Soybean aphids show resistance to insecticides for second year

By Janelle Atyeo, Tri-State Neighbor Reporter

Wading through stands of knee-high soybeans at the University of Minnesota
Southwest Research Center at Lamberton, Minn., pest management specialist Bruce
Potter pulled back lush, green leaves to reveal stems thick with yellow aphids.
Pockets where pests were thriving were easy to find, despite the plots having been
sprayed with insecticide about a week before.

The research farm is at the edge of a stretch of western Minnesota where some aphids
have developed resistance to a certain class of pesticides known as pyrethroids.
The problem might have started with producers cutting rates, using a less potent
mix of the insecticide or applying insecticide when it wasn’t needed. Those are rules that specialists such as Potter often preach. The whole idea behind the advice is to avoid issues such as resistance. “I hate it when I’m right,” Potter said.

He has been observing plots of soybeans that are part of a nematode study at the
research farm. It was first noticed last year that aphids weren’t responding to
pyrethroids. So far, the problem area is around southern Redwood County and Brown County (Minnesota), in an area that stretches about 20 to 30 miles east of Lamberton, according to Potter. He is hoping the problem doesn’t spread, but some issues with resistance are starting to turn up in areas where it wasn’t a problem last year. Researchers at other universities are keeping an eye out for similar issues in surrounding areas.

Potter suggested checking a field three to four days after applying insecticide to see
how well it’s working. If predator insects are dead but there are pockets of healthy aphids,
it could mean they are resistant to the insecticide. Spraying aphids that are already resistant to the pesticide makes the problem worse, Potter said.

In some areas, pesticides were controlling 70 percent to 80 percent of the aphid population. Potter said that’s not enough. With the high rate at which aphids reproduce, control methods need to reach almost 100 percent in order to be effective, he said. Aphids can double their population in less than two days. Soybean aphids started to reach problem numbers in some fields across the tri-state area in early August, prompting producers to treat their fields with pesticides.

Potter emphasized that it’s important to treat fields only when needed. The agreed-upon
threshold is when the aphid count is up to 250 per plant. It’s tempting to want to treat fields early and get the work out of the way. Potter said spraying early can kill the aphids’ natural enemies, then when aphids are on the move in August, they have no predators to keep their population in check. Aphid populations haven’t been as numerous as last year. Many soybean fields in the area were planted later than usual this season and aphids moved in later. But there’s still the risk of losing yield to aphids. Potter said yield loss is a problem up until soybeans are at the R6 growth stage, when the seed completely fills the pod. Producers are asked to report any problems with aphid control here.
Follow reporter Janelle Atyeo on Twitter @JLNeighbor.

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