IPMP3.0, Oregon State University, Copyright 2000 TWOSPOTTED SPIDER MITES
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Biology of the Predator Mite, Neoseiulus fallacis,
and Management of Twospotted Spider Mite

Research Progress Report - 1994
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Note: this information is considered unpublished work and should not be used as final or finished results.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.

Objective I: Study the biology of the predator mite, Neoseiulus fallacis, and management of the twospotted spider mite on mint.

Objective la: Continuation of the predator mite survey on mint in the Pacific Northwest, and on surrounding vegetation in central Oregon.

Four species of Phytoseiid predator mites have been previously identified on peppermint grown in Oregon (Hollingsworth and Berry 1982, Hadam et al. 1986). In the Willamette valley, Typhlodromus occidentalis, Amblyseius andersoni and Amblyseius brevispinus were detected in mint fields, while in central Oregon, only Neoseiulus fallacis was found. Research conducted during 1991-1992 demonstrated that N. Fallcis (NF) effectively managed Tetranychus urticae (TSSM) on mint (Morris et al. 1991, 1992, 1993, unpublished data).

A more extensive survey of the predator mites inhabiting mint in central Oregon would benefit TSSM management in several ways, including: (1) documenting the dominant Phytoseiid species inhabiting mint in this region, (2) providing assistance in the selection of a biological control candidate for inoculative releases, (3) obtaining habitat information on where in the mint system predator mites live so that disruption by agronomic practices may be minimized, and (4) using effective biological control agents to control spider mites will reduce the mint industry's reliance on conventional synthetic pesticides and reduce the cost of production for mint growers.

METHODS FOR PREDATOR MITE SURVEY

Samples of live mint foliage were collected from peppermint and spearmint fields throughout the Pacific Northwest, and from vegetation in the surrounding areas of mint fields in central Oregon. Mint foliage was examined with a 16X hand lens. Plants observed to have predator mites were collected and placed in either plastic "zip lock" storage bags or brown paper bags, depending on the mint stage of growth and the size of the sample. Small plants or single plants were placed in "zip lock" storage bags to prevent desiccation. Larger samples were maintained in brown paper bags to avoid premature decomposition (John Dunley 1991. Per Comm.). The samples were then placed into a large plastic garbage bag which was stored in a cooler with "blue ice" and transported back to the laboratory. Predator mites were collected from the foliage with a camel's-hair brush that was pre-moistened with 70% ethyl alcohol (ETOH). Specimens were stored in 70% ETOH until they could be cleared in Hoyer's solution, mounted and identified to species

Samples of vegetation surrounding mint fields in central Oregon were placed into plastic garbage bags, stored in coolers with "Blue ice" and transported back to the laboratory. Samples were processed in Tulgren Berlese funnels until dry. Specimens were stored in 70% ETOH and stored until they could be cleared in Hoyer's solution, mounted and identified to species.

RESULTS AND DISCUSSION OF PREDATOR MITE SURVEY

An overview of the survey of predator mites on mint foliage in the Pacific Northwest is located in Table 1. These results showed that N. fallacis (NF) was distributed throughout all the mint growing regions surveyed and was the only phytoseiid predator mite found in high abundance. Low densities of other phytoseiids were detected in this survey, but they were not observed to provide effective biological control.

Results of the survey of vegetation surrounding mint fields in central Oregon are found in Table 2. The results show that NF was not detected in the common high desert vegetation surrounding mint fields in this region.

These data suggest that NF is the dominant phytoseiid predator mite inhabiting mint fields in the Pacific Northwest. This study also suggest that NF does not survive well outside of the favorable micro climate provided by irrigated agriculture in central Oregon. This finding is in agreement with the literature which shows that NF is not well adapted to dry and hot environments (Croft and McGroarty 1977). For this reason, I believe that NF is able to survive on peppermint in central Oregon because of the favorable humidity and micro climate provided by abundant irrigation and the complex habitat created by this perennial crop.

Because NF has been shown to be effective at controlling TSSM on mint, and because this study demonstrates that NF is the most prevalent phytoseiid on mint in the Pacific Northwest, I believe NF should be the predator mite used in release programs on mint, unless another species is shown to be superior in some way.

Table 1: Results of the predator mite surveys conducted on mint foliage throughout the Pacific Northwest between 1991 and 1994.
____________________________________________________________________________
                                                           Sample Location
                                           CO     WV    LaG    EW    ID     MT    NC                                                           ____________________________________________________________________________
No. fields Sampled             >20     15        6        15     8      12      4

                                                No. Phytoseiids collected/field
N. fallacis                           >10    >10      >10    >10      5    >10     7
Neoseiulus spp.                    1        1           0        1      0       0      0
Galendromus occidentalis    0        0           0        0      0       0      1
____________________________________________________________________________
CO = Central Oregon, WV = Willamette Valley, LaG = LaGrande, EW = Eastern Washington, ID = Idaho, MT = Montana, NC = Northern California.

