Since You'll be Scouting for Aphids Anyway...
Look for Spider Mites Too!

With wheat harvest starting and this hot dry weather never ending…spider mites could be our next threat. Under hot, dry, windy conditions, they reproduce rapidly and infestations can spread quickly. Spider mites typically move in from the field's edge, especially from grassy weeds and wheat 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.

Figure 1 - Close-up of two-spotted spider mite adult and juvenile.

Begin by monitoring the edges of your soybean 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 (Fig. 1). Fields should be scouted weekly until mites are noticed and then scout every two to three days to stay on top of the situation.

Mites have been known to reduce yields as much as 60% if gone untreated when thresholds are reached. 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 2005-2006 version of the OMAFRA Publication 812, Field Crop Protection Guide available on the OMAFRA website and resource centres.

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