June 21, 2006

 

Soybean aphids found at low level in seven US states
 

 

Warm, humid weather has begun to increase insect activity across the US, resulting in the discovery of soybean aphids in about a half-dozen major soy-producing states so far this spring.

 

The soybean aphid (Aphis glycines) first appeared on US soil in July 2000 in Wisconsin, and has since spread to at least 22 states - from Maine to Mississippi, and Delaware to North Dakota.

 

High populations of aphids can reduce yields by as much as 50 percent, as they feed on plant sap. The pest, which is native to Japan and China, can also transmit harmful viral diseases to soybeans.

 

The first official US sighting of soybean aphids occurred May 31 on a research farm near Ames, Iowa. One soybean plant carried about 40 aphids, indicating that the colony had likely already been in place for about a week.

 

Since then, soybean aphids have been discovered in Illinois, Indiana, Michigan, Minnesota, Ohio and Tennessee.

 

Aphids have been found a bit earlier in the season than normal in some states this year, although that fact has caused little concern among soy-state entomologists.

 

"Last year, we found our first soybean aphids in Lucas County on June 1," said an integrated pest management newsletter penned by Iowa State University entomologists Marlin Rice and Matt O'Neal. "However, the population in this field reached a peak of less than 250 aphids a plant, which is below the economic threshold. So an early arrival of aphids in soybeans does not forecast a looming disaster in July or August."

 

University of Minnesota extension entomologist Bruce Potter said aphids were found inhabiting soybeans near Crookston last week.

 

"This is the earliest in the season that they had been observed in north-west Minnesota," he said.

 

Fortunately, infestation levels discovered so far have remained well below the economic threshold at which applications of chemical pesticides would be warranted - and have even failed to match 2005 levels in some areas.

 

"By this time last spring, we had already found numerous fields with developing populations of aphids, and had a single field in Wood County that had already reached threshold," said Ohio State University entomologist Ron Hammond. "Last week, we made visits to numerous fields in north-west Ohio and after much searching, located a single field where we found soybean aphids."

 

Aphid densities were extremely low, averaging less than one insect a plant.

 

"Other states have also been finding a few aphids, and it was assumed that Ohio would be no exception," said Hammond. "But observations in Ohio and these other states suggest very low populations, whereas high densities were being found at this time in 2005."

 

Aphid populations appear to fluctuate in a two-year on-again, off-again cycle, affected by a wide variety of environmental factors, such as weather and insect predators.

 

Research conducted by entomologists at Purdue University suggests that predators such as flower bug nymphs may be responsible for suppressing soybean aphid populations early in the growing season, while important late season biological control can be provided by multi-coloured Asian lady beetles.