Some regions have had a bit of rain, but others haven't seen rain in a while. Roadside grass is starting to dry up and get cut, which means spider mites move into soybean fields soon. So while you are out scouting for soybean aphids, check the field edges too for symptoms of mite damage.

Scouting for spider mites begins by monitoring the edges of your soybean fields. Initially, the mites disperse by crawling, so infestations tend to spread from the field edge. Non-mated female mites will mass at the top of the plants and spin webs that serve as a "balloon" allowing strong winds to pick them up and carry them off to another plant either in your field or into the neighbouring fields.

The upper surface of leaves may initially have white or yellow spots on them as plant cells collapse from their feeding. Eventually the leaf will dry up, curl and fall off. Turning the leaf over reveals fine webbing and from the naked eye, very small black moving dots may be visible. Shack these leaves onto a white piece of paper to see the actual mites moving around. Under a 10X hand lens you can see the mites crawling around (Figure 1).

Figure 1. Two spotted spider mite close-up (Photo courtesy of University of Missouri)

Four or more mites per leaflet or one severely damaged leaf per plant prior to pod fill is cause for concern. If mite numbers exceed the economic threshold, and the forecast calls for hot and dry conditions, an insecticide may be necessary. If caught early enough, only a spot treatment may be necessary to treat the hot spots on the edges of the field where the mites are moving in from. If rain is in the forecast, spraying should be delayed; prolonged wetness will usually reduce the number of mites to insignificant levels. Also, continue monitoring every two to three days as infestations can increase rapidly and treated fields can have re-infestations occur.

For information on chemical control options, refer to the OMAFRA Publication 812, Field Crop Protection Guide available on the OMAFRA website and resource centres.