Gary A. Herzog

Management of the Beet Armyworm (Lepidoptera: Noctuidae) in Cotton: Role of Natural Enemies

The beet armyworm, Spodoptera exigua (Hubner), has recently become a persistent and explosive pest of cotton in the southeastern United States. It is, however, attacked by a large and diverse complex of beneficial arthropods and pathogens that appear capable of maintaining beet armyworm populations below economically-damaging levels. Disruption of this complex contributes to outbreaks of S. exigua. It can also exacerbate problems with other pests because the complex of beneficial organisms attacking the beet armyworm is comprised of generalist species that also suppress other pests in the cotton production system. Management of the beet armyworm through conservation of its natural enemies, therefore, provides multiple benefits to growers by managing other pests as well.

Coccinellids in Cotton: Population Response to Pesticide Application and Feeding Response to Cotton Aphids (Homoptera: Aphididae)

Abstract Four treatments were evaluated during 1997 and 1998 to determine the effects of pesticides on coccinellid densities: (1) untreated control, (2) foliar applications of a systemic insecticide, imidacloprid (Provado 1.6 F), when spray thresholds of aphid numbers were exceeded in all plots, (3) weekly foliar applications of a fungicide, chlorothalonil (Bravo 720), and (4) weekly foliar applications of imidacloprid (Provado 1.6 F). The coccinellids observed in Georgia cotton during the study included Hippodamia convergens Guérin-Méneville, Scymnus spp., Harmonia axyridis (Pallas), Coccinella septempunctata L., and Coleomegilla maculata (Mulsant). During 1997, H. convergens and Scymnus spp. comprised 75 and 15%, respectively, of all coccinellids observed during the study. During 1998, Scymnus spp. comprised 44% of all coccinellids, and H. convergens comprised 33%. Coccinellid population densities closely tracked those of cotton aphids during both years. Cotton aphid and coccinellid densities were greatest in the chlorothalonil treatment during both years of the study. In functional feeding response experiments, fourth-instar and adult Scynmus creperus exhibited a type II functional response to A. gossypii density under laboratory conditions. Fourth instars exhibited a higher search rate and shorter handling time than adult S. creperus. Collectively, coccinellids are a valuable component of the cotton aphid’s natural enemy complex.

Rise and Fall of Cotton Aphid (Hemiptera: Aphididae) Populations in Southeastern Cotton Production Systems

Abstract The impact of natural enemies on cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae), populations in cotton, Gossypium hirsutum L., production systems in the southeastern United States was evaluated over 3 yr in irrigated commercial cotton fields. Fungal epizootics caused by the entomopathogen Neozygites fresenii (Nowakowski) Batko reduced aphid numbers to subthreshold levels in 1999, 2000, and 2001 and occurred consistently in early to mid-July in all 3 yr. Scymnus spp. were the most abundant aphidophagous predators, although other coccinellid species and generalist predators such as spiders, fire ants, heteropterans, and neuropterans also were present. Studies using arthropod exclusion cages demonstrated little impact of predators or parasitoids on aphid populations before fungal epizootics. Arthropod natural enemies were most abundant after epizootics and may have suppressed aphid populations late in the season. Seed cotton yield, and lint quality were not affected by aphicide applications in any year of the study. Implications of these findings for aphid management in the southeastern United States are discussed.

Effect of fungicide application on activity of Neozygites fresenii (Entomophthorales: Neozygitacaea) and cotton aphid (Homoptera: Aphididae) suppression

Abstract The development of resistance in aphid populations highlights the importance of biological control as a pest management tactic. Four treatments were evaluated to determine the effects of pesticides on the population dynamics of Aphis gossypii Glover and Neozygites fresenii (Nowakowski) Batko: (1) weekly applications of the insecticide imidacloprid (Provado 1.6 F); (2) weekly applications of the fungicide chlorothalonil (Bravo 720); (3) applications of imidacloprid (Provado 1.6 F) when aphid densities exceeded 30 aphids per leaf, and (4) untreated control. Differences in aphid density among the four treatments were shown only to be significant during the 1997 growing season; however, aphid densities were greater in the chlorothalonil treatment than in the other treatments during each growing season. Percentage of N. fresenii-killed aphids was most often highest in the chlorothalonil treatment as well. The fungal epizootic caused by N. fresenii was delayed ≈1 wk in the chlorothalonil treatment when compared with the other treatments. This delay allowed the aphids to temporarily escape suppression by the fungus and to continue to increase in density until the density-dependent effects of the epizootic overwhelmed the aphid population. N. fresenii also appeared to persist in the system when imidacloprid was in use and does appear responsible for initial aphid reductions. Treatment did not appear to have a large influence on yield outcome. Yield was variable from year to year and from location to location.

