Soyabean aphids, which damaged US soya crops in 2001 and 2003, are expected to cause more headaches for US Midwest producers in 2005, crop and insect experts said this week. "Right now, everything is pointing toward a pretty high soyabean aphid year," said Kevin Steffey, a University of Illinois entomologist.
The prediction is based on high numbers of aphids caught in researchers' traps across Illinois last fall, when the insects were moving from soyabean fields to their winter quarters on wild buckthorn plants in the Midwest.
A high aphid count in the fall suggests the potential for a heavy outbreak in the spring.
"In the fall of 2004, we had the highest number of aphids we've ever seen in our traps, and we also documented eggs on buckthorn in the wild," said Dave Voegtlin, an entomologist with the Illinois Natural History Survey.
An aphid outbreak would have the greatest impact in the northern Midwest, where the bulk of US soyabeans are grown.
"Michigan, Wisconsin, Minnesota, northern Iowa - this is buckthorn heaven in the Midwest. This is where the majority of over-wintering is taking place," Voegtlin said.
Soyabean aphids, which are native to Eastern Asia, began showing up in volume in the northern Midwest in 2000. The insects cut soyabean yields by sucking the sap from the plants, robbing soyabean pods of nutrients.
In 2003, US soyabean yields shrank to 33.9 bushels per acre, the lowest average since 1993, due to a drought that summer and a heavy infestation of aphids.
Aphids were not a significant problem for US crops in 2004, partly because of a high number of predator insects such as ladybugs. But the aphid population was able to rebound by the fall, setting the stage for a resurgence in 2005.
Several variables will affect the scope of the 2005 infestation, including spring planting conditions. If soyabean seeding and emergence is later than normal this spring, that would deny aphids the host plants they need for the warm-weather months.
Weather will remain key through the summer, since aphids favour temperatures below 85 degrees Fahrenheit. Both outbreak years of 2001 and 2003 had cooler-than-normal summers.
"When the temperatures climb above 85 degrees, soyabean aphid development starts to drop off rapidly," Steffey said.
"So if we have a very hot, dry summer, even if we have high numbers the preceding year, we're not sure what's going to happen," Steffey said.
Strong spring growth of aphids in the northern Midwest could trigger a wider outbreak later in the summer.
"Here in Missouri, we don't worry much about the first ones, because we don't have that many over-wintering. But we certainly do have those generations migrating in, those winged aphids in late July," said Wayne Bailey, an entomologist with the University of Missouri.
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