Table 2: Results of the predator mite survey of phytoseiids conducted on vegetation surrounding mint fields in central Oregon during 1993 and 1994.
____________________________________________________
                        Phytoseiid predator mites
Plant            Metaseiulus citri    Neoseiulus spp.
____________________________________________________
Sage                   Absent                 Absent
Juniper               Present                Absent
Rabbit brush       Absent                 Absent
Blue grass          Absent                 Present
____________________________________________________

Objective 1b: Overwintering of Neoseiulus fallacius on Peppermint in Central Oregon

The phytoseiid predator mite Neoseiulus fallacis (NF) is widely distributed across North American on a wide range of aerial and low growing crops (Croft 1990, Croft et al. 1993). It is a key biological control agent on apple and strawberry in growing regions that exhibit high humidity, for example in the midwest and east (Croft 1990, Cooley et al. 1993). In arid westem regions, NF is confined to crops that provide humid/wet microhabitats. For example, in westem Oregon valleys, which are humid except for 12 months in the surnrner, NF occurs naturally on plants like strawberry or arbororeal crops like com and hop which maintain a high crop humidity (Hadam et al. 1986, Croft 1993, Strong and Croft 1993, Croft and Croft 1994). It does not occur naturally on crops with lower canopy humidies such as pome fruits (Dowing and Moilliet 1974, Hadam et al. 1986). In more arid regions like central Oregon, NF occurs on low growing plants where supplemental irrigation is used extensively (Morris et al. 1994, unpublished). Sprinkle irrigation has been shown to irnprove biological control of spider mites on grapes by providing a more suitable microenviromnent (Kimn et al. 1972). It may also serve to physically reduce spider mites resulting in a more favorable ratio of predator mites to spider mites (Hudson and Beirne, 1970), or it may cleanse dust from plants thereby improving predator mite searching ability (McMurtry 1981).

N. fallacis is an effective natural enemy of Tetrarychus urticae on peppermint in the Pacific Northwest, USA (Morris et al. 1994, unpublished). Unfortunately, it is not always present at levels sufficent to achieve effective biological control. Possible reasons to explain the unfavorable ratios of predator mites to spider mites in the early season are: 1) mortality of overwintering adult females, 2) mortality of predators in spring or fall because of dry conditions that occur before the peppermint canopy becomes well established or, 3) a combination of 1 and 2.

A nurnber of references elude to the overwintering behavior of phytoseiid predator mites (Croft and McGroarty 1977, Metcalf and Luckman 1982, Hoy et al. 1984), but specific information on their overwintering habitats or how overwintering populations change over time is not reported. Here, we report research to evaluate several factors that may affect overwintering survival of NF on peppermint under arid conditons of central Oregon. Our objectives were to determine: 1) in which habitat elements NF overwinters, 2) how habitat elements change and densities of predators are altered, 3) whether adding or removing microhabitat materials affects survival of NF, and 4) the impact of fall pesticidal treatments on NF and TSSM the following spring.

METHODS FOR OVERWINTERING STUDIES

Study no 1: Survey of peppermint foliage for over wintering predator mites: During the fall of 1991, sections of three peppermint fields in central Oregon were identified that exhibited significant populations of Tetranychus urticae (TSSM) and Neoseiulus fallacis (NF). Two of the fields were located in Lower Bridge (B2 and B7) and the third field was located two miles south of Madras (D 1). Fields were sampled on a monthly basis beginning in December and continuing through February. Twenty plants from each field were selected in areas where NF was present that fall and examined with a 16X hand lens. The number of spider mites and predator mites present on the bottom four leaves were recorded. In fields covered with snow, the snow was removed and plants were sampled in a similar fashion.

Study no. 2a: This experiment was designed to: (1) determine in which habitat elements N. fallacis over winters in peppermint fields in central Oregon, and (2) investigate how population densities change in these habitat elements over the course of the winter.

