Continued cold weather helping to reduce their numbers BOLL WEEVILS may be shaking in their tiny little boots, dreading what possibly the coldest winter in recent years will do to populations already threatened with eradication programs.
But so far late December cold snaps throughout the Texas Plains and Oklahoma apparently resulted in a mixed bag of survival data for overwintering weevils, according to Extension entomologists.
"We're optimistic that continued cold temperatures will be instrumental in decreasing weevil survival rates," says Jim Leser, Texas Extension entomologist, Lubbock. He says survival data from caged weevils indicate a good kill in two locations but less than hoped in a third, more southern site.
"Stan Carroll, an entomologist and assistant research scientist with the Texas Agricultural Experiment Station, has collected data from three sites," Leser explained. "Those sites represent excellent overwinter habitat for weevils."
Carroll "fattens up" weevils for three weeks before he puts them in dig-up cages, which he places in an overwintering site where natural leaf drop provides cover. They check the cages every three weeks throughout the winter and into spring to judge survival rates.
"Survival in other, less protected, sites will be much lower," Leser says, "since habitat will not be as good. Also, we're dealing with `super weevils' that have more fat than most of those that survived naturally last fall."
LESER SAYS a Jan. 4 inspection of the three sites indicated significant mortality in the two northern locations but a high survival rate in the southern site, near Seminole.
Data collected near Slaton showed a 78 percent survival rate. "That seems high," Leser says, "but it's a 38 percent reduction from three weeks earlier. Also, we estimate that as many as one-third of the survivors may be dead before the next check. We count any weevil that moves at all as alive.
"Many of these were barely moving and may not live long. We're likely to see another one-third mortality from the latest cold snap."
He says survival this time last year was 68 percent at the Slaton site and attributes that mortality mostly to dry conditions. "And mortality was gradual, not the steep plunge we've seen this year."
Another site, near Plainview, showed a 53 percent survival rate. "That may seem high, too, but it indicates a drop from 85 percent just three weeks earlier. That's 30 points in just three weeks."
Weevil survival at the Seminole site was 92 percent. "That's not good," Leser says. He says the sudden drops in survival at the northern sites indicate cold weather is having an impact on weevil numbers.
Mortality likely would have been higher without snow cover.
Low temperatures at the Slaton site from Dec. 11 through Dec. 31 ranged from 29 to 16 degrees. The cage sites, however, ranged from lows of 39 degrees down to 34 degrees.
Overwinter sites near Plainview were colder, says Leser, ranging from 34 degrees to 29 degrees. "The colder temperatures apparently had an impact.
"We feel we need temperatures below 25 degrees for five to eight hours to kill weevils outright," Leser says. "I think temperatures were cold enough to cause more mortality but the snow cover helped them survive. We need another cold spell or two without snow."
He emphasizes that more natural overwintering sites will sustain even higher weevil death rates.
"These sites represent some of the better overwintering weevil habitats in the area," he says. "Many weevils do not have this much protection."
He says the trend, which he believes will continue to show increased weevil mortality if cold weather lingers, is good news for eradication and control efforts. "And we still have a lot of winter left."
Two of the sites are in areas outside active eradication zones.
"Outside the zone, lower survival means farmers will have fewer weevils to deal with next spring. And the areas that will begin eradication with a diapause treatment next fall should find costs less because of fewer weevils surviving this winter."
For growers inside active eradication zones, the high mortality rate will mean lower program costs this season and might hasten eradication status.
"We hope to get down to 5 percent survival rate, but anything below 20 percent is good."
Miles Karner, Extension entomologist in Altus, Okla., says recent cold snaps "mean money to cotton farmers.
"Even with the ice and snow cover, which offers some insulation benefits to weevils, this cold weather could be worth two or three malathion applications. We should see limited early-season weevil eradication sprays, except in a few counties (Tillman, Jackson, Harmon and Greer) that have excellent overwinter habitat."
Karner says another survival factor will be how much fat escaped weevils carried into diapause last fall.
"The cold weather has been a blessing," he says. "We've wanted a winter like this to occur in conjunction with the eradication program. This one came about three years later than we would have preferred, but it will help clean up stragglers."
Karner says other pests may be affected less by the cold weather.
"The bollworm has a number of other host plants and is very mobile. We would have to see a significant reduction in numbers all across Texas before we're certain that populations will be lower here.
"And insects have a dynamic rate of reproduction. Any little window of opportunity and they'll produce heavy populations. Most are very opportunistic."
Leser says predicting aphid survival is extremely difficult. "We have no clue as to overwintering populations," he says. "We think they survive underneath rosette type weeds, such as mare's-tail. We have no indication how weather affects populations."
He says bollworm survival also is hard to determine. "We get migration from the south in our worst infestation years. Colder temperatures in south Texas may have an impact."
He says reduced corn acreage will be a more telling factor. "We've cut corn acreage in the Northern High Plains and that will reduce boll worms. Also, farmers are planting more Bt corn, which also decreases bollworm populations."
He says farmers have expressed more concern over bollworms and beet armyworms than other insects this year. "Farmers can handle most other pests. They have the tools to take them out.
"I think a cold winter will reduce beet armyworm populations, but I've never seen back-to-back beet armyworm years anyway." He says moisture is the key for fleahopper/lygus infestations.
"If moisture generates winter weeds, these pests have a better chance to cause trouble," he says.