Soybean School: Refining the Soybean Aphid Threshold to Factor in Friendly Bugs

When soybean aphids start multiplying in soybean fields, the decision to spray is typically triggered by the number of aphids found on each plant.

The threshold for growers in Western Canada is usually reached when there are an average of 250 aphids per plant on 80 percent of the plants. The population should still be rising and the plants should not be past the R5 stage.

While this threshold is sound, the actual population where aphids will cause economic injury to soybeans is much higher, explains Jordan Bannerman, entomologist with the University of Manitoba.

“Even when field aphid populations reach that 250 aphids per plant, there’s no real guarantee those aphids will actually reach the economic injury level of 675 aphids per plant,” he explains in this Soybean School episode. “One of the primary reasons for this is the role that beneficial predators and parasites play in controlling soybean aphid populations.”

That means there are situations where the current threshold is telling farmers to spray, even when friendly bugs might be present and capable of controlling aphid numbers.

To demonstrate the effect predators can have on aphid numbers at the Manitoba Pulse and Soybean Growers’ field day on July 20, Bannerman and his colleagues set up exclusion cages in a soybean field at the U of M’s Carman research farm. As he outlines in the video, they added seven aphids to plants in and out of little tents that protect aphids from their natural enemies.

“In as little as one week, you can see a dramatic difference in the number of aphids you have on these plants,” he explains, noting the plants exposed to natural predators saw no significant increase, while the aphid numbers sky-rocketed into the 100s on the plants inside the cages.

So Bannerman and others at the U of M are trying to figure out a “dynamic action threshold” which also accounts for the populations of important predators, such as ladybird beetles, green lacewing larvae and other predatory bugs.

“The economic injury level of 675 aphids per plant remains the same,” he explains. “What you would be changing with a dynamic threshold is you would be saying ‘okay, there are 250 aphids, but there’s also an average of one ladybird beetle per plant, so maybe the threshold should be closer to 350 or 400 aphids.’”

Similar work has been done in Ontario (read more here) and the U.S., but Bannerman notes the dynamic threshold would vary with regional differences in aphid predators. Sampling techniques would also have to account for the types of predators.

“This is basically refining what is currently a fairly conservative economic threshold,” he says.

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