The experiment was conducted in two peppermint fields located one mile west of Culver Oregon. The first was a 1.5 year old field of the variety "Murray Mitcham". The second was a four year old field of the variety "Todd's Mitcham". Treatments consisted of collecting plant material from each of four strata within peppermint plots and measuring the dry weight of material collected from each strata and the number of NF present. The four peppermint strata were: (1) live foliage, (2) dead leaves (often forming a layer below the live foliage), (3) debris layer below the dead leaves (primarily composed of leaf and stem debris below the dead leaves), and (4) hollow stems remaining from the previous season. The sampling method consisted of collecting plant material from each of the four different strata and placing them individually into gallon sized "ziplock" plastic freezer bags. The bags were placed into coolers with ice and transferred back to the laboratory, located at Corvallis, Oregon, within 48 hours for processing.

Sample processing consisted of placing individual samples into Tulgren Berlese funnels, each provisioned with a bouquet of TSSM infested lima bean plants. The bean stems were placed into pint size Mason jars filled with water and attached to the bottom of the funnels. The bean foliage extended into the funnel and covered the opening of the mason jars which prevented the predator mites from falling directly into the water. The bioassay lasted from five to seven days under 40 watt light bulbs. The bean foliage was then removed from the Mason jars and each leaf was examined with a dissecting microscope for predator mites and the dry material from each strata was weighed.

This trial was conducted using a randomized complete block design with six blocks, except for November which was replicated only one time. For November, data on habitat stratification was obtained from plots as described below under study 2b. The blocking factor consisted of predator mite and spider mite populations density gradients between the two fields during the 1993 season. Blocks one through three were located in the 1.5 year old field of Murray Mitcham. In this field, predator mites had not yet reduced populations of TSSM throughout the entire field by the time the experiment was established. In this situation, N. fallacis was approaching a maximum in population density going into the winter. Blocks four through six were located in the four year old field of Todd's Mitcham. In this field, predator mites had reduced populations of TSSM to low levels and, by this time, were themselves less abundant.

For the late December sampling occasion, the plot size was 3 ft x 3 ft. On November, January, February and April, 1994 the plot size was 10 ft x 10 ft. Plots were sampled on November 3, December 30, 1993, January 25, February 28 and April 1, 1994.

Study no. 2b: This experiment was designed to determine in which habitat elements N. fallacis over winters in peppermint fields in central Oregon, during November. The experiment was conducted in a 1.5 year old field of "Murray Mitcham" variety peppermint. Treatments consisted of collecting plant material from each of the four strata described above in study no. 2a, within 2 foot x 2 foot peppermint plots. NF had been released into these plots on October 13 to augment native populations. Predator mites used for this release were collected from the same field where the experiment was conducted. The sampling method and processing was the same as for study no. 2a above, except that NF was collected into glass jars filled with 70% ETOH. For this experiment we used a completely randomized design with 7 replications.

Study no. 3: This experiment was designed to test the effects of: (l) adding or removing dead mint debris on the over wintering survival of NF on central Oregon peppermint, and (2) fall applications of carbofuran on populations of NF and TSSM the following spring.

The experiment was conducted in a three year old field of "Todd's mitchum" variety peppermint located one mile west of Culver Oregon. Treatments consisted of: (1) undisturbed plots following harvest, (2) removal of dead organic debris, (3) addition of a 0.5 inch layer of organic debris, and (4) fall application of carbofuran (2.0 Ibs ai/A).

This experiment was conducted using a completely randomized design with nine replications. Experimental units consisted of placing 15 inch in diameter by 20 inch high cylinders, made of 0.5 inch PVC pipe, over mint plants observed to have natural populations of predator mites and TSSM. Two opposing 8 inch by 4 inch rectangles were cut into each cylinder to provide ventilation. A fine mesh screen was placed over the openings to reduce immigration or emigration of predator mites. Tanglefoot brand sticker was placed two inches from the top around the inside of the cylinder to prevent escape.

Plots were established on October 10, 1993. At this time debris was either added or removed from the appropriate plots. Between October 13 and October 18, thirty additional predator mites were added to each of the PVC plots. Predators were collected from the same field where the experiment was conducted. Appropriate plots were treated with carbofuran (Furadan 4F at 2.0 Ibs ai/A) on October 20, 1993. Applications were made with an R&D CO2 backpack sprayer with a single nozzle boom. Carbofuran was applied in an equivalent of 30 gallons of water per acre.

Plots were sampled on April 5. On this date, all predator mites and spider mites on the foliage were counted in the field with a 16X hand lens. Dead leaves, stems and other debris were placed into "Ziplock" plastic bags and processed the same as in study no. 2 above.

Study no. 4: This experiment was designed to test the effects of adding predator mites in the fall, and fall applications of carbofuran (Furadan 4F at 2.0 Ibs ai/A) on TSSM populations the following spring.

