On Aug. 20, 2003, soybean plants around Lamberton, MN, were dripping with soybean aphids. Then the weather turned wet and by Aug. 23, “it was just about impossible to find a live aphid,” says Bruce Potter, a University of Minnesota (U of M) Extension integrated pest management (IPM) specialist.
What happened? Fungus amongus.
Natural insect-attacking fungi infected and killed the aphids, slashing the population from thousands per plant to nearly zero in just a few days. In moist, cool conditions, these outbreaks of friendly fungi “are as good as insecticide,” says David Ragsdale, U of M entomologist.
Beneficial fungi that attack insects play an important part in the fight against soybean aphids, two-spotted spider mites and other soybean pests. But these helpful fungi can be wiped out by fungicides used to treat soybean rust and other leaf diseases, destroying an important group of natural enemies that help control damaging bugs.
“What we're concerned about,” says Iowa State University Entomologist Matt O'Neal, “is that the use of fungicides will remove a source of aphid mortality and make a bad situation with insect pests even worse.”
It's not a new worry. “When I started 30+ years ago, IPM was just getting started in field crops,” says Craig Grau, a University of Wisconsin plant pathologist. “Questions about the use of fungicides came up, and the entomologists went ballistic. They said fungicides would disrupt fungal pathogens and could potentially give us more serious insect problems.”
In potatoes, for example, frequent fungicide applications destroy the beneficial fungi that hold down aphid numbers, O'Neal says. Farmers are forced to “spray insecticide along with the fungicide because they know there will be an aphid explosion.” Frequent insecticide exposure, in turn, has led to pesticide-resistant aphids in potatoes.
Corn and soybean growers, hoping to improve plant vigor and yields, are applying foliar fungicides more often these days — even when no fungal disease has been detected, says Alison Robertson, an Iowa State University plant pathologist. In 2008, she says, there were frequent reports of growers tankmixing a fungicide with their second glyphosate pass or with a soybean aphid insecticide. “Growers were saying, ‘We're going into the field anyway, so why not throw in a fungicide, too?’”
Research in the north-central U.S. has failed to show consistent profits from routine “plant health” fungicide applications in soybeans, Robertson says. And beyond that, she adds, “We don't know the ecological ramifications of widespread prophylactic use of fungicides.”
One result may be more robust soybean aphid populations. David Ragsdale and Karrie Koch of the U of M found that foliar fungicides cut the number of diseased aphids in soybean plots by nearly 90%, compared to untreated plots.
Related research from Cornell University showed that beneficial fungi also infect aphids in buckthorn, their winter habitat, Ragsdale says. Late in the growing season, when aphids leave soybean fields and migrate to buckthorn, “winged aphids bring the fungi with them,” he says.
Current U of M research is trying to determine if fungicide applications in the summer lead to healthier populations of aphids on buckthorn in the fall, Ragsdale says. That could result in more intense aphid pressure the following year.
Pest management experts also worry that fungicide applications could cause spider mite flares, an effect observed in Wisconsin, Grau says. Friendly fungi usually keep spider mites in check, except during droughts, when fungi don't thrive.
But in the last few years, “we're seeing more frequent spider mite outbreaks,” Ragsdale says. Entomologists aren't sure why, but some suspect that the recent trend toward prophylactic fungicide applications may be to blame, he says.
In 2008, Robertson and O'Neal looked at the effects of adding a foliar fungicide to insecticide applications at soybean growth stages R1 and R3. Yields were compared to plots where insecticide alone was sprayed when aphid numbers reached economic thresholds — the recommended IPM practice.
The trials found no advantage for adding a fungicide, Robertson says, which “confirmed our current recommendations.” In some of the seven locations where fungicide was sprayed along with insecticide, O'Neal adds, “there were more aphids later in the season than in the plots where we applied an insecticide alone.”
Potter heard complaints from growers who tried a fungicide on a few soybean fields last summer, and then found that “those were the same fields that had to be retreated for aphids.” These are just anecdotal reports, Potter notes, not experimental evidence of a link. Still, he says, it's important to remember that “there can be unexpected consequences with fungicides, just like with insecticides.” In experiments, “fungicides have been observed to interact with aphid populations by several mechanisms.”
Also keep in mind that your first line of defense against fungal diseases should not be a fungicide, but variety selection, Grau says. “Look for stability of yield over years and locations. Watch varieties in the fall. Look for those that mature when the book says they should mature, those that drop their leaves when they are supposed to. If a variety loses its leaves earlier than expected, it may be highly susceptible to leaf diseases.”
And to protect soybean aphids' natural enemies, you should avoid unnecessary fungicide applications, Ragsdale tells growers. “If you have a fungal disease at densities that cause yield losses, fungicides are appropriate. But indiscriminate use of fungicide is not pest management. It's gambling.”