, Farm Journal Seeds & Production Editor
So you've just heard farmers in the neighboring country are spraying for soybean aphids. It's tempting to call in the plane to get the jump on the pest by putting down a layer of protection on your fields too.
Wait a minute. That's just that kind of thinking that can put you on an insecticidal roller coaster that leads to costly multiple sprays, says David Ragsdale, University of Minnesota entomologist.
"Spraying early and thinking you are going to save another one to two bushel per acre is just not supported by any field study,” says Ragsdale. "And when spraying occurs on a wide scale, as it did in 2008, you run the risk of turning soybean aphid into an annual pest.”
Here's why: spraying too early can build up high aphid densities in August and allow aphids to reproduce in "enemy free space.” Ragsdale says it's important for farmers to understand the need to conserve all natural enemies in a soybean field. Widespread use of insecticides in mid-summer kills aphids AND their natural enemies and frees the remnant soybean aphid population (control is never 100%) to build back up in late summer just prior to its migration back to buckthorn. This scenario sets up soybean aphid as an annual pest. "What you end up with is a classic case of pesticide treadmill,” Ragsdale adds. "If you spray too early, you cannot break the cycle of reliance on insecticides.”
While it's tempting to pull the spray trigger rather than scout, Ragsdale and other aphid scientists, insist getting out into the field is a necessity. The economic threshold for soybean aphids is 250 aphids per plant before stage R5.5. That figure has been painstakingly worked out through collaborative, multi-state research over the past few years.
"The soybean aphid economic threshold does not move lower based on higher soybean prices either,” he says. "What changes is how quickly you need to respond once the threshold of 250 is reached. Instead of a week to 10 days, an application needs to be applied in four to five days.
"Using a lower threshold means that your ability to predict whether the pest will reach economic status is about 50% or no better than the flip of a coin. The 250 threshold is set high enough so that you know a damaging aphid population is almost a certainty. An added benefit is that by waiting to reach the 250 threshold before treatment means a single application will control aphids for the entire season.