The experiment was conducted in a two-year old peppermint field of Murray Mitcham located at Lower Bridge, Oregon. This field had a high population of TSSM and few predator mites. Treatments consisted of: (l) TSSM only, (2) addition of 30 predator mites, and (3) addition of 30 NF followed by an application of carbofuran (2.0 Ibs ai/A).

The experiment was conducted using a completely randomized design with ten replications. Experimental units consisted of five gallon pickle buckets with the bottoms removed. Stikcum brand sticker was placed around the inside of the cylinder as in study number three above.

Plots were established on October l9, 1993. On this date, 30 predators were added to the appropriate plots. Predators were obtained from a laboratory colony of NF maintained at Oregon State University. This colony was originally founded from predator mites collected from central Oregon during 1992.

Carbofuran (Furadan 4F at 2.0 Ibs ai/A) was applied to appropriate plots on October 21, 1993 in the equivalent of 30 gallons of water per acre using an R&D CO2
backpack sprayer. The mint stage of growth at the time of application was one to two inches tall. Plots were sampled on May 5. At this time, all NF and TSSM on the foliage were counted in the field with a 16 x hand lens

RESULTS AND DISCUSSION FOR OVERWINTERING STUDIES

Study no. 1: general survey: Results of the winter survey of peppermint foliage conducted during 1991-1992 are shown in Table 3. Predator mites were detected on the lower leaves in all fields on all sample dates except for Lower Bridge (BA 2) on January 3, 1992, and Lower Bridge (BA 7) on February 20, 1992. Populations of both TSSM and NF remained constant, within the range of variation, throughout the duration of this study suggesting that NF does not appreciably feed while in diapause during the winter.

Table 3. Levels of TSSM and NF on peppermint leaves in central Oregon during the winter of 1991-92.
______________________________________________________________
                                         Mean number of mites /Leaf + SD
Date             Field                       TSSM                 NF
______________________________________________________________
Dec 16    B2 Lower Bridge    1.29 + 1.45         0.013 + 0.055
Dec 16    B7 Lower Bridge    1.13 + 1.19         0.013 + 0.055
Dec 16    Dl Madras                0.45 + 0.54        0.013 + 0.055
Jan 3      B2 Lower Bridge     0.76 + 0.84        0.013 + 0.055
Jan 3      B7LowerBridge       1.10+  0.95        0.000
Jan 3      Dl Madras                0.45 + 0.54        0.013 + 0.055
Feb 20    B2 Lower Bridge     0.81 + 1.06        0.04 + 0.120
Feb 20    B7 Lower Bridge    1.24 + 1.20         0.013 + 0.055
Feb 20    Dl Madras               1.00 + 1.02         0.025 + 0.078
______________________________________________________________

Experiment nos. 2a and 2b: Results of the experiment on overwintering habitat elements and survival of N. fallacis on peppermint in central Oregon are shown in Tables 4 through 7. During the months of November, December, January and February, the density of NF per square foot was generally higher in the debris and dead leaves than in the other habitat elements (Tables 4 and 5). Although there was a trend in this same direction, there was no significant (P < 0.05) difference between either of these two strata and live leaves in December or hollow stems in January (Table 5). In February, their were significantly (P < 0.05) more NF detected in the debris (53/O) than in the live foliage (0/O). On April, there were significantly (P < 0.05) more NF detected on the new live foliage (95%) than on the other habitat elements. These data suggest that although N. fallacis will survive on all four strata throughout the winter, they prefer habitat elements that are usually closest to the ground. These results could be explained by the more favorable micro climate found in the dead leaf and debris layers. These habitat elements usually provide higher humidity and greater protection from wind and cold temperatures. There is a trend suggesting that hollow stems are preferred to live foliage during the months of January and February, often the time of most severe cold temperatures. Although the stems typically extend above the foliage, there bases would be well protected. Also, the amount of live foliage available declines appreciably due to senescence resulting from harsh environmental conditions (Table 7). By April, the majority of NF are found on the new peppermint foliage where they were observed to be feeding on TSSM.

When the results are considered on a per habitat weight basis, the results are somewhat different (Table 6). On November, December and January, there was a trend towards higher numbers of NF per gram in the dead leaves than in the other habitat elements. In December, however, the only significant (P < 0.05) difference was between the dead leaves (46%) and the hollow stems (8%). In February, there were significantly (P < 0.05) more NF per gram in the debris (56%) when compared to the live foliage (0%) which was no longer present due to killing frost and other effects of winter. Because there was no significant difference in the number of NF/gm found in the dead leaves when compared to the debris, and because there is significantly (P < 0.01) more debris per plot than dead leaves (Table 7), this suggests they primarily over winter in dead leaves. The debris layer is composed of dead leaves, dead stems, and soil particles. I believe that the majority of NF are found in the dead leaf component of the debris layer.

