulating TSSM below the treatment threshold in "hotspots" when the TSSM to predator mite ratio is below 50:1 (Tables 5, 6, and 7). For this reason, it is important to carefully and frequently monitor fields to detect changes in the ratio of TSSM motiles to predator mite motiles. Given the opportunity, predators were able to increase to effective levels even in those plots which initially had low levels of predator mites.

Comite applications appear to work most effectively when the density of predator mites increase suggesting that predators may enhance the efficacy of Comite in mint in central Oregon. However, applications of sulfur at rates above 3.0 Ibs/A may disrupt predator mites and reduce the rate at which predator mites increase to effective population levels.

Disruption of Amblyseius fallacis by Pesticides

The predator mite A. fallacis, appears to be widely distributed throughout the mint growing areas of central Oregon (Hollingsworth and Berry 1982) (Morris et al. 1991 unpublished). Populations of twospotted spider mite (TSSM) appear to be regulated by A. fallacis in central Oregon (Morris et al. 1991 and 1992 unpublished). However, predators are often absent or present at such low densities that effective biological control is not realized (Hollingsworth and Berry 1982)(Morris et al. 1991 and 1992 unpublished). This suggests that they may be disrupted by pesticides and other agricultural practices used in mint production. Because predator mites are known to be adversely affected by a wide range of agricultural pesticides, it would be advantageous to evaluate a number of those that are currently registered for use on mint for their effects on predator mite populations: new pesticides for which the mint industry in seeking registration, should also be tested.

An appropriate bio-assay program should include testing under field conditions because pesticides that are demonstrated to be disruptive in some laboratory bioassays, may be selective under actual field conditions (Jebson and Mead-Briggs in press).

The objective of this study was to evaluate the affect of three insecticides registered for use on mint, and two non-registered insecticides, on A. fallacis in central Oregon using a semi-field technique.

Materials and Methods

For all three tests, the plots were established by placing a PVC pipe enclosure (6 inch diam., 10 inch high) over mint plants that were observed to have a mixed population of TSSM and predator mites. A completely randomized design with four replications was used for each test. Plots were treated with a CO2 pressurized backpack sprayer using a single nozzle boom with a 9503 nozzle.

Plots were evaluated by collecting a number of plants from within the PVC plot and placing them in a plastic 'ziplock' storage bag. Samples were then placed in a cooler and transported back to the laboratory. All leaves were removed from the stems and the number of TSSM motiles, TSSM eggs, predator mite motiles and predator mite eggs were counted with the aid of a dissecting microscope to obtain the number of TSSM and predators/leaf.

Disruption Study no. 1

In this study, we evaluated two registered insecticides, Orthene (acephate 75 S (1.0 Ib ai/A)) and Lorsban (chlorpyrifos 4E (2.0 Ibs ai/A), and one unregistered insecticide, Asana (esfenvalerate 0.66EC (0.05 Ibs ai/A)), for their affects on TSSM and A. fallacies. Acaricides were mixed in a solution equivalent to 100 gallons/acre (GPA) and the foliage was sprayed to wet.

This trial was conducted in a four year old field of peppermint (variety Todd's Mitchum) located one mile south of Madras, Oregon. PVC pipe enclosures were placed over plants that had natural populations of both TSSM and A. fallacis. The top two inches of the pipes were coated with 'stickum' and the bottom of the pipes were sunk in the soil two inches to prevent escape or immigration by predator mites.

Treatments were applied on May 1, 1992 at 6:30 pm. The temperature at the time of application was 75F, wind speed was 0-1 NW and the mint stage of growth was 1-4 tall.

Treatments were evaluated twice; once on May 6 and again on May 13, 1992. Four plants were collected from each plot on May 6 and 10 plants were collected from each plot on May 13.

Disruption Study no. 2

This trial was set up in the same field and used the same set of treatments and procedures as in study no. 1, with the exception that the foliage in the pots was not treated to wet, but rather, the pesticides were applied at the actual field rates in 100 GPA total spray solution.

Treatments were applied on May 8, 1992, at 8:00 a.m. Temperature at the time of application was 55F, wind speed 1-3 mph NW and the mint stage of growth was 2-5 inches.

