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Hard Winter Might Freeze insect Numbers -- or Not

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Written Friday, March 21, 2003  

Hard winter might freeze insect numbers -- or not

Ohio's winter may have been a cold one with below-average temperatures much of the season, but it's still anyone's guess what impact the wintry weather will have on emerging field crop pests this growing season.

Ron Hammond, an Ohio State University research entomologist with the Ohio Agricultural Research and Development Center, said that growers may experience a variety of weather-related scenarios -- from reduced pest populations to delayed insect emergence to no impact at all.

"It's a numbers game as to how the winter weather may have impacted insect populations," Hammond said. "We expect some mortality due to the cold temperatures, but we had a lot of snow cover that may have provided protection, too. We also might see some insects emerging later in the spring, but we won't know for sure until the weather begins warming up. At any rate, we are recommending growers be aware of the potential differences and still scout, monitor, sample and take action at the appropriate times."

Pests that consistently plague field crops year after year include slugs, armyworms, alfalfa weevil, potato leafhopper, bean leaf beetle, corn flea beetle and the newly identified soybean aphid.

Some insects, like the soybean aphid and bean leaf beetle, overwinter; others, like the armyworm and potato leafhopper, migrate from the south -- leaving the field open as to how the weather will affect them. The only pest that researchers believe has a strong connection to the weather is the corn flea beetle, an insect that can spread Stewart's Wilt bacterial disease to corn.

The insect, which overwinters as an adult, tends to die off during periods of cold temperatures, reducing the severity of Stewart's disease during the growing season. Ohio State researchers use a flea beetle index to help make population predictions by reporting the average temperature of December, January and February. Index values less than 90 indicate a negligible disease threat and values over 100 indicate a severe threat.

This year's flea beetle index is reporting low insect populations and a negligible disease threat. But there's a catch.

"Our index suggests that we won't have problems on corn this year," Hammond said. "But we had more flea beetles on corn last summer, in July and August, than we've ever seen before, so it's possible that a lot of flea beetles could have survived the winter. A good kill could still leave a lot of them out there. So we are still recommending growers scout for them this spring."

Basing predictions on a previous year's populations can be a game of hit or miss. Whether soybean growers will be plagued with high bean leaf beetle populations or again escape the soybean aphid, remains to be seen.

"It was surprising to find fields full of bean leaf beetles last fall, but we can't say one way or the other that they will be there again this yea," Hammond said. "As for the soybean aphid, we expected them to be a big problem in 2002 because they were really heavy in 2001. But then they didnt show up. Our question is, what happened in 2002? We don't know. Will that happen again in 2003? That's on the top of everybody's question list."

One thing that entomologists do know is that Ohio growers will have to battle it out with slugs, as fall and winter samplings have found high overwinter populations. But the cold weather may help delay juvenile emergence come spring, giving growers the opportunity to plant early enough to escape feeding damage.

"Because of the cold winter, I would expect things to be pushed farther back for slugs. So the quicker growers can get planted, the better off they will be," Hammond said. "But then again, if the weather turns warm very quickly, the cold weather won't have that much of an impact."

The third week of March has already seen sunny days and springlike temperatures, unseasonably warm for this time of year in the Ohio valley.

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