WOOSTER, Ohio – Lower populations of the soybean aphid might be in store for growers this growing season, but based on last year's unusual activity, Ohio State University Extension entomologists are not ruling out any surprises.
"We had an unusual summer in 2009 with scattered aphid populations through the state, cooler-than-normal summer temperatures, extremely high aphid populations on buckthorn in the fall, a pathogen which wiped the populations out, and then very few eggs on the buckthorn," said Ron Hammond, an OSU Extension entomologist with the Ohio Agricultural Research and Development Center. "The big question is, are we going to have a low aphid year, and this year we can't say for certain if our predictions will be accurate."
The soybean aphid, whose voracious appetite can greatly damage untreated soybean fields, came on the scene in Ohio in 2001. Since then, it has taken growers on a rollercoaster ride of high populations one year and low populations the next. Last season was predicted as a "high" year, with aphids expecting 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 had 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," said Hammond. "By contrast, we found aphids in southern Ohio for the very first time and beyond threshold numbers. Aphid populations throughout central Ohio were practically nonexistent, so we are not sure where those populations in the south came from."
Hammond said that other states throughout the Midwest were experiencing the same phenomenon: exploding populations across southern counties with the central part of the state generally void of the pest.
In addition, cooler-than-normal summer temperatures in Ohio pushed the flights of winged aphids into late August/early September, when populations normally take flight to their overwintering host of buckthorn in mid-August.
"In a normal ‘high' aphid year, winged aphid populations would die down and we wouldn't see much on buckthorn, suggesting a ‘low' aphid year the following season," said Hammond. "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 that this would break the cycle of high populations one year and low populations the next."
But then a curious thing happened. 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 have been found after all.
"My colleagues and I have found one, maybe two, small aphid colonies in our buckthorn sampling in Ohio and that's it," said Hammond. "There are not a lot of aphids out there. Generally this year would be characterized as a low population year, but we are predicting low populations for reasons we didn't anticipate."
As a result, entomologists are not really sure what to expect as the season progresses and will be keeping a close eye on what happens with soybean aphid populations.
"Most of our aphid problems occur in mid-summer with aphids migrating from more northern areas, like Michigan. However, our colleagues in other Midwest states are seeing the same thing we are seeing with little to no aphids being found," said Hammond. "Our general thought is that we will have low aphid populations this year, but we just can't be 100 percent sure."
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. 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 and that will determine if we will follow a typical low year," said Hammond. "We know now that 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 the next time around."
The C.O.R.N. newsletter can be accessed at the following Web site: http://agcrops.osu.edu.