Since we’ve been getting a bit of rain across the province in the last week I’ve been asked about how the rain will affect spider mite populations in soybeans. Rains are very beneficial to help keep populations of spider mites down but don’t assume that because it rained our spider mite problems are over. If mite populations in the field have been quite high, one rain event will not be enough to reduce high mite numbers below threshold. We will need some continuous cooler weather with higher humidity to give the beneficial fungi a chance to make a dent in the mite numbers. I’ve seen some very healthy mite populations this week in fields that had a good amount of rain on the weekend and with continued warm weather, these populations are likely to continue to increase. To determine if your mites are still alive, check to see if they are still moving around on the leaves. If the mites are not moving even when disturbed then they are most likely dead.
Mite Control Options
Insecticides registered for spider mite control in soybeans are Cygon 480 and Lagon 480, both contain dimethoate as the active ingredient and typically give about a 7 day residual for spider mite control. Drought stress will affect absorption and trans-location of the insecticide in the plant so even though this active is systemic, good coverage is essential for the product to be effective. Using higher water volumes can help improve coverage of leaf area and will give the product a better chance to work. Dimethoate does not control spider mite eggs so in situations where there is a heavy infestation a follow up treatment may be needed when the eggs hatch.
Stress on soybean plants during pod fill can affect yield so if mite populations are above threshold then treatment may be necessary. The entire field may not need to be sprayed; perimeter treatment or treatment of hotspots may be all that is needed to get the problem under control.