U.S. cotton producers are spraying less for insect pests due to Bollgard and boll weevil eradication. But the resulting “low spray environment,” in conjunction with conservation programs and reduced tillage, is creating the opportunity for so-called secondary cotton pests to emerge.
The changing cotton ecosystem “has created new challenges” for growers, according to Roger Leonard, Experiment station entomologist with the LSU AgCenter. Leonard presented a paper on the subject during the 2002 Beltwide Cotton Conference in Atlanta.
While the concept of low spray environment is relatively new, it will require producers to expand their insect control strategies beyond field borders while at the same time doing a better job of targeting pests within a field.
Two broad categories are effecting changes in cotton pest management — pest management strategies and cropland use patterns, according to Leonard. He pointed to six factors contributing to the change in the density and diversity of pests in a cotton field.
Boll weevil eradication — Prior to the eradication program, many of the broad-spectrum insecticides applied for boll weevils maintained secondary pest densities below injurious levels, noted Leonard. However, following eradication efforts, the reduction of insecticide sprays for boll weevil have allowed tarnished plant bug and stinkbug species to become more consistent pest problems.
Bollgard cotton cultivars — Bollgard cotton varieties are now planted on a significant portion of the cotton acreage in the United States. Bollgard cotton controls two of the most important caterpillar pests, tobacco budworm and pink bollworm, while supplemental control is needed to control bollworm, soybean looper, armyworms and other non-caterpillar pests.
“The widespread use of this technology has created a low-spray environment by reducing the frequency of foliar applications for lepidopteran pest control over 50 percent across the Cotton Belt,” Leonard said. “In Louisiana, insecticide applications have been reduced to two to four applications per year compared to four to eight applications per year prior to the availability of Bollgard.”
As a result, tarnished plant bug and stinkbug species have become more serious pests, Leonard said.
“The trend to reduce foliar insecticide treatment will continue with the commercialization of Bollgard II cotton cultivars. These cultivars are more toxic to bollworm, soybean looper and armyworms. This technology will likely eliminate at least one more treatment for caterpillar pests in exchange for additional insecticide use for non-caterpillar pests.”
Target-specific insecticides — Leonard noted that federal and state regulatory agencies have forced agro-chemical industries to pursue the commercialization of extremely target-selective insecticides.
“Most of the insecticides registered during the last decade control a more-restricted range of pests compared to the pyrethroids, organophosphates and carbamates. Populations of some pests that would have been inadvertently controlled with broad-spectrum insecticides are affected very little or not at all with many of the new products. Within these conditions, minor pests have the opportunity to become significant problems.”
Successful management of an insect complex with these new products usually relies on the co-application (tank mixes of multiple insecticides, noted Leonard.
Conservation tillage and herbicide tolerant cultivars — The availability of herbicide-resistant cotton varieties like Roundup Ready and BXN cotton has increased the adoption of conservation tillage practices by providing an effective and economical weed management tool.
“The resulting increase in surface residue from crops and weedy vegetation provides a favorable microenvironment for soil-swelling insects. In some instances, insect densities increase to higher-than-expected levels in these fields before cotton is available as a host.
“The reduction of tillage and use of winter cover crops have made cutworms, cotton aphids and false chinch bug more-common problems that require insecticides.”
Improper timing of herbicides on resistant cotton could exacerbate the problem, according to Leonard. “If Roundup is applied too early or against weeds larger than the susceptible stage, Roundup is an ineffective treatment. Spring plant species not controlled by Roundup can be non-crop hosts for cutworm, tarnished plant bug, false chinch bug, thrips and cotton aphid.
“In addition, the general timing of preplant herbicide applications in cotton has shifted closer to the time of planting compared to the timing of tillage practices. With herbicides, producers can delay fieldwork for vegetation management much longer than if tillage is used. This delay in terminating some weed species provides a refuge for insects until cotton becomes available. Therefore, insect pests have another opportunity to increase population densities.”
Producers are changing from a monoculture system of cotton production to a diversified multi-cropping system to include more corn, rice, grain sorghum and soybeans.
“Multiple-cropping systems can influence insect populations across an entire region over which an individual producer has little control.” This impact has not been well-studied, according to Leonard.
“Field corn can serve as an alternate host for insect pests such as cutworm, bollworm and stinkbug, similar to the non-crop plants within cotton fields and on field margins.
“Bollworm densities in cotton fields usually are much higher in those states with appreciable acreage of field corn than in areas that have little field corn acreage.”
The stinkbug has become a particularly bothersome pest in recent years. “Stinkbugs typically prefer soybeans over cotton and usually significant stinkbug migration into cotton does not occur when reproduction stage soybeans are available,” Leonard said.
“However, cotton could potentially become a more frequent host for stinkbugs due to the decline in acres planted to soybeans. Since 1990 in Louisiana, soybean acreage has decreased from approximately 1.8 million acres to 700,000 acres in 2001. A similar situation can be observed in some areas with field corn.”
Cotton fields in close proximity to federally subsidized programs like the Conservation Reserve Program and Wetlands Reserve Program also help create alternate non-crop hosts for many cotton insects, according to Leonard.
“In Louisiana, the parishes of greatest cotton production also are the same parishes in which large amounts of acreage are devoted to CRP. Fifty-six percent of the total acreage planted to cotton occurs in five parishes (Tensas, Morehouse, Franklin, Richland and Madison). These same parishes account for a large percentage of the CRP acres in the state.
How should farmers adjust to this changing environment? “Cotton IPM strategies must become more holistic and consider pest control decisions not only within individual field units but also across farm landscapes,” Leonard said.
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