INTRODUCTION

As growers consider using conservation tillage and other cultural practices for cotton and soybean production, it is essential that their potential effects on pest and beneficial insects be known. Since crop residues may favor the build up of some insect pests, other cultural practices may need to be manipulated to better manage species of pests and reduce grower dependence on traditional pest control methods, especially the use of broad-spectrum pesticides. Using natural control methods will increase grower profits, make conservation tillage and other practices more acceptable, and, in turn, create a cleaner environment and increase the sustainability of our agricultural systems.

OBJECTIVE

To examine the effects of doublecropping, conservation tillage, crop rotation and row width on population dynamics and management of arthropods (pest and beneficial) in cotton and soybean.

APPROACH

Populations of arthropods were monitored for three years (1997-2000) at the Pee Dee Research and Education Center (REC). Weekly samples of major insect pests and their natural enemies in cotton were taken from May through August of each year. Insecticides applied included aldicarb and l-cyhalothrin in treated plots. Cotton treatments included rye/disk, rye/no-till, monocrop/disk, monocrop/no-till, rotated with corn/disk, and rotated with corn/no-till. For the rotation treatment, corn was grown in 1997 and 1999. The effects of surface tillage, row width, and crop rotation on soybean pest and beneficial arthropod populations were also determined in insecticide-free plots. Weekly samples were taken from July through early October of each year in soybean. Doublecroppped soybean were grown using all combinations of row width (7.5" or 30") deep tillage (deep tilled or none) and surface tillage (disked or none). A combination of insect sampling techniques were used, including visual examination for cotton bollworm eggs and larvae, beat pan for predaceous arthropods and insect pests in cotton, and sweep net for all arthropods in soybean. To determine incidence of parasitism, insect samples were transported to the laboratory at Coastal REC and reared individually on an artificial pinto bean/wheat germ/soy protein diet in 30-ml plastic cups.

RESULTS

Insect pests detected in cotton included thrips, tobacco budworms, cotton bollworms, soybean loopers, beet armyworms, fall armyworms, and cotton aphids. Bollworm eggs were parasitized by Trichogramma spp. at rates reaching 50% during late July. The most abundant predator was the red imported fire ant. The ants were more abundant in the rye/no-till treatments (Table 1). There were also high numbers of ants in 1998 in the rye/no-till treatment with aldicarb (Table 2). Other major predators included bigeyed bugs and lady beetles. More bigeyed bugs were found in the disked treatments. More lady beetles occurred in the monocrop/disk treatments. In addition, hooded beetles, lacewings, spiders, and pirate bugs were detected. These results indicate that conservation-tillage systems can have an effect on the population density of predaceous arthropods in cotton.

While the green cloverworm was the most prevalent lepidopterous pest detected in the soybean experiment, other potential pests included soybean looper, corn earworm, velvetbean caterpillar, and fall armyworm. Green cloverworm populations peaked during early September at 64.0 per 100 sweep net samples in disked, deep tilled, 30" row width treatment (the Check). There were no significant differences in the mean number of green cloverworms per treatment, ranging from 10.7 to 21.3 per 33 sweeps. The southern green stinkbug was prevalent during late September and early October at pod-fill. Mean population density ranged from 4.7 to 13.3 per 33 sweep net samples (Table 3). Population density reached 40.0 per 100 sweeps in the two treatments that were rotated with corn. The stinkbugs remained below the treatment threshold of one/row-ft. A parasitic fly in the Family Tachinidae was reared from stinkbugs with very low incidence of parasitism. The three-cornered alfalfa hopper was first detected during late August and remained through early October (Table 4). No trend was recognized in populations among soybean treatments. The most abundant predaceous arthropods were spiders and nabids. Red imported fire ant mounds were quite numerous in the soybean plots; however, the number of ants detected in the canopy of the plants was minimal. Parasitoids of lepidopterous larvae included the braconids Cotesia marginiventris (Cresson) and Meteorus autographae Mues. A polyembryonic wasp, Copidosoma truncatellum, also represented the Family Encyrtidae. Parasitoids were more prevalent during September, as their success is often dependent upon high populations of the host insect.

CONCLUSIONS

No surface tillage decreased the densities of adult and immature thrips and thrip injury to cotton plants, compared with disking. Effects of rye and crop rotation with corn on densities of thrips and thrip injury to plants were variable and no definite trends were found. Cropping and surface tillage practices had no effect on densities of bollworm/tobacco budworms or damage to cotton fruit by these pests. An inverse relationship appeared to exist between densities of ants and densities of some other species of predators, indicating that conditions, which favor an increase in densities of ants, may result in decreased densities of bigeyed bugs, lady beetles, and possibly other species. Cotton aphid densities have been reported higher in conservation tillage, and the increased fire ant populations could possibly attribute to this phenomenon since the ants farm the aphids for honeydew. Cotton yields were increased by no surface tillage, compared to disking, and by aldicarb compared with no aldicarb (please see Research Results - Cotton), indicating that thrips may have a substantial impact on cotton yield. No consistent conservation tillage, row width, or crop rotation treatment effects were found on pest or beneficial arthropods in doublecropped soybean experiment.