The soybean aphid outbreaks were memorable because it was hard to miss the presence of the aerial applicator airplanes. Although Iowa is a heavy agricultural area with a long history of pesticide use, the added visibility of the aerial applicators caused a real uneasiness among the people who live here as they wondered what was sprayed and if they were exposed to something unusual.
When I looked at the decline of the Poweshiek skipper I wondered if somehow pesticide applications to control soybean aphids were to blame. The problem with that idea is that pesticides, including insecticides, have been used for over 50 years in Iowa, and generally it is thought that pesticides are used more safely now than back then, the pesticide residues don’t hang around as long as in the past, and the Poweshiek skipper did survive the first 35 or 40 years of pesticide use with apparently no problems.
I tried to get some pesticide use information, to compare what was sprayed in 2001 or especially 2003 (the year of the largest soybean aphid outbreak) with other years, and my conclusion was that what information that was available was not good enough.
In 2003, 2.9 million acres of soybeans were treated for soybean aphids. That is about 8% of the total land mass of Iowa. However, there were counties where either aphids or total number of soybean acres were more concentrated. In some of the northern counties of Iowa the percent of the total land mass where pesticides were applied to control aphids was closer to 30%.
Consider how insecticides kill aphids. First of all, they kill a broad spectrum of insects—they don’t just kill aphids. They kill butterflies or their caterpillars as well. Secondly, they must do more than kill on contact—if you miss the aphid with the spray, the active ingredient in the spray must be still able to kill the aphid. So the pesticide evaporates, and the gas is taken up by the respiratory system of the aphid, killing it. (There can be other routes of exposure, but the pesticides used on an emergency basis for aphid control mostly work this way.) So if there is a vapor concentration in the soybean field that is strong enough to kill all aphids within that field, what happens when the wind blows or the vapors disperse?
If a third of the landscape is sprayed with the insecticide, will it still be strong enough to kill other, non-target insects when it spreads to the remaining 2/3? Or if 10% of the landscape is sprayed, is it still strong enough to kill insects on the remaining 90%?
When a pesticide applicator applies a pesticide with the wind blowing, and some of the spray gets somewhere it was not intended to go, the result is called “spray drift.” This generally happens due to the carelessness of the applicator, and may be a violation of the law.
When a pesticide applicator applies it in accordance with the label requirements, and vapors arise from the application area they can spread to areas not intended to be treated. The result is called “vapor drift”, and is usually not considered to be due to carelessness, nor against the law.
We don’t know what wiped out the Poweshiek skipper in Iowa and Minnesota. We certainly cannot rule out vapor drift, especially from the application of pesticides to control soybean aphids. In fact, vapor drift is a fairly strong candidate.