The dry season is leaving the edible bean and soybean crops susceptible to two-spotted spider mites, especially now that wheat harvest is wrapping up. In some years, spider mites will also feed on corn leaves. Spider mite damage and management are very similar for soybeans and edible beans. Because of the dry conditions and likely earlier harvest of edible beans this season, get out and scout for spider mites now.
A single, un-mated spider mite can be the start of a new colony and under hot conditions infestations can grow quickly. Spider mites will move into the edge of bean fields after wheat is harvested, and can then be moved further into fields on the wind using a “balloon” made of spun webbing. They are barely visible to the naked eye, and would require a 10x lense to see. Two spotted spider mites are yellow-brown with two dark spots on their abdomen, or are orange in colour if they are overwintering females. Eggs are found on the underside of bean leaves and are small, round and translucent. When scouting, pull a leaf off the plant and shake it over white paper to see the spider mites moving.
Damage is visible in the form of white stippling (dots) on the upper leaf surface from the sucking mouth parts, and plants appear sand blasted or dusty on the underside of the leaves. From the road a soybean field may look grey if there is an infestation, or brown if the leaves are dropping off and the plants are shutting down and dying.
Action Threshold for beans: 4 mites per leaflet or 1 severely infested plant, prior to pod fill.
Dimethoate (Cygon/Lagon) is the only product registered for spider mites on soybeans and edible beans. Note that there is a 30 day harvest interval after application, so scout fields now and keep this time frame in mind. DO NOT use Matador, it does not control spider mite and will kill beneficial insects which include ladybird beetle, thrips, and predaceous mites.
We are all wishing for rain and analyzing the forecast; if our hopes come true and we are lucky enough to have prolonged leaf wetness, spider mite numbers may drop significantly.