Toxic and antifeeding actions of melittin in the corn earworm, Heliothis zea (Boddie): Comparisons to bee venom and the insecticides chlorpyriphos and cyromazine

The acute and sublethal effects of melittin were compared to whole bee venom, chlorpyriphos and cyromazine injected into the corn earworm, Heliothis zea (Boddie). Melittin had twice the toxicity of crude venom, but only 3% that of chlorpyriphos. Melittin significantly reduced growth rate, diet consumption and food utilization efficiencies of fourth instar larvae. Bee venom and insecticides reduced these parameters to a lesser extent.

FMRF-Amide in the Corn Earworm (H. Zea): Immunoreactivity in the Midgut and Cerebral Nervous System

The midgut is the functional interface between insects and their food. Although the digestive and excretory functions of the midgut likely are highly regulated, surprisingly little is known about the chemical mediators — neural, endocrine, or paracrine — which coordinate these vital processes.

Peptide Toxins from Arthropod Venoms Disrupt Feeding and Utilization of Diet in the Cotton Bollworm

The broad spectrum insecticides in current use cause indiscriminate mortality among pest and beneficial species alike. Resistance to synthetic insecticides continues to be a major problem. At least twenty-three insect species have developed resistance to the newest class of insecticides, the pyrethroids (Georghiou, 1986). As an alternative to these synthetic insecticides, we propose the development of highly selective insect-specific control systems based on peptides with new target sites and/or novel modes of action. Peptide toxins with activity specific to insects are of great interest in this respect. Candidate peptides would conceivably disrupt feeding or utilization of food in addition to any acute toxic effect they have. The goal of this study was to identify model peptides which reduce overall gastroenteric activity or efficiency in the corn earworm, Heliothis zea.

Baseline Susceptibility of Tobacco Hornworms (Lepidoptera: Sphingidae) to Acephate, Methomyl and Spinosad in Georgia

The tobacco hornworm, Manduca sexta (L.), is a serious defoliating pest of flue-cured tobacco in Georgia. Isolated producer reports of increased difficulty in controlling this pest with standard in...

Differential effects of Vespula wasp venom and its components, mastoparan, and bradykinin: toxicity and feeding tests with the cotton bollworm

Abstract 1. The abilities for sublethal concentrations of mastoparan, bradykinin, and Vespula venom to disrupt ingestion and utilization of food were determined in the fourth instar Heliothis zea . 2. Mastoparan and Vespula venom severely decreased the metabolic efficiency for conversion of nutrients to biomass, thus, reducing larval growth rate. Although bradykinin reduced larval feeding, a stable growth rate was maintained due to increased metabolic efficiency. 3. The ld 50 s of mastoparan and Vespula venom in H. zea larvae were 40 and 41 μg/larva, respectively. Each caused a spastic-contractile paralysis when injected, suggesting neurotoxicity.

Toxicity of the venoms of two scorpions (Androctonus australis and Centruroides sculpturatus) and a sea anemone (Condylactis gigantea): Acute and antifeeding actions in the cotton bollworm (Heliothis zea)

Abstract 1. Single injections of 12.5–100 μg/larva Androctonus australis and Centruroides sculpturatus scorpion venoms and Condylactis gigantea sea anemone powder decreased larval weight and fecal production of fourth instar Heliothis zea . 2. Although the venoms decreased overall feeding, diet consumption was proportional or increased compared to larval weight. 3. The three compounds increased digestive ability of treated larvae but inhibited conversion of the digested food to biomass; the resulting growth rate of envenomed larvae was significantly decreased.