During December and January, blocking significantly (P < 0.05) improved the precision of this experiment for both density of NF per square foot and per gram of habitat element. Significantly (P < 0.05) higher populations of N. fallacis were found during the early winter in the 1.5 year old field compared to the four year old field. While the same trend existed during February and April, the results were not significant (P > 0.05). These data suggest that higher populations of N. fallacis over winter in peppermint fields where TSSM have not been reduced to low levels. The lower levels detected in the four year old field are probably due to the effects of starvation's and/or dispersal. Reductions in populations of NF on other crops have been correlated with crashes of TSSM populations, primarily through the actions of dispersal and starvation (Croft and McGroarty 1977).

Because predator mites were detected at high numbers in new fields with less debris, the ability of NF to over winter successfully in peppermint is encouraging. As fields age, they typically contain more debris unless they are heavily flamed or tilled. Also promising was that the winter of 1994 was very dry and there was no insulating snow pack. This suggests that NF can tolerate dry winters in central Oregon by over wintering in peppermint fields. The grower did, however, irrigate up until October when water was no longer available.

Populations of N. fallacis / square foot showed about a 50% decline from the end of January until the latter part of February (Table 5). This could be due to over wintering mortality and dispersal of NF, or decay and movement of the debris and dead leaves (Table 7). There was no appreciable decline in the number of predator mites per gm of habitat units over time (Table 6).

Table 4: Results of the 1993 study on habitat partitioning by NF on peppermint in central Oregon during early November. Results are reported as numbers of NF per square foot of soil surface. Treatments include numbers of NF per square foot in peppermint debris, dead leaves, live foliage, and hollow stems.
______________________________________________________
Mean No. N. fallacis motiles per square foot of soil surface
                               Sample Date
Treatment                 November
______________________________________________________
Debris                        6.86 a1
Dead lvs                    5.64 a
Live lvs                      2.21 b
Hollow stm                0.43 b
SE                              0 .94
______________________________________________________
1Treatment means compared with FPLSD (P<0.05).

Table 5: Results of the 1993-1994 study on over wintering survival of NF and habitat partitioning on peppermint in central Oregon. Results are reported as numbers of NF per square foot of soil surface. Treatments include number NF per square foot m peppermint debris, dead leaves, live foliage, and hollow stems.

____________________________________________________________________________
                         Mean No. N. fallacis motiles per square foot of soil surface
                                                        Sample Date
Treatment    November 31    December 30     January 25    February 28     April 1
____________________________________________________________________________

Debris                0.120                 0.185 a            0.153 a            0.185a       0.030b
Dead lvs             0.070                 0.183 a           0.162 a            0.013ab     0.000b
Live lvs              0.020                 0.090 ab          0.022 b            0.000b       0.600a
Hollow stm        0.015                  0.037 b           0.073 ab          0.035ab     0.000b
SE                                                0.040              0.038               0.035          0.593
____________________________________________________________________________
1 Experimental units were not replicated in November. Other dates compared with FPLSD (P<0.05).

Table 6: Results of the 1993-1994 study on over wintering survival of NF and habitat partitioning on peppermint in central Oregon. Number of NF per gram of strata. Treatments include numbers of NF per gram of debris, dead leaves, live foliage, and hollow stems.

____________________________________________________________________________
                               Mean No. N. fallacis motiles per gram of strata
                                                       Sample Date
Treatment    November 3    December 30     January 25    February 28     April 1
____________________________________________________________________________

Debris              0.050               0.060 a b           0.110               0.250 a             0.003 b
Dead lvs           0.080               0.120 a             0.220 a             0.080               0.000 b
Live lvs            0.040               0.060 a b           0.040 b             0.000 b            0.450 a
Hollow stm       0.030               0.020 b             0.140                0.120               0.000 b
SE                                            0.025                0.046                 0.060               0.033
____________________________________________________________________________
1 LSD comparison of high value against lower values, "F" test was not significant. Other sample dates were tested with FPLSD.

Table 7: Results of the 1993-1994 study on over wintering survival of NF and habitat partitioning on peppermint in central Oregon. Dry weight of habitat strata per square foot. Treatments include number of NF per square foot of debris, dead leaves, live foliage, and hollow stems.

