Farmers are finding aphids in their soybean fields across Missouri, especially along Interstate 70 east of Columbia.

“Don’t panic. Scout your fields before you spray,” said Wayne Bailey, University of Missouri Extension entomologist. While aphids can be devastating, few of the infestations this year are at economic thresholds.

“Spraying too early could be a waste of money,” Bailey said.

Economic infestations of aphids have been found in east-central Missouri around Montgomery City, in central Missouri around Fulton and, to a lesser extent, south of Nevada, Mo.

Most fields are below the economic threshold, 250 aphids per plant, Bailey said. The level at which aphids cause yield loss is more than 1,000 aphids per plant.

“Just because neighbors are spraying doesn’t mean you have enough aphids to invest in control,” he said.

The aphids, which can fly in from soybeans fields in northern states, usually cause little problem in Missouri late in the season, Bailey said. “Scouting fields before spraying is always recommended, but especially now.”

Most seasons, beneficial insects such as ladybugs keep aphids under control. The weather this year has reduced the beneficial insects, but cool summer temperatures have proven favorable for aphid survival.

“When aphids reach 250 per plant, there is time to prepare to spray,” Bailey said. “It takes an aphid population from five to seven days to reach the 1,000-aphid level.”

Pay attention to the growth stage of the soybean plants, Bailey advised. Late-planted beans that are blooming are at risk. If the beans are at pod-filling stage, the threat of economic loss is much less.

“Spraying when aphid count is below 250 probably won’t pay,” he said. “There are some 30 research studies to confirm that.”

Bailey said that early spraying often does more harm to the beneficial insects than to the aphids.

In many of the areas without problems, soybean plants already have passed the R5 (partial pod fill) stage of growth. “No U.S. study shows an economic gain from spraying aphids on plants past pod fill,” he said.

“We’ve scouted lots of fields across central Missouri without finding aphids at economic-threshold levels,” he said. “We find aphids, but heavy infestations are highly localized.”

One thing the MU Extension scouts noted is that infestations are more likely to appear in fields with soils low in potassium. Bailey speculates that applying more potassium might help prevent problems in soybean fields next year.