US soyabean crop faces summer threat from aphids

A tiny sap-sucking pest that infected US soyabean fields two years ago and eroded crop yields is expected to return this summer after surviving the mild winter in good shape, entomologists said. The soyabean aphid made it through the winter in the US Corn Belt by surviving on buckthorn, a common wild plant found from northern Illinois into Minnesota, Wisconsin and Michigan. "This is the first time we've been able to find them over-winter in Ohio. It's leading us to believe that perhaps this will be another bad year for them," said Ron Hammond, an entomologist at Ohio State University.
Soyabean aphids were also found in northern Illinois near Rockford, in several areas in McHenry County near Chicago and in western Illinois around Moline, entomologists said.
Typically, aphids first appear in northerly portions of the Midwest where there is an abundant supply of buckthorn, a favourite winter host of the tiny pest.
Once eggs hatch in the spring, aphids eventually move over to soyabeans by late May or early June, producing several more generations and migrating southward as the summer progresses.
Soya aphids were first spotted in US fields in 2000.
"The pattern seems to be a light year followed by a heavy year followed by a light year. Last year was a light year," said Robert O'Neil, an entomologist with Purdue University in Lafayette, Indiana.
The heaviest infestation of aphids came in the summer of 2003. US soyabean yields were slashed as the crop was hit by drought as well as the tiny insects. The result was the smallest US soya crop in eight years, leading to the smallest US stockpile of soyabeans in 27 years by August 2004.
In Minnesota, for example, in 2003 farmers sprayed 3.5 million acres with insecticide to fight aphids. But growers still lost about $120 million due to the infestation, according to data from the University of Minnesota.
Aphids thrive in dry conditions. So a wet summer could still limit their numbers in coming months. High populations of predators like the lady beetle - an insect that feasts on aphids - could also limit their numbers, entomologists said.
Once aphid populations reach 250 per plant, farmers must spray their bean fields to save yields. Typically, it takes until late July or early August for aphid populations to reach that level in the heart of the Midwest.