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Will soybean aphids break out this year, forcing growers to spray?  Our advice – stay tuned to the Ag news in Illinois to see if aphids are breaking out.

At the moment things are quiet and we have to wait and see what happens. However, take notice that aphids have been sighted in states like the Dakotas, Minnesota and Wisconsin. So there is a chance that populations will show up in Illinois, and in the northern part of the state first. Sightings in nearby states during the first half of July reveal relatively low numbers so far.

The aphid has been around for 15 years now and it is not the threat it once was. A decade ago aphids were boom and bust. They would breakout one year; those populations were knocked back by pesticides and predators and the overwintering population was quite small, so the next season there was little chance of a breakout until the populations built again. Thus a 2-year boom followed by bust cycle. Forward a decade and we no longer see the boom or bust cycle.

Aphids have settled in to stay but there are a lot of predators now who feed on them and particularly the meddlesome ladybugs that have also settled in and occupy homes and buildings in the winter. The boom and bust years don’t happen now as predators keep their numbers under control. But now they can be a pest annually instead of biennially and breakouts over wide swaths of multiple states isn’t common anymore.

Courtesy of University of Illinois Extension

The key to managing aphids is scouting for their presence, monitoring populations relative to 250 aphids per plant action threshold and being prepared to spray. A recent article out of Michigan State University confirms that 250 aphids per plant is still the right threshold. Dr. Christina DiFonza wrote, “The bottom line is that it takes hundreds of soybean aphids per plant to do measurable damage, and even more to cause economic damage. The threshold of 250 soybean aphids per plant is stuck in the minds of growers, so entomologists continue to use it. However, it is on the conservative side (erring towards spraying rather than not), and if you have ready access to a sprayer, you can wait until the population is nearly double before spraying.” That is exactly what we were told 10 to 12 years. The 250 number gives you a few days to organize spraying.

Aphids move into soybeans in July or August and usually there is only one infestation and an insecticide treatment can protect the crop for 2 to 3 weeks. Monitor the population as it approaches 250 and see if it increases. Sometimes the population will stabilize or fizzle. Always check again in a couple days to see if the population is increasing. If you are at R5 or approaching R6 and are approaching 250 take a wait and see approach. If population stabilizes at 500 or below it doesn’t pay to spray.

However, when you reach R6, when soybeans are moving toward maturity, spraying is no longer economically justified.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.