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Spider Mite Watch 2009

We are starting to dry up in Ontario and the heat is turned on this week. Along with wheat harvest comes spider mite infestations in both soybeans and seed corn. And windy days help spread them further into fields before you know it. So while you are out scouting for soybean aphids in soys and western bean cutworm eggs in corn, check the field edges too for symptoms of mite damage. Though they can infest field corn too, rarely do they cause enough injury in Ontario to warrant a spray.

Scouting for spider mites begins by monitoring the edges of the fields. Initially, the mites disperse by crawling, so infestations tend to spread from the field edge. In both soybeans and corn, the upper surface of leaves may initially have white or yellow spots (stippling) on them as plant cells collapse from their feeding. Turning the leaf over reveals fine grayish webbing and from the naked eye, very small black moving dots may be visible(Figure 1). Eventually these leaves will dry up, curl and fall off. In corn, infestations tend to start at the bottom of the plant, making the plants appear as though it is burning up along the field's edge but left unchecked, the entire field can start to wilt, impacting kernel development.

Figure 1. Spider mites on the underside of a corn leaf
(T. Baute, OMAFRA)

If management is necessary, dimethoate is the only chemistry currently registered in corn and soybeans that will control spider mites. If a pyrethroid, like Matador, has been recently used in a field that had spider mites present, this can initiate a flare up, causing the mite population to explode in the absence of their predators that were successfully killed off by the pyrethroid.

For more information on scouting and thresholds, refer to the OMAFRA Publication 811, Agronomy Guide for Field Crops at: http://www.omafra.gov.on.ca/english/crops/pub811/13soybean.htm#spider

For chemical control options, refer to the 2009-2010 OMAFRA Publication 812, Field Crop Protection Guide at: http://www.omafra.gov.on.ca/english/crops/pub812/2spider.htm