It’s a numbers game with soybean aphids

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Managing soybean aphid is a numbers game—starting with the skyrocketing population of a growth curve of the pest and working through cumulative aphid days, a calculation that tallies the number of aphids on a plant over time.

“Yield loss occurs when populations grow very quickly,” says entomologist Brian McCornack at Kansas State University. “If you quickly accumulate aphid days over a short period of time, it causes a lot of stress to the plants. When stress occurs and for how long is very critical in determining final yield.”

click image to zoomOhio State University“If you can slow an aphid down by even half – instead of reaching 250 aphids per plant by mid- July, it takes a couple of weeks longer – then the plant is still growing, so it’s much bigger, and there are more natural enemies around,” says Chris DiFonzo, Michigan State University That’s especially true during the R1- to-R4 stages of the crop, when blooms and young pods are highly susceptible to stress.During that period, racking up a high number of aphid days can cause infested plants to abort flowers or pods, McCornack explains. On the other hand, the crop may withstand a longer period of feeding if the population is growing slowly.

Soybean aphids are masters of reproduction, real-life monsters that seem to have been lifted straight out of a science fiction movie. All the soybean aphids you see in your crop are female. They’re born pregnant, and they can pop out as many as eight offspring per day for 10 to 20 days. When the weather is favorable – in the 70s, with moderate humidity – soybean aphid populations on the average crop can double every 72 hours.

As a result, the key tools for managing soybean aphid revolve around beating the numbers. Relatively early in the last decade, entomologists set a treatment threshold of a field average of 250 aphids per plant, with 80 percent of the plants infested. From that number, populations could easily explode to yield-damaging levels – 675 aphids per plant – in a few days. Below the injury level, beneficial insects, fungal disease or vigorous crop growth can often defeat the aphids. Above it, foliar insecticide treatments can protect yields. The 250-aphid-perplant economic threshold helps growers know when to take action to avoid losses, says McCornack.

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