Treatments were evaluated twice; once on May 15 and again on May 26, 1992. Four plants were collected from each plot on May 15 and 9 plants were collected from each plot on May 26, 1992. Disruption Study no. 3
This trial was conducted in the same field as in study no. 2. In this experiment we evaluated one registered insecticide, Vydate (oxamyl 2L (1.0 Ib ai/A) and one unregistered insecticide, Capture (bifenthrin 2EC (0.06 Ibs ai/A), for their affects on TSSM and A. fallacis. All treatments were applied at actual field rates in 100 GPA total spray solution. Plots were treated on June 12, 1992, at 7:00 pm. The temperature at the time of application was 69F, wind speed 0-2 mph from the NW and the mint stage of growth was 1 5-20 inches.

Plots consisted of 10 inch diam PVC pipe enclosures cut to a height of 15 inches. The larger PVC pipes were used to accommodate the expanded mint growth.

Because Vydate and Capture have acaricidal activity, treatments were applied after the TSSM populations had declined but prior to decline of the predator mites. By doing this, a reduction in predator mite populations in the treated plots, when compared to the untreated plots, would not be correlated with a reduction in TSSM populations in the treated plots, but would be related to a treatment effect resulting from application of the pesticides.

Plots were evaluated once on June 20, 1992 by collecting six plants from each plot.

Results

Disruption Study no. 1

Results of the first disruption study comparing three different Insecticides for their effect on populations of T. urticae to A. fallacis are shown in Tables 8 and 9. None of the pesticides tested significantly (P < 0.05) reduced populations of TSSM motiles or eggs compared to the untreated check (Table 8).

On May 6, 1992, there was a significant (P<0.05) reduction in the mean number of predator mite motiles detected in the Lorsban and Asana treatments compared to either the Orthene treatment or the untreated check (Table 9). Predator mites were reduced by 63% in the Lorsban treatment and 100% in the Asana treatment when compared to the untreated check. There was no significant (P<0.05) difference in the mean number of predator mites observed between the Orthene treatment and the untreated check. There was a significant (P<0.05) reduction in the mean number of predator mite eggs in plots treated with Lorsban and Asana when compared' to the untreated check: 88% and 91% respectively (Table 9).

On May 13, 1992, significantly (P<0.05) fewer predator mite motiles were detected in the Lorsban and Asana treatments compared to either the Orthene treatment or the untreated check. Compared to the untreated check, Lorsban reduced populations of motiles by 94% while Asana reduced populations by 100%. Predator mite eggs were significantly (P<0.05) reduced in plots treated with Orthene, Lorsban and Asana, resulting in reductions of 68%, 94% and 100% respectively.

Table 8. Evaluation of insecticides on populations of T. urticae (TSSM), Madras, 1992.
___________________________________________________________________________
                                            Mean No. / Leaf
                   May 6 Post Treatment         May 13 Post Treatment
                       Rate           TSSM         TSSM         TSSM            TSSM
Treatment    Ibs ai/A    No. Motile    No. Eggs    No. Motile      No. Eggs
____________________________________________________________________________
Untreated          -                 2.16a         5.36a          0.69a           2.31a
Orthene 75S    1.00              2.21a         2.47a         1.64a            3.62a
Lorsban 4E      2.00              2.91a         5.28a         1.33a            5.6 a
Asana .66E      0.05               2.26a        5.70a          0.58a           4.31a
____________________________________________________________________________
Means with same letter are not significantly (P<0.05) different. FPLSD.

Table 9. Evaluation of insecticides on populations of A. fallacis, Madras 1992.
____________________________________________________________________________
                                                                      Mean No. / Leaf
                                                May 6 Post Treatment       May 13 Post Treatment
                            Rate            Predator         Predator      Predator      Predator
Treatment            Ibs ai/A       No. Motile       No. Eggs     No. Motile     No. Eggs
____________________________________________________________________________
Untreated                  -                  0.16 a          0.25 a            0.25 a           0.21    a
Orthene 75S          1.00                 0.15 a          0.11 ab          0.13 ab         0.02    b
Lorsban 4E            2.00                 0.06 b          0.03 bc          0.02 bc          0.01    b
Asana .66E             0.05                0.00 b          0.02 bc          0.00 bc          0.00    b
____________________________________________________________________________
Means with same letter are not significantly (P<0.05) different. FPLSD.

Disruption Study no. 2

Results indicate that there were no significant differences in the mean number of TSSM motiles or eggs between pesticide treatments (Table 10). Predator mite motiles were significantly (P<0.05) reduced in the Asana treatment when compared to the untreated check; by 100% in this study. There were no significant differences in the mean number of predator mites motiles detected between the Orthene, Lorsban and control treatments. Significantly (P<0.05) fewer predator mite eggs were detected in the Orthene and Asana treatments when compared to the untreated check; 79% and 100% respectively.

