John W. Van Duyn

Effect of Insecticides on Trichogramma exiguum (Trichogrammatidae: Hymenoptera) Preimaginal Development and Adult Survival

Abstract The effect of insecticides on Trichogramma exiguum Pinto & Platner emergence, adult survival, and fitness parameters was investigated. Insecticides tested were lambda cyhalothrin, cypermethrin, thiodicarb, profenophos, spinosad, methoxyfenozide, and tebufenozide. All insecticides, with the exception of methoxyfenozide and tebufenozide, adversely affected Trichogramma emergence from Helicoverpa zea (Boddie) host eggs when exposed at different preimaginal stages of development (larval, prepupal, or pupal). Regardless of the developmental stage treated, none of the insecticides tested had a significant effect on the sex ratio or frequency of brachyptery of emerged females. However, the mean life span of emerged T. exiguum females significantly varied among insecticide treatments, and was significantly affected by the developmental stage of parasitoid when treated. Based on LC50 values, spinosad and prophenofos were the most toxic compounds to female T. exiguum adults, followed by lambda cyhalothrin, cypermethrin, and thiodicarb. Insecticides field-weathered for four to 6 d on cotton leaves showed no activity against female T. exiguum adults.

Life History Traits of Helicoverpa zea (Lepidoptera: Noctuidae) on Non-Bt and Bt Transgenic Corn Hybrids in Eastern North Carolina

Abstract Transgenic varieties of field corn that express the Cry1Ab B. thuringiensis (Bt) toxin in ear tissue present the potential of reducing ear feeding by the corn earworm, Helicoverpa zea (Lepidoptera: Noctuidae), and for reducing the size of populations of the insect infesting other host crops. Life history parameters of H. zea feeding on ears of conventional and Bt field corn varieties were measured in field plots in eastern North Carolina in 1997 and 1998. Transformation events investigated were Mon-810 and Bt-11. Bt corn was found to cause a steady mortality of larvae during development, but permitted ≈15–40% survival to the prepupal stage compared with non-Bt corn. Mortality of prepupae and pupae from Bt corn was also higher than from non-Bt corn, reducing overall adult production by 65–95%. The larvae that did survive grew more slowly on Bt than on non-Bt corn, and produced pupae that weighed 33% less. Pupation and adult eclosion were delayed by 6–10 d by feeding on Bt corn ears. Corn varieties expressing Bt in ear tissue have the potential to reduce H. zea ear feeding by up to 80%, and the potential to reduce populations emerging from ear-stage corn fields to infest cotton, soybean and other crops by around 75%. To have a measurable effect on area-wide populations, Bt corn varieties would need to be planted in large proportions of corn fields. Extensive planting of varieties such as those tested here, having only moderate effects on H. zea, would raise concerns about rapid evolution of resistance.

Impact of Small Fitness Costs on Pest Adaptation to Crop Varieties with Multiple Toxins: A Heuristic Model

A deterministic two-locus model was used to examine how small fitness costs to individuals carrying resistance alleles could impact the risk of panmictic insect pest populations adapting to crop varieties that produced two distinct toxins. Parameters examined were (1) level of toxicity of each toxin, (2) initial frequencies of alleles for adaptation to the toxins, (3) percentage of population feeding on nontoxic plants, and (4) level of fitness cost associated with adaptation to each of the two toxins. Resistance to each toxin was assumed to be biochemically independent, controlled by a resistance coding allele at a single locus, and inherited as a partially recessive trait in the field. When plants are extremely toxic to the pest, effective refuge size is 10%, and there is a fitness cost to resistance alleles only when in homozygous form (5%), the pest population is never predicted to adapt to either toxin as long as the initial frequencies of the resistance alleles are below 0.05. Even if the initial frequency of the allele for adapting to one toxin is 0.95 when a two-toxin cultivar completely replaces a one-toxin cultivar, the model predicts that a low equilibrium allelic frequency will develop for both resistance alleles, as long as the frequency of the allele for adapting to the second toxin is initially 0.001 or less. If cultivars with one and two toxins are planted, the model predicts that resistance will develop. Nonrandom mating and stochastic variation within subpopulations also could lead to evolution of resistance.

Trichogramma releases in North Carolina cotton: why releases fail to suppress heliothine pests.

Abstract Field studies were conducted in 1996 and 1997 to determine the fate of naturally oviposited F3 heliothine eggs in cotton plots treated with augmentative releases of Trichogramma exiguum Pinto & Platner and nontreated plots. Four cohorts of newly oviposited eggs (<24 h old) were followed in 1996 and two cohorts in 1997. In 1996, mean ± SD percent parasitism, estimated by in-field studies following the fate of naturally oviposited eggs, ranged from 7 ± 7 to 61 ± 8% in T. exiguum release plots and 0 ± 0 to 35 ± 13% in control plots. The mean ± SD percent of eggs hatched in T. exiguum release plots ranged from 1 ± 2 to 11 ± 4% and 7 ± 4 to 28 ± 10% in control plots. In 1997, mean ± SD percent egg parasitism ranged from 27 ± 4 to 40 ± 3% in T. exiguum release plots and 15 ± 18 to 25 ± 8% in control plots. The mean ± SD percent of eggs hatched in T. exiguum release plots ranged from 7 ± 3 to 12 ± 2% and 18 ± 6 to 28 ± 8% in control plots. Despite increased parasitism and reduced egg hatch in T. exiguum release plots, overall, there was no significant difference in larval density (all instars combined) between T. exiguum release and control plots. Combined analysis of the heliothine larval populations and egg fate data revealed that the additional egg mortality produced by released T. exiguum was offset by lower larval mortality in release plots. Because of the occurrence of compensatory mortality, the egg stage of heliothines is not an appropriate target for biological control using Trichogramma wasp releases.

