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Soybean growerswatch for unintended consequences of pesticide application

Agriculture.com Staff 07/31/2006 @ 12:45pm

Sudden population explosions of soybean pests like spider mites, bean leaf beetles and soybean aphids can create itchy trigger fingers for pesticide application among growers in the western Corn Belt, and rightfully so.

But, when it comes to combating a potentially damaging infestation, it's best to err on the cautious side and keep in mind a few key points when selecting and applying chemical pesticides, according to Virgil Schmitt, Iowa State University Extension crops field specialist in Muscatine County, Iowa.

It's important, first of all, to understand the consequences of a chemical application on any beneficial insects that are present in a field. While a particular type of chemical, like synthetic pyrethroid, may work well for one pest, it may create an unintended explosion in numbers in another.

"Synthetic pyrethroids kill many beneficials but have little effect on spider mites," Schmitt says. "If a field has a low level of spider mites and a synthetic pyrethroid is used for another insect, the destruction of the beneficials can cause spider mites to flair up. Especially in some of the drier areas, be sure to inspect the field closely for evidence of spider mites before treating with a synthetic pyrethroid."

If spider mites are present in a given field, Schmitt advises opting for a chlorpyrifos product like Lorsban® to avoid an unintended population boost.

Dimethoate products also can be very efficacious on one pest but actually foster the growth of another. While very effective on spider mites and others, applying dimethoate can inadvertently cause a bloom in the soybean aphid population. In the presence of both aphids and spider mites, Schmitt also recommends using a chlorpyrifos-based product.

When it comes to residual effects, conditions on the ground and not always the chemicals themselves can influence application decisions. A good rule of thumb, Schmitt says, is to use dimethoate products, which have short residual effects, as a "knock-down" only, saving chlorpyrifos and synthetic pyrethroid products for longer residual activity. Even then, long-term efficacy depends on the conditions.

"Chlorpyrifos products are generally considered to have shorter residual activity than do synthetic pyrethroids," he says. "However, in Illinois in 2003, in areas where the weather was very hot (above 95 degrees), the experience was that chlorpyrifos had a longer residual life."

Sudden population explosions of soybean pests like spider mites, bean leaf beetles and soybean aphids can create itchy trigger fingers for pesticide application among growers in the western Corn Belt, and rightfully so.

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