____________________________________________________________________________
                             Mean weight (gms) per square foot of soil surface
                                                        Sample Date
Treatment    November 31    December 30     January 25    February 28    April 1
____________________________________________________________________________
Debris                    4.85                    3.34 a           2.62 a             0.76 a         1.52 a
Dead lvs                1.54                    1.64 b            0.78 b             0.27 b         0.00 c
Livel vs                  0.97                   1.63 b            0.58 b             0.00 c          0.28 bc
Hollowm stm         0.88                    2.13 b            0.54 b             0.33 b         0.31 b
    SE                                                 0.27              0.26                0.04             0.10
____________________________________________________________________________
1 Experimental units were not replicated in November. Other dates compared with FPLSD (P<0.05)

Experiment no. 3: Results of the experiment evaluating the effects of adding or removing debris in the fall, and fall application of carbofuran, on over wintering survival of N. fallacis and TSSM are found in Table 8. There were significantly (P < 0.05) fewer TSSM/leaf in all treatments when compared to the plots receiving an application of carbofuran. There were no significant (P < 0.05) differences in the number of TSSM/leaf between the other three treatments.

There were significantly (P < 0.05) fewer NF in the plots treated with carbofuran and where debris was removed when compared to the other treatments (Table 8). Carbofuran completely eliminated NF while removal of debris resulted in a 68% reduction in NF compared to the untreated control and 78% fewer NF when compared to the treatment where debris was added. The addition of debris resulted in significantly (P < 0.05) more NF/plant when compared to the untreated control (Table 8).

These data demonstrate that applications of carbofuran in the fall can result in economically damaging populations of TSSM the following spring. At least partially because carbofuran was used for eight years under a section 18 emergency exemption for root weevil control during the late 1970's and early 1980's, TSSM has become a more severe pest problem on mint. Disruption of predator mites, I believe, is the primary reason for this increase in TSSM populations, but enhanced fecundity due to the effects of carbofuran may also play a role.

The results also suggest that adding a debris layer, and/or allowing a debris layer to remain in the field, can result in higher over wintering survival of NF compared to fields where the debris layer is somehow reduced. The type of ground cover present and how it is manipulated has been shown to effect the survival of predator mites (Croft and McGroarty 1977, Smith et al. 1989). Mint growers often disk and till their fields to control weeds and to thicken their stands, these practices may have an impact on the predator mites living there. Allowing a layer of dead leaves and debris to remain throughout the fall and winter would provide a more suitable environment for the over wintering survival of NF as well as provide necessary habitat elements in which they can over winter.

Table 8: Results of the 1993-1994 study investigating the effects of removing or adding debris, and applications of Carbofuran (2.0 lbs ai/A), on the over wintering survival or N. fallacis and TSSM. Trial conducted one mile west of Culver, Oregon.
____________________________________________________________________________

Treatment            Mean no. TSSM      Treatment            Mean no. Predator mite
                                motiles/plant                                           motiles/plant
____________________________________________________________________________
Carbofuran 4F        37.00 a               Add debris                           7.33 a
Untreated control    6.33 b                Untreated control                4.88 b
Add debris               6.22 b                Remove debris                   1.56 c
Remove debris        4.11 b                Carbofuran 4F                     0.00 c
____________________________________________________________________________
                            SE = 2.81                                                         SE = 0.84

Experiment no. 4: Results of the study investigating the effects of adding predators in the fall, or fall applications of carbofuran, on populations of TSSM the following spring are found in Table 9. Results indicate that fall applied carbofuran eliminated predator mites and resulted in significantly (P < 0.05) higher populations of TSSM the following spring. These results agree with those in experiment no. 3. Although there was a trend towards higher levels of NF in plots not treated with carbofuran or where NF were not added, the differences were not significant (P < 0.05). Predator mites were able to contaminate the plots where no predators were added which provided confusion to the results. The intention was to see if carbofuran alone could result in increased TSSM populations during the duration of this study.

Table 9: Results of the 1993-1994 study investigating the addition of predators alone or followed by applications of carbofuran, on TSSM population levels the following spring. The trial was conducted at Lower bridge Oregon.

____________________________________________________________________________
Treatment            Mean no. TSSM           Treatment         Mean no. Predator mite
                               motiles/plant                                             motiles/plant
____________________________________________________________________________
Carbofuran                 1.310 a              Predators added              0.023 a
No predators added   0.067 b              No predators added         0.005 a
Predators added        0.032 b              Carbofuran                       0.000 a
____________________________________________________________________________
                                 SE = 0.31                                                 SE = not significant
______________________________________________________________________________

Objective 1c: Disruption of Neosiulus fallacis with Pesticides

Spider mites are primarily secondary pests that become of primary concern when their natural enemies are disrupted (Croft and Mcgroarty 1977, Helle and Sabalis 1985, Croft 1991). The use of non selective pesticides are known to be a major reason for this disruption (Croft and Nelson 1972, Bower and Kaldour 1980, Croft l991, Malezieux et al. 1992). Conversely, predators may be influenced by factors other than pesticides; for example ground cover management (Smith et al. 1989).

