Director of Research, Iowa Soybean Association

Soybean aphid is the most significant above-ground pest that farmers face today.  Aphids reduce soybean yield and farmer profitability. They may even transmit viral diseases that can reduce seed quality at harvest. They’re born pregnant so populations can explode almost as quickly as you can turn your head.

Farmers could be facing another year of heavy aphid populations.  According to Matt O’Neal, Iowa State University soybean entomologist, he and his Midwest colleagues logged record numbers of aphids headed to overwintering sites in 2009.  “The numbers of aphids found on buckthorn last fall were so high that leaves were falling off the plant,” says O’Neal.  However, a follow-up visit to those same areas found very few eggs. “This is a bit of a mystery to us,” admits O’Neal. He also admits they’ve seen this before and if history repeats itself, farmers will be spraying for aphids in late July or early August.

“One well-timed foliar application applied after the aphid population exceeds the economic threshold will protect yield and increase profits in most situations,” says O’Neal.

Timing is very important, however.

“Adding insecticide to an early-season glyphosate application as ‘insurance’ doesn’t make sense, says David Ragsdale, University of Minnesota entomologist. Weeds need to be controlled early because they compete for water, nutrients and sunlight. Moreover, aphid populations don’t reach the economic threshold until much later.

The timing of a fungicide application may overlap with the management of soybean aphid. However, many fungicides are toxic to naturally occurring fungi that attack aphids and their use could lead to a subsequent increase in aphid numbers. Farmers who apply fungicide either alone or in a tank mix should monitor those fields for aphid population growth.

Photo: Iowa State University

Some marketing programs encourage the prophylactic use of fungicides and insecticides as a tactic for managing soybean. Although yield gains using this tactic have been recorded, the results have been inconsistent and thus the benefit this management tactic remains unclear. Ragsdale believes there is a better way.

“Aphid management requires multiple tactics,” states Ragsdale.  He advises farmers against becoming too reliant on the “quick fixes” when making management decisions. “Using multiple tactics including cultural, chemical, and biological control is the most effective aphid management plan for long-term, profitable soybean production,” he concludes.

Both Ragsdale and O’Neal recommend regular scouting and a 250-aphid-per-plant threshold for spraying to conserve the natural enemies that prey on aphids. Scout once or twice a week beginning in late June or early July, no later than the R1 (beginning bloom) soybean growth stage, and continue through pod fill. Pay particular attention to late-planted fields, or fields under moisture stress.

That’s your soybean checkoff. Delivering results.  For more information on effective management of the soybean aphid access www.planthealth.info

-David Wright

-Funded through the Soybean Checkoff