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Watch for midsummer insects

It’s July and not a fun time to scout for insects in soybean fields, but you really should go look. Of course, the first insect to watch for is Asian soybean aphid, our most economically important soybean pest. Next you need to watch for Japanese beetle, a relatively new pest in some parts of Iowa.

Finally, check for the old standbys: green cloverworm, bean leaf beetle and defoliators, such as grasshoppers and the imported longhorned weevil.

Soybean aphid made its appearance in Iowa in 2000 in the northeast part of the state. We all dealt with it in 2001 as we (agronomists and farmers) struggled to determine treatable population levels and treatment programs.

University researchers in the Midwest worked hard to determine the economic potential of this new soybean pest. After a few years, the economic injury level, or EIL, was determined to be about 675 aphids per plant. The EIL is the point at which yield reductions equal the cost of treatment. Nobody wants to reach EIL.

The economic threshold, or ET, for soybean aphid is 250 aphids per plant. That’s the threshold you should use to make treatment decisions. The entomologists assumed it would take roughly a week for a population of soybean aphids to go from 250 to 675 aphids per plant. So using 250 aphids per plant as the ET would allow enough time for farmers to line up the pesticide and apply the treatment before fields reached the EIL.

Although the EIL is a dynamic number and will decrease with high soybean prices, the ET remains at 250 aphids per plant. A word of caution: ET is the point at which a treatment decision is made. Granted, many farmers will treat when aphid populations are much less than the ET. They reason that aphid populations will always continue to increase, so why not treat now and stop scouting?

Treating before ET

Here are three reasons to not treat before the aphid population reaches ET:

More frequent pesticide applications can promote insect resistance.

Aphid populations are highly variable during summer and could naturally decline before the ET is reached.

Treating too early could result in multiple applications, which will increase the overall production costs for that field.

The ET provides a balance between increasing aphid populations, protecting yield and saving growers unnecessary application costs.

University entomologists and graduate students continue to evaluate host plant resistance to soybean aphid. The Rag1 gene is currently available in several soybean varieties. This gene is slowing aphid development on plants but not stopping it.

Several other host plant resistance genes are being evaluated by university researchers, including Rag2, Rag3 and Rag4. They find that these genes also slow aphid development but do not stop it.

Besides single gene expression, breeders are finding ways to combine Rag genes for improved expression. When plants contain both the Rag1 and Rag2 genes, researchers have found that additional protection was provided over a single gene.

Another insect to watch for is the Japanese beetle. These beetles are defoliators and will infest soybeans in July and August. For several years, Japanese beetle has been a problem in southeast Iowa and in scattered locations in central Iowa. This pest seems to be spreading to new areas.

Japanese beetle’s hangouts

Japanese beetle is often a problem in vineyards, fruit trees and ornamental plants. Farmers with bean fields close to vineyards or fruit trees should watch for Japanese beetle. The traditional economic threshold for treatment of soybeans after bloom is 20% defoliation.

The final pests to watch for this July include the traditional defoliators such as bean leaf beetle, grasshoppers, green cloverworm and imported longhorn weevil. Check ISU Extension ET’s and treatment recommendations.

An excellent resource is the Soybean Disease and Pest Management Field Guide, available from ISU Extension and the Iowa Soybean Association.

Holmes is the ISU Extension field agronomist at Clarion in north-central Iowa. ISU Extension entomologist Erin Hodgson contributed to this month’s column.


TINY BUT MIGHTY: Soybean aphids are a tiny pest but can cause big yield losses. Entomologists recommend soybean growers start scouting fields for aphids before soybeans bloom.

This article published in the July, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.