A number of predator mites are tolerant or have developed resistance to a number of pesticides used in agriculture today (Croft and Meyer 1973, Croft and Whalon 1983, Babcock and Tanigoshi 1988, Croft 1991, Hoy 1990). For this reason, not all pesticides are incompatible with IPM programs. For example, propargite is known to be "soft" on predator mites (Hoy and Conley l989, Croft 1991)

As discussed above under the predator mite survey, it is known that a number of predatory mite species inhabit mint fields, and based on the above population dynamic studies, it was shown that N. fallacis (NF) is able to regulate populations of spider mites on mint. For these reasons, the mint industry would benefit from knowing which of the pesticides registered for use on mint, or in the registration process, are selective on NF. Because releasing predator mites may be costly and time consuming, it is important to understand which factors are harmful to them. With this understanding, growers may be able to modify their pesticide use patterns and encourage the survival of beneficial biological control agents such as predator mites. This strategy would also reduce the cost of production to mint growers.

I believe 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 situations due to ecological escape (Jepson and Mead-Briggs in press). Those pesticides that are found to be more selective could then be tested for physiological selectivity in the laboratory.

The objective of this study during 1994 was to: (1) evaluate the affects of Mocap (ethoprop), Tilt (propiconizole) and Grarnoxone extra (paraquat) on NF in central Oregon using a semi-field technique, and (2) determine whether selectivity to Orthene (acephate) and Lorsban (chlopyrifos) is ecological or physiological.

METHODS FOR PREDATOR MITE DISRUPTION STUDY

Experiment no. l: Semi- field evaluation of Mocap Tilt and Gramoxone: During 1994, three pesticides were evaluated for their effect on NF in central Oregon. Treatments included: (1) a non-treated check, (2) the nematicide ethoprop (Mocap 6EC) at 6.0 lbs ai/A, (2) the fungicide propiconizole (Tilt 3.6 EC) 8.0 oz/A, and the herbicide paraquat (Grarnoxone extra 2.5 EC) 16.0 oz/A. Plots consisted of 15 inch PVC pipes cut to a height of 15 inches. Each pipe was placed over mint plants that had a natural population of both TSSM and NF. Each PVC pipe was ringed with stickum 2 inches from the top to avoid emigration of predator and spider mites. Each plot was inoculated on May 2 with 25 predators that were collected from the same field where the study was conducted. Plots were arranged in a completely randomized design with 9 replications. Treatments were applied on May 4 at 10:00 AM with a R&D CO2 backpack sprayer with a single nozzle boom equipped with a Teejet 95004 flatfan nozzle. The ambient temperature at the time of application was 63F, the wind was blowing 1-3 MPH from the west and the mint stage of growth was 2-5 inches tall.

Plot evaluation consisted of collecting 20 plants from each PVC plot and placing them into a plastic "ziplock" storage bag. Samples were placed into a cooler and transported back to the laboratory. Six leaves were removed from each plant, 2 from the bottom, 2 from the middle and 2 from the top, and the number of TSSM motiles, TSSM eggs, predator mite motiles and predator mite eggs were counted with the aid of a dissecting microscope. Final evaluation occurred on May 15, 1994.

Experiment no. 2: Laboratory bioassay of Orthene and Lorsban. Also during 1994, laboratory bioassays were conducted to determine if the selectivity by NF to acephate (Orthene) and Chlopyrifos (Lorsban) was due to physiological insensitivity or because NF was able to escape in space or time from the toxic effects of these insecticides.

Tolerance to acephate and Chlopyrifos was determined by bean leaf disc bioassays using an air brush application method (Miller, et al. 1985). Two rates of acephate ( 1.0 and 0.5 lbs ai/A) and one rate of chlopyrifos (1.0 lb ai/A), each mixed in the equivalent of 20 gallons of water per acre, were applied along with a water only check used to assess control mortality. Three replications of 15 adult female NF were each tested at each of the concentrations over four time periods, they were: (1) time 0 (spray was directed over NF that were placed on the discs 24 hours prior to application, (2) NF were placed on the discs two hours after application, (3) 48 hours after application and (4) 72 hours after application.

