No. 20 Article 4/August 5, 2005

Continued Concerns About Control of Twospotted Spider Mites

The article "Spider Mite Problems Intensify in Northern Illinois," published as an "Alert" to the Bulletin online on July 29, 2005, continues to have relevance for many people who have been battling twospotted spider mites in soybeans for some time. We continue to receive reports of unsatisfactory control after second (occasionally third) applications of the same or two miticides, chlorpyrifos and dimethoate. Although we have been sharing these reports, we may have overlooked the obvious in some situations. Keep in mind that application of a miticide to control twospotted spider mites in soybeans will not make the yellowing disappear; injured leaves will not turn green after a miticide is sprayed. This seems like an overobvious statement, but we have not reiterated this caution about assessing performance of miticides this season. If you have sprayed a miticide more than once, make certain that you detect a definite lack of performance (i.e., numbers of spider mites did not decrease, or numbers of spider mites increased after the second application) before assuming that the product did not control spider mites. Injury caused by spider mites will remain obvious for the remainder of the season.

For people who are not certain what to do if mites persist after a second application of a miticide, there may be some help from Mother Nature. In the two untreated plots (each replicated four times) in our spider mite control experiment near Tolono, densities of spider mites have "crashed." From July 15 to 29, the combined average density of spider mites in the untreated checks declined from a peak of 1,122 to 194 per five leaflets, a decrease of about 83% in 2 weeks. From July 22 to 29, the combined average density of spider mites in the untreated checks declined from 544 to 194 per five leaflets, a decrease of about 64% in 1 week. In one of the two checks, the decline was from a peak of 1,383 spider mites per five leaflets on July 15 to only 58 mites per five leaflets on July 29, a decrease of about 96% in 2 weeks. The average densities of spider mites in all treated plots (with one exception) also declined from July 22 to July 29, although the decline was less dramatic in treated plots than in the untreated checks. In fact, the average density of spider mites in the untreated checks now is lower than the average density of spider mites in all of the treated plots (with one exception).

We are not certain why the spider mite densities have crashed in our experiment, but we have retained samples for examination. It is possible that natural enemies not controlled previously with applications of miticides could be suppressing twospotted spider mite populations. It is also possible that the recent higher humidity has enabled establishment of a fungal organism that is resulting in an epizootic. Regardless of the cause, the crash should come as good news for people who have become frustrated with controlling spider mites in soybeans. It may be wise to wait if you are contemplating a third miticide application this summer.--Kevin Steffey and Mike Gray

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