Table 10. Impacts of insecticides on populations of T. urticae (TSSM) and A. fallacis in Madras, Oregon, 1992.
____________________________________________________________________________
                                                         Mean No. / Leaf
                                                      May 26, Post Treatment
                       Rate         TSSM           TSSM         Predator      Predator
Treatment    Ibs ai/A    No. Motile    No. Eggs     No. Motile    No. Eggs
____________________________________________________________________________
Untreated           -              1.73a           4.07a             0.32a           0.2a
Orthene 75S    1.00            1.16a           7.96a             0.20a          0.06bc
Lorsban 4E      2.00            1.21a           2.96a             0.20a          0.21ab
Asana .66E      0.05            1.27a           4.75a             0.00b          0.00cd
                                                                               SE = 0.05    SE =0.06 ____________________________________________________________________________

Means with same letter are not significantly (P<0.05) different. FPLSD.

Disruption Study no. 3

Results of the third predator mite disruption study showed that there were no significant differences between treatments in the mean number of TSSM motiles or eggs detected (Table 11).

There were significant (P<0.05) reductions in the mean number of predator mite motiles observed in the Vydate and Capture treated plots when compared to the untreated check plots; 84% and 100% respectively. There were no significant differences in the mean number of predator mite eggs detected between treatments.

Table 11. Evaluation of insecticides on populations of T. urticae (TSSM) and A. fallacis in Madras, Oregon, 1992.
___________________________________________________________________________
                                                            Mean No. / Leaf
                                   June 20 Post Treatment     June 20 Post Treatment
                       Rate         TSSM          TSSM               Predator      Predator
Treatment    Ibs ai/A    No. Motile      No. Eggs        No. Motile      No Eggs
___________________________________________________________________________
Untreated         --               0.000a           0.01a              0.13a         0.0075a
Vydate 2L        1.00           0.000a            0.00a              0.02b         0.0025a
Capture 2E       0.06           0.005a           0.00a              0.00b         0.0000a
                                                                                  SE = 0.02
__________________________________________________________________________
Means with same letter are not significantly (P<0.05) different. FPLSD.

Discussion

These data suggest that applications of Orthene and Lorsban adversely affected predator mites, and should only be applied when target pest populations (i.e., cutworms, adult strawberry root weevils) are determined to be above the economic threshold. However, Orthene appears to be less toxic to A. fallacis than Lorsban when the foliage was sprayed to wet. When Orthene and Lorsban were both applied at field rates, only Orthene significantly reduced predator mite eggs under these test conditions. These results agree with evaluations of Orthene and Lorsban in larger field plots.

Both Asana and Capture were very toxic to A. fallacis motiles and eggs under these test conditions, and if registered, could disrupt A. fallacis and result in secondary outbreaks of TSSM. However, no resurgence of TSSM was observed in this study; perhaps because of the short evaluation period.

At present, two large field plots are being studied in central Oregon to evaluate the possible disruption of A. fallacis, in the fall, by Vydate, Asana and Furadan (carbofuran). These studies will investigate the effects of these treatments on both A. fallacis and TSSM through the fall of 1992 and into the winter and spring of 1993.

Acaricide Efficacy

Because of documented resistance in twospotted spider mite (TSSM) in mint to dicofol, and the potential this product has for disrupting predator mites, the mint industry should begin developing alternative selective acaricides to Kelthane that are complimentary to Comite. If Comite is relied upon as the mint industry's only effective acaricide, resistance to this product may soon follow; if it has not already occurred.

The objective of this series of experiments was to begin screening perspective selective acaricides for possible inclusion into a spider mite management program on mint. A second objective was to determine the effectiveness of currently registered acaricides that the mint industry depends on to manage TSSM outbreaks.

Materials and Methods

Six acaricide efficacy trials were conducted on peppermint in Marion, Deschutes and Jefferson counties during 1992. The acaricides that were evaluated were: (1) Kelthane MF (dicofol) registered for use on mint, (2) Comite (propargite 6.55 EC) registered for use on mint and relatively non-toxic to predator mites; (3) abamectin which is presently not registered for use on mint but may be worth pursuing because preliminary results indicate that low rates of abamectin are "soft" on predator mites; (4) Apollo (clofentezine 4 SC) which is currently not registered for use on mint, however, this acaricide would be a valuable tool for mint production because it is "soft" on predator mites and has ovicidal activity; (5) Vendex (fenbutatin oxide 4L) which is not registered for use on mint, but is selective for predator mites; and (6) Capture (bifenthrin 2EC) which is not currently registered for use on mint but has been reported to disrupt predator mites.

