Agriculture Online
07-31-06
Soybean growers: watch for unintended consequences of pesticide application
Chemicals effective on one pest can sometimes promote another, specialist says
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."