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Soybean aphid exhibits unusual behavior, perplexes expert

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Written Thursday, December 03, 2009  

Just when entomologists think they have the soybean aphid figured out, the tiny sapsucker throws a monkey in the wrench.

Ron Hammond, an Ohio State University Extension entomologist, said 2009 was such an unusual year for the soybean pest that he’s hesitant to predict what’s in store for growers next season.

“For the first time, at this point, I’m not really sure what will happen in 2010,” said Hammond.

The soybean aphid, whose voracious appetite can greatly damage untreated soybean fields, came on the scene in Ohio in 2001. Since then, the insect has taken growers on a rollercoaster ride of high populations one year and low populations the next. This season was predicted to be a high year, with aphids expected to settle in fields across northern and central Ohio as they migrated south from Canada and Michigan. But, the season didn’t turn out quite as Hammond and his colleagues predicted.

“We started seeing aphids early on in the season and thought they’d build up in the northern part of the state. The aphids built up heavy in northeast Ohio and in some locations along the lake, but nowhere near what we had expected,” Hammond said. “By contrast, we found aphids in southern Ohio for the very first time and beyond threshold numbers. Populations throughout central Ohio were practically nonexistent, so we are not sure where the southern populations came from.”

Hammond said other Midwestern states were experiencing the same phenomenon—lower-than-anticipated aphid populations in the north, exploding southern populations and virtually non-existent populations in the central part of the state.

Aphids normally take flight in mid-August, to find buckthorn plants on which to overwinter. However, cooler-than-normal summer temperatures in Ohio pushed flight back to late August and early September.

“In a normal high aphid year, winged aphid populations would die down and we wouldn’t see much on buckthorn,” Hammond said. “That would suggest a coming season of low aphid populations, but the late flights lasted so long that we suddenly had incredible numbers on buckthorn and we anticipated seeing a lot of eggs. We thought this would break the cycle of high populations one year and low populations the next.”

However, a fungal pathogen, at the right place and the right time, wiped out much of the winged aphids on buckthorn. The result: Very few eggs were found after all.

“So, we are heading into winter not really having a clue what’s going to happen,” Hammond said. “Are we going to continue our two-year cycle where we won’t have problems in 2010, or was there enough overwintering to potentially generate high populations next year?”

Hammond said that growers should stay tuned to OSU Extension’s Crop Observation and Recommendation Network (C.O.R.N.) newsletter for their cue to begin scouting for soybean aphids in their fields next year.

The best way to manage the soybean aphid is to become more knowledgeable about the insect’s biology, know when to scout and to carefully time foliar insecticide applications if treatments are warranted. The economic threshold of aphids is 250 insects per plant.

“We’ll begin sampling fields in June to see if we have early aphid populations. That will determine if we will have a typical low year,” Hammond said. “We now know we have to pay more attention to southern Ohio. In the past we didn’t worry about southern counties, but we are not going to be able to take that approach this time.”

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