October 19, 2001
SINCE THE advent of Bollgard® cotton varieties, what once were brushed off as secondary pests are now pushing their way into the spotlight. Leading the charge is the tarnished plant bug.
“The primary problem insects for cotton growers in 2001 were plant bugs, plant bugs and more plant bugs. This insect so overshadowed everything else that it seemed to be the only game in town,” says Gordon Andrews, Extension entomologist with Mississippi State University.
“Delta farmers also faced infestations of stinkbugs and saltmarsh caterpillars this year, but more often than not what triggered spraying were plant bugs.”
With the high number of plant bugs and stinkbugs in cotton fields, Andrews says those farmers with Bollgard cotton did reap some benefits.
“The early season worms like garden webworms and leafrollers that can cause terminal damage didn't affect Bollgard cotton,” he noted. “Also, the non-Bt cotton was likely treated mid-season with a tank mix that had to include worm materials in addition to plant bug sprays.”
Another variable affecting cotton insect pressure in the Mid-South was the boll weevil eradication program, according to Andrews, who is based at the Delta Research and Extension Center in Stoneville, Miss.
The success of the eradication program in the region has meant fewer weevils making their homes in cotton fields, which means fewer insecticide sprays made by the program for boll weevils. Less than 6 percent of the cotton fields in Mississippi required treatment for boll weevils in 2001, the Mississippi Boll Weevil Management Corp. reports.
“We didn't get the help from the eradication program that we did in the last few years,” Andrews says. “One consequence of the overall reduction in number of sprays is that stinkbugs now have more opportunity to become established in cotton fields and to cause damage.”
Historically, organophosphate and/or pyrethroid sprays targeted against other pests coincidentally controlled stinksbugs and prevented them from becoming a problem, according to the Mississippi State cotton insect report. In low spray environments, however, stinkbugs can become a serious problem during mid and late season.
It is especially important to be aware of the potential for stinkbug problems in cotton once alternative crops, like corn, sorghum and early soybeans begin to mature. When this occurs, high numbers of stinkbugs can migrate into cotton fields quickly and cause significant damage to young maturing bolls
In Louisiana, Ralph Bagwell, entomologist at the Scott Research and Extension Center in Winnsboro, says growers in his state began battling insects early in the growing season.
“We had quite a lot of treatments go out for early season insects. The problem is that there's no single magic bullet available that will control every pest out there.”
In many cases, Bagwell says, instead of high populations of one pest, farmers are trying to control a handful of different insect species in each crop field. And, while each pest may not have reached threshold, the various species combined are triggering control treatments.
“Often, it's not a problem with any one pest, it's all the pests out there. It's seems to be the same everywhere across the Mid-South. The prevalent species may change from area to area, but they all seem to be out there in the crop fields at some level,” he says.
Despite the cold winter temperatures that many thought would reduce the insect pressure facing farmers in 2001, cotton growers throughout the Mid-South reported high early-season numbers of saltmarsh caterpillars, thrips and armyworms.
“We usually see second generation insects in crop fields by about the first of June. This year, we were probably already into the second generation of insects by the middle of April,” says Tommy Baird, area Extension agent in Indianola, Miss. “The early temperature warm-up may have sped up the insects' reproduction cycles, which produced a second generation of insects a little earlier and a little heavier than usual.
“We're going to continue to have early season insect problems until we have a couple of winters in a row with extremely cold temperatures. By extremely cold, I mean the ground is going to have to freeze at least 6- to 8-inches deep each year for several years,” Baird says.
Among the more unusual early season cotton pests that found their way into cotton fields this year were the three-cornered alfalfa hopper (Spissistilus festinus), the sugarcane beetle (Euetheola rugiceps), and the false chinch bug.
As the 2001 cotton season progressed, so, too, did the number of insects growers had to battle. While pressure from budworms and bollworms was, for the most part, light to moderate, pressure from armyworms, plant bugs and stinkbugs was quite high in many areas.
“Reports from the field indicate the Bollgard cotton held up quite well under pressure from these other pests.
Three different stinkbug species, including the green stinkbug, the brown stinkbug, and the southern green stinkbug, were reported infesting many Delta cotton fields.
Although all three of these species are shield-shaped and possess the same identifying odor, the green species (Acrosternum hilare) and the southern species (Nezara viridula) are bright green. A narrow, orange-yellow line also borders the body of the green stinkbug. The brown stinkbug (Euschistus servus) is slightly smaller and brownish in color.
While the brown stinkbug is a little more tolerant to pyrethroids, none of three species are difficult to control with insecticides, according to Blake Layton, an entomologist at Mississippi State University in Starkville, Miss.
Besides being an increasing pest in cotton, more stinkbugs are finding their way into soybean fields.
Jim Hamer, an entomologist with the Mississippi Soybean Management by Applied Research and Technology (SMART) program, says, “Stinkbugs, left untreated, can cause quality and yield losses, especially in soybeans that have not yet reached the maturity stage where the soybeans are filling the pod cavity.
“While the nymphs are not as damaging as adults are, the eggs mass cluster often contains many eggs which, after hatching, can feed heavily in one concentrated area. Then, once the nymphs mature into adults and they get wings, plant damage often increases as the stinkbugs move across a field to feed on developing pods,” Hamer says.
“When stinkbugs feed on soybean plants that have reached that later growth stage, the soybean seed will continue to mature out, although it may be of lower quality. Such resulting quality losses include a black discoloration of the seed and lower soybean oil content.
Stinkbug damage to younger soybean plants, however, can cause significant yield loss, according to Hamer. “Stinkbugs can feed on that immature seed and blank that seed completely out. And because the pods continue to look normal but are simply empty, the soybean plant can stay green trying to fill out that pod, which can be one cause of Green Bean Syndrome.”
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