IPMP3.0, Oregon State University, Copyright 2000

BIOCONTROL OF TWOSPOTTED SPIDER MITE

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Research Report - 1993 to the Oregon Mint Commission

Note: this information is considered unpublished work and should not be used as final or finished results.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.

Disruption of Amblyseius fallacis with Pesticides

Mark  A. Morris, A. M. Todd Company

Spider mites are primarily secondary pests that become of primary concern when their natural enemies are disrupted (Croft and McGroarty 1977, Helle and Sabelis 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 1991, 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). 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 1989, 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 these predator mites; especially NF. Because releasing predator mites is both costly and time consuming, it is important to understand which factors are harmful to them. With this understanding, growers would perhaps be able to modify their production practices in order to 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 (Jebson 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 was to evaluate the affect of different insecticides registered for use on mint, or potentially considered for registration, on AF in central Oregon using a semi-field technique.

Materials and Methods

In 1993, I evaluated five insecticides for their selectivity on NF in central Oregon. The treatments were: (1) a non-treated check, (2) methomyl (Lannate 1.8L) at 0.9 lbs ai/A, (3) sulfur (Thiolux 80 DF) at 6.0 lbs ai/A, (4) carbofuran (Furadan 4L) at 2.0 lbs ai/A, (5) malathion 5 EC at 1.0 lbs ai/A and (6) oxamyl (Vydate 2L) at 1.0 lbs ai/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 also inoculated on June 14 with 25 predators that were collected from the same field where the study was conducted. Plots were arranged in a completely randomized design with 6 replications.

Pre-treatment samples were collected just prior to application of the pesticides which occurred on June 16, 1993 at 10:00 am. Treatments were applied using an R&R C02 back pack sprayer with a single nozzle boom equipped with a Teejet 95004 flatfan nozzle. The ambient temperature at the time of application was 75F, the wind was blowing 3-5 MPH from the NW and the mint stage of growth was 10-14 inches in height. Sampling consisted of collecting 10 plants from each plot and counting the number of TSSM and predator mite motiles and eggs that occurred on six leaves per plant: 2 from the bottom, 2 from the middle and 2 from the top for a total of 60 leaves per sample. The final evaluation occurred on July 3, 1993.

Results and Discussion

Results of the studies on disruption of NF on mint in central Oregon are found in figure 4 and tables 16, 17 and 18. Pre treatment counts taken on June 16, indicated that there was no significant (pc0.o5) differences in the numbers of TSSM motiles/leaf between plots (Table 16). The only significant difference in pre treatment counts of NF/leaf, were in the plots that were to receive an oxamyl treatment. In this case, there were more NF/leaf than in the other plots (Table 17).

The post treatment evaluation conducted on July 3, showed a significant (P<0.01) difference in the mean number of TSSM motiles and eggs/leaf in the plots treated with carbofuran compared with the other treatments (Table 16). There were no significant differences among the remaining treatments for either TSSM motiles or eggs. Also on July 3, there were significantly (P<0.05) fewer NF motiles/leaf in plots treated with carbofuran and oxamyl when compared to the untreated check (Table 17). On this same date, there were significantly fewer NF motiles/leaf in the methomyl treatment compared with either plots treated with sulfur or malathion (Table 17).

In table 18, the effect of the different insecticide treatments on the ratio of TSSM motiles to NF motiles is found. On both July 3 and August 1, this ratio was significantly (P<0.01) higher in the plots treated with carbofuran compared to the remaining treatments (Table 18). Although there was a trend towards higher ratios in the methomyl, malathion and oxamyl plots, they were not significantly higher than the untreated check.

These data suggest that applications of carbofuran, methomyl, and oxamyl can result in a resurgence of TSSM on mint. Also, applications of methomyl for foliar insects and oxamyl for nematodes, also can result in increased TSSM injury due to disruption of predator mites. This list can now be included with the pyrethroids tested in 1992 as insecticides to beware of due to TSSM resurgence. This doesn't mean that the other insecticides can be used without similar problems. Hollingsworth and Berry 1983, catalogued a number of insect predators on mint that prey on TSSM. The insecticides malathion, acephate and chlorpyrifos are all known to be toxic to predatory insects (Croft 1991), so should be used only when absolutely necessary.

Table 16: Evaluation of insecticides on populations of A. fallacis and Tetranychus urticae. Study was conducted in Lower Bridge Oregon, 1993.
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                                                            Mean No. / Leaf
                                     June 16 Pre-Treatment         July 3, Post Treatment
                      Rate               TSSM         TSSM                  TSSM         TSSM
Treatment    Ibs ai/A        No. Motile     No. Eggs         No. Motile     No. Eggs
____________________________________________________________________

Untreated         -                   6.75 a       14.71 a                 4.22 b         25.44 b
Methomyl       0.9                 4.42 a          8.13 a                6.67 b         28.98 b
Malathion      1.0                  6.43 a       15.57 a                 4.00 b         14.17 b
Thiolux          6.0                 7.26 a        12.14 a                 5.29 b         31.20 b
Carbofuran    2.0                 9.78 a         25.97 a              15.61 a         162.13 a
Oxamyl         1.0                  5.97 a        22.76 a                4.58 b        13.23 b
____________________________________________________________________
P value                              P<0.05         P<0.05                  P<0.01         P<0.01

____________________________________________________________________
Stand Error                         1.38              3.19                      1.87           13.15
____________________________________________________________________
Means with same letter are not significantly different. FPLSD.

Table 17: Evaluation of insecticides on populations of N. fallacis. Study was conducted in Lower Bridge, Oregon 1993.
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                                                          Mean No. / Leaf
                                   June 16. Pre-Treatment       July 3. Post Treatment
                      Rate         Pred mite    Pred mite         Pred mite    Pred mite
Treatment    lbs ai/A     No. Motile     No. Eggs         No. Motile     No. Eggs
____________________________________________________________________
Untreated        -              0.052 ab       0.072 a                0.360 ab     0.25 a
Methomyl      0.9            0.042 b         0.022 a                 0.145 bc     0.09 a
Malathion     1.0             0.078 ab       0.038 a                 0.402 a       0.12 a
Thiolux         6.0             0.030 b         0.015 a                 0.383 a       0.18 a
Carbofuran   2.0             0.028 b         0.027 a                 0.013 c        0.01 a
Oxamyl        1.0              0.107 a         0.050a                  0.119 c        0.03a
____________________________________________________________________
P value                            P<0.05         NS                        P<0.01         NS
____________________________________________________________________
Stand Error            0.019         0.072         0.076         0.066
____________________________________________________________________
Means with same letter are not significantly different. FPLSD, NS=not significantly different

Table 18: Evaluation of insecticides on populations of TSSM and Amblyseius fallacis in Lower Bridge, Oregon 1993. Ratio's of TSSM motiles/leaf to A. fallacis/leaf.
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                      Rate             Mean Ratio TSSM / leaf to A. fallacis / leaf
Treatment    lbs ai/A        Post treatment July 3    Post treatment August 1
__________________________________________________________________
Untreated           -                          13.45 b                        11.50 b   
Methomyl         0.9                        46.81 b                       100.59 b
Malathion        1.0                        12.88 b                       100.30 b
Thiolux            6.0                       70.50 b                          20.53 b
Carbofuran      2.0                 11,220.83 a                      2346.67 a
Oxamyl           1.0                        83.84 b                        400.00 b
__________________________________________________________________
P value                                       P<0.01                         P<0.01
__________________________________________________________________
Stand Error                               1771.04                           209.76
__________________________________________________________________

Means with same letter are not significantly different. FPLSD.