For all tests, acaricides were applied in 30 GPA using a CO2 pressurized backpack sprayer equipped with a four nozzle boom with Teejet 9504 nozzles. Treatments were replicated four times in a randomized complete block design; untreated plots served as checks. Plots were evaluated by collecting 10 plants from each plot and placing them into a brown paper bag which was then put into a plastic garbage bag. The plastic garbage bags were placed in a cooler with blue ice and transported back to the laboratory. Three leaves from each plant, one from the top, middle and bottom, were removed from each of the ten plants collected from each plot, and examined under a dissecting microscope. The number of TSSM motiles, TSSM eggs, predator motiles and predator eggs were counted.

Acaricide Efficacy Study no. 1

In this study, the objective was to determine how well six acaricide treatments, applied in the early spring, would effectively manage spider mites on mint. The acaricide treatments were: (1) Kelthane 40.0 oz/A, (2) Comite 40.0 oz/A, (3) abamectin 8.0 oz/A, (4) Apollo 16.0 oz/A, (5) Comite 40.0 oz/A + Apollo 8.0 oz/A, and (6) Comite 20.0 oz/A + Apollo 8.0 oz/A.

The trial was conducted in a peppermint field located in Gateway, Oregon. Treatments were applied on May 1, 1992, at 8:30 a.m. The temperature at the time of application was 48F, wind was 0-1 mph from NW and the mint stage of growth was 1-4 inches.

Acaricide Efficacy Study no. 2.

The objective of this study was to evaluate the effectiveness of several acaricide treatments that are selective for TSSM and "soft" on predator mites. The acaricide treatments were: (1 ) Apollo 16 oz/A, (2) Comite 40.0 oz/A, (3) Comite 20.0 oz/A + Apollo 8.0 oz/A, (4) Vendex 48 oz/A, (5) Vendex 48 oz/A + Apollo 8.0 oz/A, and (6) Vendex 32 oz/A + Apollo 4.0 oz/A.

The trial was conducted in a peppermint field located near Lower Bridge, Oregon. Treatments were applied on June 19, 1992, at 8:00 a.m. The temperature at the time of application was 73F, wind was 0-1 mph from NW and the mint stage of growth was 5-12 inches.

Acaricide Efficacy Study no. 3

The objective and set of treatments for the third acaricide efficacy study were the same as for acaricide efficacy study no. 2, with the exception that the trial was conducted in the Willamette Valley.

The trial was conducted in a peppermint field located two miles east of Jefferson, Oregon. Treatments were applied on June 19, 1992, at 8:00 p.m. The temperature at the time of application was 76F, wind was 2-4 mph from NNW and the mint stage of growth was 24-36 inch long mint stems that had lodged.

Acaricide Efficacy Study no. 4

The objective of this study was to do a general screening of different acaricides to see how well they managed populations of TSSM in mint.

The study was conducted in a peppermint field located in Gateway, Oregon. Treatments were applied on June 17, 1992, at 7:30 a.m. The temperature at the time of application was 70F, wind was 0-1 mph from the west and the mint stage of growth was 8-16 inches.

Results and Discussion

Because resistance to Kelthane has been documented in TSSM on mint, and because Kelthane has been found to be disruptive to predator mites, it would be advantageous to register several new selective acaricides for use on mint. Also, if the mint industry continues to depend solely on Comite to manage outbreaks of TSSM, then resistance to this acaricide is likely to occur; if it has not already done so.

During 1992, the performance of four acaricides were evaluated for their effectiveness to reduce populations of TSSM on mint in central Oregon and the Willamette Valley: 1 ) abamectin, 2) Apollo, 3) Vendex, and 4) Capture. Results of these experiments are summarized in Tables 12 through 18. Abamectin was very effective at reducing populations of TSSM in mint (Tables 12, 13, 17, 18). Apollo, while it was not often effective alone, was consistently among the best treatments when applied in combination with a one half rate of Comite or mixed with Vendex (Tables 12, 13, 14, 15, 16). Vendex was not as effective as either abamectin or Comite when applied by itself (Tables 14, 15, 16, 17, 18), however, it was quite efficacious in combination with Apollo (Tables 14, 15, 16). Because Vendex is known not to disrupt predator mites, this product may be worth pursuing if a registration for Apollo is also possible. Capture also significantly reduced populations of TSSM (Tables 17, 18), however, this product has also been shown to be very disruptive to predator mites (Table 1 1 ) and could result in resurgence of TSSM following application.

