Hot, dry weather provides ideal conditions for spider mite infestations. Typically, this damaging pest moves from grassy plants into soybeans fields when wheat is harvested or when grass ditch banks are cut. 

Scouting in soybeans

To scout for spider mites, check the grassy edges along the perimeter of your fields. You may also find them along fencerows or in tree lines. Once spider mites are found in a field, reoccurrence is likely in the same area in the future. Take note of where you find spider mites – those are good spots to start scouting next time you grow soybeans in that field.

Spider mites are very tiny and hard to see with the naked eye. One clue is to look for stippling –  small white or yellowish dots – on the underside of the leaf. A hand lens makes it easier to identify the little pest. If you don’t have a lens, an easy alternative is to take a soybean leaflet and tap it over a white piece of paper. If little specks of dirt start moving around the page, you can be certain you have spider mites.

Threshold for control

The threshold for spider mite control is four mites per leaflet or one severely infested plant, prior to pod fill. Quite often, because they tend to start on the edges of the field, you can manage the population by just spraying once around the perimeter of the field. This keeps them from moving in – or at least delays them from infesting the field further.

If you’ve met the threshold for an in-crop application, Dimethoate (Cygon®/Lagon®) is the only product registered for spider mites on soybeans. Insecticides such as Endigo® or Matador® 120 EC, which offer effective control of soybean aphids, will not kill spider mites. Select your insecticide carefully as you don’t want to harm the beneficials that prey on spider mites and, ultimately, cause the pest population to increase. 

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