Linda F. Benedict, Snodgrass, Gordon, Costello, Richard
Richard Costello, Eugene Burris, Gordon Snodgrass and William Scott
The tarnished plant bug has always caused problems in cotton, but in recent years the problems have escalated. Data from 1990 to 1995, before the advent of transgenic Bt cotton, put the cost per acre to control the tarnished plant bug at $3.19 compared to $12.02 from 1996 to 2002, after Bt cotton was introduced. The total loss in cotton because of the tarnished plant bug was 0.41 percent from 1990 to 1995 compared to 1.07 percent from 1996 to 2002.
Successful boll weevil eradication and the adoption of Bt cotton, while reducing the need for insecticide applications to control the boll weevil and tobacco budworm, provided an opportunity for tarnished plant bugs to flourish. Resistance to insecticides such as the pyrethroids, organophosphates and carbamates has contributed to control problems. Resistance to these insecticides increases during the cotton-growing season.
Not only do these factors contribute to higher numbers of tarnished plant bugs, but the broad range of crops and wild hosts that this pest can use for survival is huge. The tarnished plant bug is reported to have a host range of about 390 species of plants, giving it the broadest feeding niche of any of the arthropod pests. In the Delta, research has indicated 169 hosts representing 36 families of plants. These hosts often live next to cotton. The seasonal flowering patterns of these species give the plant bug a consistent habitat from late winter through spring.
U.S. Department of Agriculture researchers have shown a 46 percent reduction in tarnished plant bugs in cotton fields in areas where wild hosts have been controlled. Research also has indicated a significant economic return on investment, of up to $10, in areas where wild hosts were controlled with herbicide applications.
LSU AgCenter researchers conducted an experiment in Tensas Parish to evaluate the potential of reducing tarnished plant bugs by managing weed hosts in field border areas adjacent to cotton fields before planting cotton. Each experimental area encompassed about 620 acres in 2000, 1,200 acres in 2001, and 1,700 acres in 2002. One test area in each year was treated with herbicide to control broadleaf weeds in field borders and ditches close to cotton fields.
Herbicide applications were initiated February 23, 2000, and March 19, 2001. Strike 3, a combination of 2,4-D, mecoprop and dicamba, was applied at 2 quarts per acre to kill broadleaf weeds. The weed density and species data were taken before the herbicide treatments and three to four weeks post treatment. Tarnished plant bug adults and nymphs were collected within each area using a standard 15-inch sweep net. Field border sampling was initiated in February before herbicide treatments, and sweep net data from the field borders were collected weekly continuing through the first week of June. Up to 500 sweeps were made in randomly selected cotton fields beginning in mid June through the end of July.
Throughout this study, tarnished plant bug numbers were considerably lower in the areas in which the broadleaf hosts had been removed. For the most part, populations followed a similar cyclic pattern each year. In each year there were definite peaks, although the peaks occurred on different dates. In general, tarnished plant bug adults began to increase in early to mid April and nymphs in mid to late April.
In 2000, differences in the number of tarnished plant bugs caught in field borders were higher from early to mid April throughout May. Adult numbers in the treated area did not exceed two insects in 25 sweeps, and nymph numbers remained below one in 25 sweeps. In the nontreated area, adult numbers began to increase in early April and peaked on May 23 at 16 in 25 sweeps. In contrast, nymph numbers in the nontreated area peaked in mid April and declined through May.
Differences in adult tarnished plant bugs between the treated and nontreated areas were evident by April 9 in 2001. Tarnished plant bug nymph populations did not begin to build until the end of April, and differences between the treated and nontreated areas were not distinguishable until May 3. Tarnished plant bug numbers in the treated area were no higher than 1.5 insects in 25 sweeps at any rating date. In the nontreated area, numbers peaked at 10 and 11 in 25 sweeps for adults and nymphs, respectively.
In the treated area in 2002, adult and nymph numbers were below three in 25 sweeps. Adults captured in 25 sweeps peaked at 17 on April 11 in the nontreated area. In general, nymph numbers in 2002 were lower than in the other years. The highest number of nymphs, seven in 25 sweeps, was caught April 2.
These data indicate that destruction of wild hosts from field borders has the potential to reduce tarnished plant bug populations; however, migration of these insects from other areas such as conservation and wetland reserve areas and corn fields, along with the inability to eliminate all wild hosts, also contributed to plant bug infestations. If used as part of an overall management strategy, wild host destruction will provide another tool to reduce insect pressure and provide economic value to the producer.
Richard Costello, Post Doctoral Researcher, Northeast Research Station, St.Joseph, La.; Eugene Burris, Professor, Northeast Research Station, St. Joseph, La.; Gordon Snodgrass and William Scott, both Research Entomologists, USDA-ARS, Stoneville, Miss.
(This article was published in the spring 2003 issue of the Louisiana Agriculture magazine.)