Farmers scouting fields for Asian soybean rust may find another pest from the Far East first, said a University of Missouri entomologist. Soybean aphids, formerly known as Chinese aphids, are in low numbers across Missouri.

"Winged stages of the aphid are being found, indicating they migrated from northern states," said Wayne Bailey, with MU Extension. "We have a very limited winter population found on buckthorn shrubs, an alternate host."

Northern states and Canada had higher than usual overwintering populations this year, Bailey said. Aphids arrived in Missouri earlier than usual.

"We should see our largest migrations in the third and fourth weeks of July," Bailey said.

Huge colonies, with thousands of aphids per plant, can build up quickly under favorable conditions. Aphids suck juice from soybean plants reducing yields or even killing the plants.

Most of the year aphids are wingless. However, when aphids reach some stress level in a field they develop wings and fly up where winds carry them to new fields. "We don't know what triggers migration, which can be for hundreds of miles," Bailey said. "Some entomologists think it has to do with population density and day length.

"If we have a few days of north winds, we could see a huge flight of soybean aphids into the state."

Iowa, Minnesota and Wisconsin have had crop losses from the aphid in past years, but Missouri has been lucky, Bailey said. "With huge populations north of us, we may get our first heavy infestations this year."

The economic threshold for spraying the crop for pest control is 250 aphids per soybean plant. "We can reach damaging numbers in only a few days," Bailey said. "The aphids give live birth and produce a new generation every couple of days."

Scouting for aphids requires a hand lens with 10X or greater magnification. The light green, almost colorless, aphids appear first on bottoms of lower leaves and on plant stems.

"Soybean aphids are distinguished by two black 'tail pipes,'" Bailey said. "They are the only aphids that colonize soybeans."

Aphid colonies produce honeydew that grows a black mold. "Walking though an infected field can result in black smudges on pant legs," Bailey said.

The only thing that might slow the aphid invasion is hot, dry, weather. "Aphids prefer cooler, moister conditions than we usually have in Missouri," Bailey said.

Areas that received rain are more likely to see aphid colonies develop first. "Aphids prefer lush growth," Bailey told regional extension agronomists on a crop teleconference from the MU campus in Columbia.

So far, beneficial insects have kept aphid infestations in check. Ladybugs and insidious flower bugs, also known as minute pirate bugs, eat aphids as they arrive on the soybean plants.

"A huge migration could overwhelm our biological defense," Bailey said. "This is just one more thing that soybean growers don't need." Drought, followed by spider mites, has reduced growth of soybeans in a band of counties across the state from the southwest to the northeast.