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Issue No. 19, Article 5/July 29, 2005

Spider Mite Problems Intensify in Northern Illinois

We have received several reports of lack of efficacy of miticides applied (especially those applied for the second time) for control of twospotted spider mites in soybeans. In a previous issue of the Bulletin (issue No. 18, July 22, 2005), we indicated that our data from a miticide efficacy trial near Tolono (Champaign County) suggested that a second application of the same product (either Dimethoate 4E or Lorsban 4E) would not be advisable. The most recent data we extracted from the trial verifies our initial concern. From July 15 to 22, numbers of spider mites increased from 543 to 782 per five leaves in plots that had been treated two times with Dimethoate 4E. Numbers of spider mites increased from 281 to 971 per five leaves in plots that had been treated two times with Lorsban 4E. During the same time period, numbers of mites in the two untreated checks decreased from 860 to 487 mites per five leaves and 1,383 to 600 mites per five leaves, respectively.

It is apparent that using the same product the second time is not advisable. If dimethoate was applied the first time, consider applying chlorpyrifos the second time, and vice versa. Some individuals have done this and still believe that control of twospotted spider mites has not been acceptable.

It is very interesting--and important--to note the significant decrease in numbers of spider mites in plots that have not been treated this summer. We are not certain yet what has caused this reduction, but we can speculate that in the absence of miticides, natural enemies may be suppressing spider mite populations.

For people who have given up on using dimethoate and chlorpyrifos for control of twospotted spider mites in soybeans in Illinois, the only other labeled options are Proaxis and Warrior, both pyrethroids, and both labeled for "suppression only." The rate of application for both products is 3.84 fluid ounces per acre. We do not have any data regarding the effectiveness of either product. The only other option at this time is to make the decision not to spray again, with hopes that the natural enemies will suppress increasing populations to below-economic levels.

In fields of soybeans that have been or will be treated for control of soybean aphids, the application of pyrethroids not labeled for control of twospotted spider mites (i.e., Asana, Baythroid, Mustang Max) could exacerbate spider mite problems. Fields treated for control of soybean aphids should be inspected for twopotted spider mites (after the legal re-entry time indicated on the insecticide label) to make certain that spider mites don't get away from you. This will most likely occur in the drought-stricken areas of Illinois where spider mite populations are growing.

At this time, we cannot explain the increases in numbers of twospotted spider mites in plots that have been treated twice with the same product this summer. We intend to investigate this phenomenon further to try to obtain some answers.--Kevin Steffey and Mike Gray

Authors:
Kevin Steffey
Mike Gray

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