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Cold temps might cut insect pests

Labe studies show boll weevils often die if subjected to at least six continuous hours of below-freezing temperatures.

The 2000-01 winter has provided much of the Delta with day-after-day of below-freezing temperatures. While these bitter cold temperatures aren't very conducive to outdoor recreation, they may ultimately benefit the areas cash-strapped farmers come this spring.

According to Bart Freelund with the Delta Research and Extension Weather and GIS Data Center in Stoneville, Miss., the two-month period of November and December 2000 was the coldest on record at that location. In addition, December 2000 was the fourth coldest December ever in Stoneville, Miss.

According to Freelund's records as of Feb. 1, weather station readings recorded constant temperatures below 20 degrees F this winter for six days in Stoneville, Miss., 12 days in Starkville, Miss., 14 days in Tunica, Miss., and 20 days in Memphis.

Those frigid statistics should prove to be good news for farmers' pest-control budgets this spring, especially when it comes to treating for boll weevils.

Blake Layton, an entomologist at Mississippi State University, says previous lab studies have shown that overwintering boll weevils often die if subjected to at least six continuous hours of below-freezing temperatures. “We've seen that this year, so we should have a pretty good mortality of weevils this year.”

It's generally believed that constant temperatures below either 5 degrees F with low humidity, or below 20 degrees F with high humidity, will drastically reduce boll weevil populations, according to Layton. “Those temperature levels are ambient temperatures, which means it may not have to get quite that cold to kill boll weevils overwintering in piles of leaves or underground,” he says.

Aubrey Harris, an entomologist at the Delta Research and Extension Center in Stoneville, Miss., and consultant for the state's boll weevil eradication program, says he expects the boll weevil eradication effort in the north half of Mississippi to benefit greatly from the 2000-01 winter.

The 2000-01 winter has, so far, provided what could be five times the number of degree-days under 32 degrees F in December and January, compared to the previous four years, Harris says.

LSU AgCenter entomologist Ralph Bagwell agrees the cold winter weather should rid the Delta's cotton fields of some boll weevils this growing season.

“The boll weevil is fairly sensitive to overwintering conditions,” he says. “You get some hard freezes, especially in conditions with a lot of ice, and you get massive mortality of those insects.”

Delta growers could also see fewer armyworms this spring due to below-freezing temperatures. However, reducing spring and summer armyworm populations has more to do with how cold it gets in south Texas and Florida and less to do with Mid-South thermometer readings. “Armyworms do not overwinter in the Delta, but the colder it gets farther south of us, the further they will stay away from us this spring,” says Layton.

While the cold winter weather bodes well for a spring with fewer boll weevils, the same may not hold true for other insects, such as plant bugs, budworms and bollworms.

Because plant bugs are able to successfully overwinter in Canada where it is much colder, the winter weather the Delta has experienced will do little to reduce plant bug populations, according to Layton.

Budworms and bollworms are also likely to have survived the below-freezing temperatures this winter. “While there is the possibility budworms and bollworms could be affected by the cold winter weather, it's not likely because they are pretty well insulated in the soil,” he says.

Bagwell says that even if many insects die during the winter, favorable spring conditions can still result in very high numbers.

“You could have massive mortality in the winter, but a good spring will wipe that away,” he explains.

“Spring conditions are normally going to be wet, and a lot of plants will be flourishing,” he says. “That's what those insects will start off with and build up to very high numbers.”

Among those insects affected more by early spring weather conditions than by the winter cold are thrips and aphids. “The low temperatures have little effect on these insects unless their host plants fail to green-up in the early spring due to cold early-spring temperatures,” Layton says.

Another factor affecting spring insect populations, Bagwell says, is winter and early-spring rainfall. “A lot of rain will actually do more to reduce overwintering populations than the cold weather would.”

One example of this principle, he says, is the buck moth caterpillar. “They're going to be more sensitive to a lot of water — where you're getting standing water, they actually drown.”

Much of the Mid-South has experienced drought conditions for the past couple of years, and that has led to higher numbers of insects, according to entomologists.

“The drought increases survival,” Bagwell says. “When it's dry, you get a higher survival. But more often than not, it's the spring conditions that impact the kind of year you're going to have that following year during the summer months.”

One insect that will likely be found in large numbers throughout the Delta this spring, despite the cold winter temperatures, is the mosquito.

“Mosquitoes can survive just about everywhere,” Bagwell says, adding that the biting bugs can be found even in the arctic tundra. “They survive that overwintering there, where they'll get freezes that go down 2 and 3 feet in the soil.

“Most insects are very well-adapted to just about any situation,” he says. “They can just about freeze and still survive the process without any problems whatsoever.”

For more weather data, visit Delta Research and Extension Center's weather site on the Web:


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