Friday, July 29, 2011

Drought favors spider mites and grasshoppers

Although some parts of Iowa have had adequate moisture (or even flooding!), other areas could use some rain. Drought stress combined with high temperatures is good news for field crop pests like grasshoppers and spider mites. If your area is hot and dry, consider scouting fields now and throughout August. 

The two-spotted spider mite thrives in hot, dry weather. Spider mite injury to soybean can resemble herbicide injury or a foliar disease; however, characteristic signs are tiny yellow spots, or stipples, on leaves. As the injury becomes more severe, leaves turn yellow, then brown, and finally die and drop off. Spider mite injury can reduce soybean yields by 40 to 60 percent, and cause pod shattering, wrinkled seed, and early maturity.

Heavy spider mite damage to corn and soybean will cause leaves to look stippled. Photo by Whitney Cranshaw.

In corn, prolonged spider mite feeding will turn leaves yellow with stippling on the upper surface. Heavy infestations can cause premature drying, which results in the loss of leaf tissue, stalk breakage, and kernel shrinkage. 

Under dry conditions, foliar treatments are recommended when corn or soybean have substantial stippling or leaf-yellowing and spider mites are active. Although spider mites are not insects, they are often treated with insecticides. Pyrethroid chemicals are not very effective at reducing outbreaks, so consider using organophosphates. Using pyrethroids to control other pests (e.g., soybean aphid) may actually flare spider mites in the field. 

Warm and dry weather also favors grasshoppers growth and development. When late summer grasshopper damage does occur, it usually is related to drought conditions and is frequently, but not always, restricted to field edges. There are two common grasshoppers in soybean in Iowa, the differential grasshopper and the redlegged grasshopper. 

Young grasshopper nymphs eat irregular-shaped holes in tender leaf tissue and may consume the entire seedling. Older nymphs and adults can consume all of the leaf except the tougher veins. Grasshoppers chew through green soybean pods (which bean leaf beetles will not do) and destroy the seeds within. They can also feed on developing corn ears and destroy kernels. 

Grasshoppers chew through soybean pods and can damage corn kernels. 
Photo by Marlin E. Rice.

Reducing grasses and other weeds within and around fields will discourage adults from feeding and mating in that area. The economic thresholds are based on leaf area consumed or percent defoliation. In soybean, a foliar treatment may be justified if defoliation exceeds 40 percent before R1 (full bloom) or 20 percent after R1. Consider an insecticide application in corn if grasshoppers are clipping silks or ear tips, or are removing foliage above the ear leaf. Border treatments are recommended if infestations are restricted to field edges.

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