Spider mites attacking soybean fields, creating double whammy

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The recent dry weather in Ohio has resulted in concern with two-spotted spider mites (TSSM). Numerous calls have been received indicating the presence of mites in many fields.

Visits to northeastern Ohio revealed a few fields with ongoing TSSM outbreaks that warrant treatment. We observed other fields with increasing mite populations that are waiting to explode. These fields have areas, especially along field edge, with very large egg populations.

The presence of TSSM along with continued drought conditions are a cause for concern.

What to watch for. Where TSSM is a problem, discoloration of foliage will be evident. TSSM feeding sites will exhibit speckling, and all stages of mites (adults, nymphs, and eggs) will be readily detected by inspecting the underside of impacted foliage with a 10X hand lens.

The decision to treat should be based on the following concerns:

* Does a soybean stand exhibit significant stress and foliage discoloration that can be linked to the mites and not attributed primarily to drought or herbicide stress?

It should be noted that either drought or herbicide stress may enhance spider mite development.

* Is the healthy green foliage showing signs of TSSM feeding such as stippling of the upper-side of leaves and expanding discoloration of the under-side of leaves?

If fresh signs of TSSM feeding are widespread and easily detected throughout a field, then the infestation is well established and economic losses are likely to result if hot and dry growing conditions continue.

* Finally, are all stages of mites present? A key factor is the easy detection of mite eggs which will require the use of a hand lens.

What to look for. If one inspects the underside of a leaf below a point of leaf discoloration and many eggs are detected, then a significant hatch is forthcoming and the TSSM population is about to multiply leading to additional foliage injury.

Under a hand lens, the eggs appear like little marbles and should be readily detected when the adult mites (with two black spots) and nymphs (with 6 or 8 legs) are active.

If TSSM is found only in hot spots or mites mostly along the field’s edge, the entire field should also be examined for the presence of mites. If eggs or light populations are found throughout the field, growers should give strong consideration to spraying the entire field.

Aphid worry. A concern this year relates to the spraying of TSSM in fields also having soybean aphids. Although aphid populations are probably not at levels warranting treatment, our concern is that spraying for TSSM might allow the aphids to increase in population size.

This could occur for various reasons, especially if beneficials insects, such as lady beetles, are killed. There is evidence that lady beetles greatly assist in keeping aphids at low levels.

Because the aphid situation is so new, we do not know what will happen.

Although numerous materials have mites on their labels, the two most likely ones that will be used against TSSM are Lorsban and dimethoate. Some of the other materials are listed for suppression only.

In unreplicated field tests in Michigan in 2000, Lorsban appeared to be slightly more effective that dimethoate in lowering aphid numbers. However, another state reported similar efficacy with both materials.

If soybean aphid is common in fields that will be treated for TSSM, Lorsban might be a better choice than dimethoate based on this information.

We would advise that growers keep a close watch on TSSM-treated fields that also have soybean aphid present.

(The authors are entomologists at Ohio State University.)

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