Pest alert – Two-spotted spider mites in soybeans

Farm Forum

In spite of the abundant rainfall and the relatively mild temperatures, some Minnesota soybean fields have populations of two-spotted spider mites (TSSM) at or near economic damaging levels and mites can be found at lower levels in others.

TSSM infestations have been observed in several counties. In Sibley County, problem fields were soybeans planted into alfalfa. It makes sense that these fields would have higher TSSM populations because the mites winter on perennial plants. TSSM problems are typically associated with drought. What is unusual, and causing concern and some confusion are the high TSSM populations in an area of abundant rainfall and without a period of prolonged hot, dry weather.

A Brown County soybean field was observed with disturbingly abundant TSSM. The mites and their damage had already progressed to the tops of the plants in small pockets in the field. As is typical, the field border was more heavily infested but the mites had been present for some time, most likely before the adjacent road ditch was mowed. We also received a report of a TSSM infestation in a Carver County soybean field. Both of these areas have had above average rainfall.

How can we have a spider mite infestation in soybeans without drought stress?

It could be related to impaired root systems from root disease and/or SCN leading to poor water uptake, even though soil moisture is good. Rhizoctonia root rot and SCN were both obvious in the Brown County field.

Neozygotes, a fungus that normally controls mites when weather is cool and wet, was apparently not effective in these fields. Insecticide and fungicide applications can remove the beneficial arthropods and fungi that typically keep TSSM populations low.

Some entomologists suspect that seed and foliar applications of pesticides containing neonicitinoid insecticides (look for thiamethoxam, imidacloprid or clothianidin on the label) can increase the probability of spider mite problems.

If you applied a seed insecticide, foliar insecticide or foliar fungicide to a soybean field earlier this year, you might want to check to see if you unintentionally created another problem.

Dimethoate, chlorpyrifos, bifenthrin and products containing these insecticides are labeled for TSSM control in soybean. When TSSM are present in a field, be careful with any insecticide applications for soybean aphid. The pyrethroid insecticide bifenthrin (e.g. Brigade, Hero, Tundra) is labeled for two-spotted spider mite control. However, other pyrethroid insecticides are not very effective on mites, and some pyrethroid insecticides can actually increase TSSM reproductive rates.

If you treat a field for spider mites or insects, make sure you evaluate control 5-7 days later. The hatching of two-spotted spider mite eggs and any immature or adult mites surviving poor spray coverage can rapidly re-infest a field.

Resistance to pesticides is always a concern with mites. A chlorpyrifos (e.g. Lorsban) resistant mite population was documented in Redwood County in 2012. It is not known how widespread this resistance is, or even if it is still present in Minnesota mite populations. TSSM populations resistant to chlorpyrifos and/or to bifenthrin are known to occur in other parts of the country. Currently, we do not recommend tank-mixes of insecticides for TSSM control in MN. The use of tank mixes of chlorpyrifos and bifenthrin insecticides is best reserved until mite populations cannot be controlled with the individual products.

At this time, there are relatively few reports of TSSM. We do not yet know the extent of this year’s problem. However, it surprising that we have any reports of significant mite infestations given this year’s weather patterns. If the weather turns hot and dry, we could be fighting TSSM problems for the next two months. Treat the soybean pest problems you know you have and can be effectively controlled. Insurance pesticide applications, in the form of insecticides (of any kind) or foliar fungicides, are probably not the best idea in fields with spider mite populations.

Spider mites populations are present at low levels in most, if not all, soybean fields every year. Adding a “just in case”, low rate of chlorpyrifos, or other product with TSSM activity, to a tank mix of insecticide(s) with poor performance on TSSM is a recipe for pesticide resistance problems. Instead of “fixing” any gaps in insecticide performance, you may end up fixing pesticide resistance in your TSSM population…a very bad thing.