Table 12. Efficacy of acaricides on populations of T. urticae (TSSM) motile growth stages infesting a peppermint field near Gateway, Oregon, 1992.
__________________________________________________________________________
                                             Mean No. TSSM Motiles / Leaf
                                                 Evaluation Dates 1992
                      Rate     Pre-Treat     Post-Treat     Post-Treat     Post-Treat
Treatment    oz's/A    April 30         May 17         May 29           June 10
__________________________________________________________________________
Untreated         -             6.19a          10.44a            6.91a              4.10a
Kelthane      40.0            4.65a           1.61b             2.13a              3.64ab
Comite         40.0            4.51a           1.86b             2.08a              2.67abc
Abamectin     8.0            2.82a           1.67b             1.21b              2.59abc
Apollo          16.0            2.62a         14.23a             6.45a              2.21bc
Com + Apol  40 + 8        3.23a           1.84b             1.03b               1.13c
Com + Apol  20 + 8        2.47a           1.28b             1.02b               1.44c
                                   SE =1.36       SE =1.97       SE = 0.52          SE = 0.61 __________________________________________________________________________
Means with same letter are not significantly (P<0.05) different. FPLSD.
Com = Comite, Apol = Apollo.

Table 13. Efficacy of acaricides on populations of T. urticae (TSSM) eggs in a peppermint field located near Gateway, Oregon, 1992.
__________________________________________________________________________
                                                   Mean No. TSSM Eggs/ Leaf
                                                      Evaluation Dates 1992
                      Rate         Pre-Treat     Post-Treat      Post-Treat     Post-Treat
Treatment    oz's/A         April 30        May 17           May 29           June 10
  __________________________________________________________________________
Untreated        -                51.37a          14.76a             19.36a             6.10bc
Kelthane       40.0             37.64a            8.08abc         13.48a           10.86a
Comite          40.0             36.69a            3.2bc               9.64a            9.41ab
Abamectin      8.0             26.66a            1.12c               4.86a            4.58abc
Apollo           16.0             46.09a          14.5a              12.50a             4.03c
Com + Apol   40 + 8         37.36a            5.44bc              7.03a            3.39c
Com + Apol   20 + 8         35.80a            9.3ab               9.13a             2.29c
                                       SE =16.9      SE =2.40         SE = 3.69           SE = 1.36
__________________________________________________________________________
Means with same letter are not significantly (P<0.05) different. FPLSD.
Com = Comite, Apol = Apollo.

Table 14. Efficacy of acaricides on populations of T. urticae (TSSM) motile growth stages infesting a peppermint field in Lower Bridge, Oregon, 1992.

___________________________________________________________________________
                                              Mean No. TSSM Motiles / Leaf
                                                   Evaluation Dates 1992
                      Rate       Pre-Treat    Post-Treat     Post-Treat    Post-Treat
Treatment    oz's/A      June 19         July 3           July 14          July 28
___________________________________________________________________________
Untreated        -               3.78a           7.78a            13.64              2.36a
Apollo            16.0           2.18a           1.33cd          10.48              1.12a
Comite           40.0           2.63a           1.44cd            5.70              0.55a
Com + Apol    20 + 8        3.67a            0.44d            2.18 *            1.42a
Vendex          48.0           4.41a           4.56b            12.77              1.65a
Ven + Apol    48 + 8        2.68a           2.19cd            3.72              1.05a
Ven + Apol    32 + 8        3.61a           3.08bc            6.68              1.00a
                                    SE = 0.74     SE =0.80       SE = 3.43      SE = 0.62
____________________________________________________________________________
Means with same letter are not significantly (P<0.05) different. FPLSD.
* Significantly different than the untreated check (LSD P<0.05).
Com = Comite, Apol = Apol, Ven = Vendex

