SOMETIMES, eliminating one problem in your soybean crop could help encourage the development of another. That's what two university researchers in the upper Midwest say occurred last season in soybean test plots where they applied fungicides.

The two researchers, David Ragsdale, an entomologist at the University of Minnesota, and Craig Grau, a plant pathologist at the University of Wisconsin, were working on how to best address Asian soybean rust with fungicides, should rust move into their respective areas. In the process, they found that the same fungicides used to kill pathogens, such as rust, also kill all beneficial fungi, which help control damaging insects. With fewer beneficial fungi, insects such as soybean aphids and mites were able to flourish.

Healthier insect populations

“The fungicides slowed the disappearance of the aphids,” Ragsdale says. “In untreated plots, aphid populations crashed quickly, whereas in plots treated with fungicide, aphids seemed to stay around longer, even into early September.”

Grau saw similar results for two-spotted spider mite populations in one of his soybean fungicide test plots. Furthermore, Grau harvested lower yields at one plot where fungicides were applied, most likely because mite populations had increased.

“We had 35 to 37 bu./acre in the fungicide-treated plots, versus 41 bu./acre in the side-by-side, untreated control plots,” Grau says. Insecticides were not applied for mite control.

As a result of the research, Ragsdale hypothesizes that fungicides, if used indiscriminately, could negatively affect multiyear insect populations in soybeans, essentially increasing their prevalence.

“Although difficult to test in small plot studies, the scenario that is likely to occur if fungicides are used too frequently is that fungicides will result in healthier aphid populations late in the season,” he says. “As aphids leave soybeans to return to their winter host, buckthorn, a healthier aphid population will result in more eggs laid and contribute to an even higher aphid population the following season.” Both Ragsdale and Grau plan to repeat their fungicide research trials this year.

Sound IPM practices

Ragsdale and Grau encourage producers to follow established guidelines when making fungicide treatment decisions. “As with any pest, treatment should only be applied when the pest is present and when fungicide (or insecticide) application will prevent further yield loss equal to the cost of the treatment,” Ragsdale says. “It is a sound IPM practice to use fungicides when the application is targeting a specific pathogen.”

Ragsdale points out that the focus of the Minnesota research was to determine if insect-pathogenic fungi were suppressed when fungicides targeting soybean pathogens were used at recommended rates and frequency. The key finding of this one-year study is that fungicides likely to be used against soybean rust do have the potential to harm beneficial, insect pathogenic fungi.

Ragsdale's advice is to “use fungicides only when you know you have a particular disease problem that can be effectively managed with fungicides.”

Grau adds, “This reinforces that we need to maintain good IPM practices here. Consultants and growers need to accurately assess the potential for all pests and pathogens before making fungicide applications.”