Trichogramma exiguum (Hymenoptera: Trichogrammatidae) Releases in North Carolina Cotton: Evaluation of Heliothine Pest Suppression

Abstract Field studies were conducted in 1996 and 1997 to reevaluate the use of augmentative releases of Trichogramma wasps for heliothine management in cotton. In 1996, nine releases of Trichogramma exiguum Pinto & Platner, spaced 3–4 d apart, were made into three 0.4-ha cotton plots. Six weekly releases were made in 1997, each containing two T. exiguum cohorts developmentally staggered by 45°C degree-days. Field release rates, estimated from laboratory and field quality control data, averaged 108,357 T. exiguum ♀♀ per hectare per cohort per release in 1996 and 193,366 ♀♀ per hectare per cohort per release in 1997. In 1996, mean ± SD adult emergence under laboratory conditions for released cohorts was 92 ± 7%; 62 ± 5% of emerged adults were females, 3 ± 2% of females displayed brachyptery (nonfunctional wings), mean female longevity under laboratory conditions was 15 ± 4 d, and mean ± SD field emergence was 97 ± 2%. Quality control measurements were similar in 1997. In 1996, mean ± SD percent parasitism of heliothine eggs in field plots on the sampled dates ranged from 67 ± 4 to 83 ± 5% in T. exiguum release plots and 25 ± 9 to 55 ± 8% in control plots. In 1997, parasitism levels ranged from 74 ± 4 to 89 ± 5% in T. exiguum release plots and 18 ± 18 to 69 ± 11% in control plots. Despite increased parasitism levels in T. exiguum release plots, there were no significant differences in density of fifth instars, boll damage, or yield between T. exiguum release and control plots. Therefore, it is concluded that Trichogramma augmentation is not an effective heliothine management tool in North Carolina cotton.

Do Refuge Requirements for Biotechnology Crops Promote Economic Efficiency? Some Evidence for Bt Cotton

We examine producer behavior, resistance evolution, and returns under alternative refuge requirements in an eastern North Carolina region with multiple corn, cotton, and soybean fields infested by a mobile pest. Returns are highest, pyrethroid sprays occur least frequently, and pyrethroid resistance evolution is delayed most effectively with no refuge requirement. Complying with the current 20% refuge requirement costs the producer

Influence of Cotton Microhabitat on Temperature and Survival of Trichogramma (Hymenoptera: Trichogrammatidae) within Cardboard Capsules

8.67 per cotton acre, or

Relationship between external stink bug (Hemiptera: Pentatomidae) boll-feeding symptoms and internal boll damage with respect to cotton lint gin-out and fiber quality.

34.21 per non-transgenic insecticidal (Bt) cotton acre. Returns are highest under each refuge requirement when one-toxin Bt cotton is not phased out; however, removal of the technology at the earliest phase-out date minimizes regional pyrethroid sprays.

Changes in insecticide efficacy against Helicoverpa zea (Boddie) on soybean due to larval size and plant phenology

Abstract Field studies were conducted in 1996 and 1997 to determine the most suitable release sites in cotton for inundatively released encapsulated Trichogramma exiguum Pinto & Platner. Atypical applications of a plant growth regulator, mepiquat chloride (Pix), were used to manipulate cotton plant size and canopy closure to produce a range of plant sizes that might be present in North Carolina when Trichogramma releases for suppression of third-generation (F3) heliothines were made. Pix treatments had a significant effect on canopy closure, which significantly influenced the number of hours soil surface temperatures between rows were ≥35°C. The mean daily number of hours temperatures were ≥35°C was greatest on the soil surface between rows, followed by in the canopy, then soil surface within rows. A significant correlation between the number of hours preimaginal Trichogramma were exposed to temperatures ≥35°C and Trichogramma emergence was found in 1997. Consequently, suitability of the soil surface as a release site for Trichogramma capsules depended significantly on the level of canopy closure, location of capsules relative to the center of rows, and length of time capsules remained in the field before parasitoid emergence. Our results indicate that these factors should be considered when implementing augmentative releases of Trichogramma wasps in cotton.

Spatial Processes in the Evolution of Resistance in Helicoverpa zea (Lepidoptera: Noctuidae) to Bt Transgenic Corn and Cotton in a Mixed Agroecosystem: a Biology-rich Stochastic Simulation Model

ABSTRACT Cotton, Gossypium hirsutum L., bolls from 17 field locations in northeastern North Carolina and southeastern Virginia, having 20% or greater internal boll damage, were studied to determine the relationship between external feeding symptoms and internal damage caused by stink bug (Hemiptera: Pentatomidae) feeding. In 2006 and 2007, two cohorts of 100 bolls each were sampled at all field locations. The first cohort was removed as bolls reached approximately quarter size in diameter (2.4 cm). External and internal symptoms of stink bug feeding were assessed and tabulated. Concurrent to when the first cohort was collected, a second cohort of quarter-size-diameter bolls was identified, tagged, examined in situ for external feeding symptoms (sunken lesions), and harvested at the black seed coat stage. Harvested bolls were assessed for internal damage and locks were categorized (undamaged, minor damage, or major damage), dried, and ginned. Lint samples from each damage category were submitted for high volume instrument and advanced fiber information system quality analyses. Significant, moderately strong Pearson correlation coefficients existed between number of external stink bug feeding lesions and internal damage. Pearson correlation of total external lesions with total internal damage was stronger than any correlation among the other single components compared. Predictability plots indicated a rapid increase in relationship strength when relating external stink bug lesions to internal damage as the number of external lesions increased. Approximately 90% predictability of internal damage was achieved with four (2006) or six (2007) external lesions per boll. Gin-turnout and fiber quality decreased with increasing intensity of internal stink bug damage.