Table 15. Efficacy of acaricides on populations of T. urticae (TSSM) eggs in a peppermint field in Lower Bridge, Oregon, 1992.
____________________________________________________________________________
                                             Mean No. TSSM Eggs / Leaf
                                                 Evaluation Dates 1992
                     Rate      Pre-Treat     Post-Treat    Post-Treat    Post-Treat
Treatment    oz's/A     June 19        July 3           July 14          July 28
____________________________________________________________________________
Untreated       -              23.84a         46.09a         34.85a            4.70a
Apollo          16.0           17.09a         29.06ab        33.97a         10.14a
Comite         40.0           19.24a        18.00b          19.94a             8.65a
Com +Apol   20 + 8       20.83a        28.33ab        20.91a          19.65a
Vendex        48.0           23.05a        42.16a           39.43a            5.33a
Ven +Apol    48 + 8      17.53a        30.45ab        25.38a           5.19a
Ven +Apol    32 + 8      19.94a        42.68a          28.03a           7.77a
                                    SE = 4.36   SE =6.08     SE = 5.35        SE =3.83
____________________________________________________________________________
Means with same letter are not significantly (P<0.05) different. FPLSD.
Com = Comite, Apol = Apollo, Ven = Vendex.

Table 16. Efficacy of acaricides on populations of T. urticae (TSSM) motile stages and eggs in a peppermint field located near Jefferson, Oregon, 1992.
_____________________________________________________________________________
                                                 Mean No. TSSM / Leaf
                                                  Evaluation Dates 1992
                     Rate     Pre-treatment-June 16         Post-treatment- July 8
Treatment    oz's/A      Motiles       Eggs                 Motiles         Eggs
_____________________________________________________________________________
Untreated       -               2.78a         9.54a                   5.31a          9.60a
Apollo          16.0           4.58a        19.96a                   3.18a           9.99a
Comite         40.0           4.87a        17.30a                   0.87cd         3.27a
Com + Apol  20 + 8       5.37a        13.71a                   0.27d         1.27a
Vendex        48.0           3.10a        14.09a                   2.50bc         9.46a
Ven + Apol   48 + 8       3.40a       11.71a                    0.67cd       4.70a
Ven + Apol   32 + 8       3.97a      17.65a                   1.34bcd      4.87a
                                   SE = 0.57   SE=2.35               SE =0.69     SE = 2.26
_____________________________________________________________________________
Means with same letter are not significantly (P<0.05) different. FPLSD.
Com = Comite, Apol = Apollo, Ven = Vendex.

Table 17. Efficacy of acaricides on populations of T. urticae (TSSM ) motile developmental stages infesting a Peppermint field in Gateway, Oregon, 1992.
_____________________________________________________________________________
                                                     Mean No. TSSM Motiles / leaf
                                                          Evaluation Dates 1992
                    Rate         Rate        Pre-Treat     Post-Treat    Post-Treat
Treatment    oz's/A    Ibs ai/A         June 17         July 1         July 18
_____________________________________________________________________________
Untreated       -                                12.25a            6.95              0.25a
Kelthane     40.0                              15.28a            0.04*             0.01a
Comite        40.0                              12.74a           0.31*             0.03a
Abamectin    4.0                             13.65a             2.24              0.90a
Abamectin    8.0                             15.57a           0.83*              0.09a
Vendex        32.0                            10.07a           3.84              0.46a
Vendex        48.0                              8.84a           4.15              0.64a
Capture        3.8            0.06              8.11a            0.81*             1.00a
Capture        6.4            0.10           14.15a             0.34*             0.69a
                                                     SE=2.84       SE =1.72      SE = 0.36
_____________________________________________________________________________
Means with same letter are not significantly (P<0.05) different. FPLSD.
*Means are significantly different from the untreated check (LSD P<0.05).

Table 18. Efficacy of acaricides on populations of T. urticae (TSSM) egg stages in a peppermint field in Gateway, Oregon, 1992.
______________________________________________________________________________
                                                    Mean No. TSSM Eggs / Leaf
                                                        Evaluation Dates 1992
                     Rate       Rate            Pre-Treat     Post-Treat     Post-Treat
Treatment    oz's/A    Ibs ai/A         June 17         July 1         July 18
______________________________________________________________________________
Untreated        -                                 6.78a          16.87a            0.65
Kelthane      40.0                               7.97a            1.07b            0.45
Comite         40.0                             11.69a            2.28b            0.17
Abamectin     4.0                               7.31a           9.73ab          2.53
Abamectin     8.0                               7.52a            1.91b            0.32
Vendex         32.0                              6.76a            3.84ab          1.51
Vendex         48.0                              9.34a           13.32a           3.23
Capture         3.8         0.06                 6.46a             7.17ab         6.17*
Capture         6.4         0.10                 7.83a             2.54b           4.08
                                                      SE = 2.03    SE = 3.55    SE = 1.54
______________________________________________________________________________
Means with same letter are not significantly (P~0.05) different. FPLSD
*Means are significantly different from the untreated check (LSD P<0.05)