The predator mites were collected from laboratory colonies with a small camel's hair brush and transferred to the bean leaf disks. Laboratory colonies of NF originated from central Oregon peppermint fields. Each leaf disc was provisioned with at least 20 TSSM collected from a laboratory colony. Disks were stored at 20C for 48 hours prior to evaluation. Mortality was assessed at 48 hours by lightly touching the predator mites with a fine camels-hair brush; significant movement by the predator mite was considered survival.

RESULTS AND DISCUSSION OF PREDATOR MITE DISRUPTION STUDY

The results of the two studies evaluating the effects of pesticides on NF are found in Tables 10 and 11. Under the semi-field test conditions of study no. 1, both paraquat (Gramoxone Extra 16.0 oz's/A) and ethoprop (Mocap 6.0 lbs ai/A) significantly (P < 0.05) reduced populations of NF motiles compared to the untreated controls, 76% and 100% respectively (Table 1). There was a trend for NF eggs with gramoxone reducing egg numbers by 86% while Mocap reduced the number of NF eggs by 100% (Table 10). There was no significant (P < 0.05) difference in the number of NF eggs or motiles between the untreated controls and the Tilt treatment. These data suggest that if Mocap 6EC is registered, then methods of application will have to be employed to use it selectively, otherwise outbreaks of spider mites can result. If gramoxone is used during the spring or fall when predator mites are active, this herbicide can also severely impact NF and result in more problems with TSSM. Observational data suggests that applications of Gramoxone extra during the dormant season does not severely impact NF.

In the second study, designed to determine if the selectivity of Orthene and Lorsban is due to physiological or ecological mechanisms, the results demonstrate that selectivity is mainly ecological. All application regimes, except the water only controls, resulted in 100% mortality of NF for at least 48 hours (Table 11). After 72 hours, both the 0.5 and 1.0 lb ai/A rates of Orthene showed limited survival, 39% and 7% respectively. The 1.0 lb ai/A rate of Lorsban continued to result in 100% mortality after 72 hours and was not significantly (P < 0.01) different than the high rate of Orthene (Table 11). These results can be explained by the lower degradation half life of Orthene compared to Lorsban, 3 days versus 30 days respectively (Agricultural chemical statistics 1990).

These data suggest that both Orthene and Lorsban should be applied only when necessary because both products can be lethal to beneficial predator mites. Orthene is less harmful than Lorsban when both are applied in a broadcast manner. Orthene degrades more rapidly than Lorsban and allows predator mites to escape in areas where coverage is not thorough, often on the bottom of leaves down lower in the canopy. Lorsban, when applied using chemigation, is dilute enough to not severely effect NF (Morris, personal observation).

Table 10. Evaluation of insecticides on populations of TSSM and NF in Lower Bridge, Oregon 1994.
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                                                         Mean No. / Leaf
                                         Post treatment evaluation on May 15. 1994
                       Rate           TSSM         TSSM         Pred mite    Pred mite
Treatment    lbs or oz's     No.Motile    No.Eggs    No.Motile     No.Eggs
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Tilt 3.6EC        8.0oz's/A     0.004 a        1.07 a         0.29 a          0.33 a
Untreated           --             0.040 a         2.80 a         0.25 a         0.39 a
Gramoxone  16.0oz's/A     0.104 a         0.06 a         0.06 b         0.04 b
Mocap           6.0 lbsai/A    0.009 a         0.00 a         0.00 b         0.00 b
P value                              NS                 NS         P < 0.05      P < 0.05
Stand Error                                                                0.04            0.06
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Means with same letter are not significantly different. FPLSD. NS= not significant (P < 0.05).

Table 11. Evaluation of insecticides on populations of NF in the laboratory.
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                                          Mean No. live Neoseiulus fallacis / Leaf
                                                          Application times
                              Rate
Treatment           lbs ai/A         0 hours      2 hours         48 hours      72 hours
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Water only              --              15.33 a        13.33 a         14.33 a          13.66 a
Orthene75 SP      0.50                0.00 b          0.00 b           0.00 b           5.33 b        
Orthene75 SP      1.00                0.00 b          0.00 b           0.00 b           1.00    c
Lorsban4EC         1.00                0.00 b          0.00 b           0.00 b           0.00    c
P value                                  P<0.01           P<0.01           P<0.01         P<0.01
Stand Error                           SE = 0.17     SE = 0.33     SE = 0.33      SE = 0.55
____________________________________________________________________________
Means with same letter are not significantly different. FPLSD. NS= not significant (P < 0.05).
Pesticides were applied directly to NF on bean leaf discs.