Soybean aphids are sneaky. Attacks seem to come out of nowhere, and recently, the pest has also sho wn an increasing ability to withstand insecticidal controls.
That’s why entomologists such as University of Minnesota’s Bob Koch are encouraging growers to consider the sap-sucking insects when selecting soybean varieties.
Relying on insecticides alone is an increasingly questionable strategy, according to Koch. All three of the classes of insecticides used against the soybean aphid are facing problems, either from insect resistance or regulatory scrutiny.
Aphid-resistant soybean varieties are still somewhat scarce, but Koch said growers should give them a try. “They don’t have to dive entirely into one variety,” he said. “We’re encouraging growers to do a little experimenting on their farms and see how they do.”
A rise in the use of aphid-resistant varieties could help push the industry to approach aphid management in a more sustainable way, Koch noted. “In many other crops, pest-resistant varieties are the cornerstone for integrated pest management, but that hasn’t caught on for soybeans yet,” he said.
WHAT’S OUT THERE
Aphid-resistant genes — dubbed Rag1, Rag2, and so on — are most effective when they are stacked together. A number of university breeding programs, including the Universities of Minnesota, Illinois and Michigan, are working to produce “pyramided” soybean lines with combinations of Rag genes, Koch said.
In the meantime, a handful of varieties are already commercially available with either Rag1, Rag2, or a combination of those two genes. You can see a list of some (mostly suited to northern Midwest growers, such as Minnesota or northern Iowa) in this University of Minnesota chart here: http://bit.ly/….
Keep in mind that single-gene varieties are not as robust a protection against the aphid, whose rapid and asexual reproduction increases the chances for the development of resistance. “Unfortunately, we have already had different populations or biotypes of aphids detected that can overcome single-resistance genes,” Koch said.
HOW TO EXPERIMENT
This year, Koch hopes to plant some demonstration plots with aphid-resistant soybean lines, with the goal of producing data on their performance for farmers.
Growers can do the same, of course, with strip trials or small acreages.
Research from Iowa State has shown that the Rag genes do not produce a yield drag, in and of themselves. But growers must be careful to pick varieties well adapted to their region, and compare them against susceptible soybean lines with similar genetics in order to get valid results, Koch cautioned.
WHY WE NEED IT
The goal is to have a robust line-up of aphid-resistant varieties that can be used alongside insecticides to prolong the life of both, Koch said.
In 2016, farmers once again reported pyrethroids failing to control aphids in parts of Minnesota and northern Iowa. University of Minnesota researchers have confirmed resistance in some of these populations in southwest and northwest Minnesota. “That marks two years in a row with performance issues and laboratory documentation of some levels of resistance,” Koch said.
The levels of resistance Koch and his colleagues have documented have varied in severity, but with persistent aphid populations and regular insecticide treatments in this region, the problem is likely to intensify, he said.
“If insecticides really do start breaking, we’re going to need other options,” he said. Growers have only two options beyond pyrethroids — organophosphates and neonicotinoids. The most effective organophosphate against aphids, chlorpyrifos (Lorsban), is facing a proposed ban by the EPA, and other organophosphate insecticides provide less consistent control of aphids, Koch noted.
Neonicotinoids such as imidacloprid (Gaucho), clothianidin (Poncho) and thiamethoxam (Cruiser) are also facing regulatory scrutiny, and EPA is scheduled to release risk assessments on them before the end of the year.
“So that’s three different groups of chemicals for aphid management and each is being threatened in different ways, and if we lose any one, it really affects resistant management strategies,” Koch said.
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