K. Clint Allen

Risk assessment for Helicoverpa zea (Lepidoptera: Noctuidae) resistance on dual-gene versus single-gene corn.

ABSTRACT Recent Environmental Protection Agency (EPA) decisions regarding resistance management in Bt-cropping systems have prompted concern in some experts that dual-gene Bt-corn (Cry1A.105 and Cry2Ab2 toxins) may result in more rapid selection for resistance in Helicoverpa zea (Boddie) than single-gene Bacillus thuringiensis (Bt)-corn (Cry1Ab toxin). The concern is that Bt-toxin longevity could be significantly reduced with recent adoption of a natural refuge for dual-gene Bt-cotton (Cry1Ac and Cry2Ab2 toxins) and concurrent reduction in dual-gene corn refuge from 50 to 20%. A population genetics framework that simulates complex landscapes was applied to risk assessment. Expert opinions on effectiveness of several transgenic corn and cotton varieties were captured and used to assign probabilities to different scenarios in the assessment. At least 350 replicate simulations with randomly drawn parameters were completed for each of four risk assessments. Resistance evolved within 30 yr in 22.5% of simulations with single-gene corn and cotton with no volunteer corn. When volunteer corn was added to this assessment, risk of resistance evolving within 30 yr declined to 13.8%. When dual-gene Bt-cotton planted with a natural refuge and single-gene corn planted with a 50% structured refuge was simulated, simultaneous resistance to both toxins never occurred within 30 yr, but in 38.5% of simulations, resistance evolved to toxin present in single-gene Bt-corn (Cry1Ab). When both corn and cotton were simulated as dual-gene products, cotton with a natural refuge and corn with a 20% refuge, 3% of simulations evolved resistance to both toxins simultaneously within 30 yr, while 10.4% of simulations evolved resistance to Cry1Ab/c toxin.

Baseline Susceptibility of Lygus lineolaris (Hemiptera: Miridae) to Novaluron

Abstract Tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), populations were collected from field locations in the Mississippi River Delta of Arkansas, Louisiana, and Mississippi. Third-instar F1 nymphs from each field location, in addition to a laboratory colony, were screened for susceptibility to novaluron. Both a glass vial bioassay and a diet-incorporated bioassay used dose-response regression lines to calculate LC50 and LC90 values for novaluron. Mean LC50s for glass vial bioassays ranged from 44.70 ± 3.58 to 66.54 ± 4.19 µg/vial, while mean LC50s for diet-incorporated bioassays ranged from 12.10 ± 0.77 to 17.63 ± 2.42 µg/200 ml of artificial diet. A comparison of LC50 values from the same field population screened using both bioassay methods failed to show a relationship. LC50 values from field locations were compared with a historically susceptible population from Crossett, AR. Results indicated that considerable variability in susceptibility to novaluron exists within field populations of tarnished plant bugs across the Delta, including some locations with lower LC50 values than a historically susceptible population.

Comparative Susceptibilities of Different Life Stages of the Tarnished Plant Bug (Hemiptera: Miridae) to Three Classes of Insecticide

Abstract. Insecticidal control of the tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), is targeted at the adult and nymphal stages, but there is little information on relative susceptibilities of these insects to insecticides. Tarnished plant bug adults were collected from various locations in Mississippi during 2008–2009, and different life stages of their progeny were tested for susceptibilities to three classes of insecticides. A glass vial bioassay was used to evaluate four populations of tarnished plant bugs for susceptibility to the pyrethroid insecticide, permethrin, and two populations to the organophosphate insecticide, methamidophos. Additionally, a diet-incorporated bioassay was used to evaluate the susceptibility of the various life stages of a single population of tarnished plant bugs to a neonicotinoid insecticide, thiamethoxam. For all but one population and insecticide combination, the estimated LC50 values ranked from least to greatest were: 1st instars, 2nd instars, 3rd instars, adults, 4th instars, and 5th instars. The tolerances of 5th versus 1st instars were greater for populations of tarnished plant bugs exposed to permethrin compared to methamidophos or thiamethoxam. The results of this study indicated that the effectiveness of an insecticide application for the control of tarnished plant bugs depends on the age-structure of the population being treated.

Supplemental Control with Diamides for Heliothines1 in Bt Cotton

Abstract. Supplemental control with diamides for bollworm, Helicoverpa zea (Boddie), in Bt cotton is becoming more frequent, but there is little information on net returns to growers. The value of spraying Bt cotton with commercially available diamides was evaluated at seven locations in the Mississippi Delta cropping region in 2014 and 2015. Plots of Bollgard II®, Widestrike 3®, and non-Bt cotton varieties planted at each location were divided into five subplots. Treatments randomly assigned to subplots of each cotton cultivar were: 1) lowest labeled rate of chlorantraniliprole applied at threshold, 2) lowest labeled rate of flubendiamide applied at threshold, 3) lowest labeled rate of λ-cyhalothrin + chlorantraniliprole applied at threshold, 4) sprayed check, which was the maximum labeled rate of chlorantraniliprole applied on a 3-week schedule and initiated before first bloom to minimize damage from heliothines, and 5) nontreated check. Each plot was monitored weekly for larval abundance and plant damage by examining 100 plants at random. Supplemental control applications were initiated when larval abundance (≥4 per 100 plants) and plant damage (>2% damaged bolls with larvae present) met economic thresholds outlined by the Mississippi State University Extension Service. Following first bloom, all non-target pests were controlled with broadcast sprays of insecticide with low or no known toxicity to heliothines. No differences were detected in yield among treatments for any given Bt cotton variety. In non-Bt cotton, all threshold treatments and the sprayed check yielded significantly more lint cotton per hectare than did nontreated plots. Mean larval numbers per 100 plants ranged from 0.75–5.95 in non-Bt cotton to 0.04–0.8 and 0.44–1.17 in Bollgard II and Widestrike 3 cottons, respectively. Non-Bt cotton required approximately one additional diamide application for heliothine control as compared to Bt cotton plots. Results indicated no economic benefit to supplemental spray of diamides for bollworms in Bt cotton at larval abundance observed during the study. Bt cotton alone seemed sufficient to control bollworms. This should provide confidence for producers to avoid overspray at near threshold densities of bollworms.

Influence of Maize and Pigweed on Tarnished Plant Bug (Hemiptera: Miridae) Populations Infesting Cotton

Abstract. The influence of maize, Zea mays L., and pigweed, Amaranthus spp., on the abundance of tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), adults infesting cotton, Gossypium hirsutum L., in the Mississippi Delta was studied using stable-isotope analyses. Cotton fields adjacent to maize and those farther than 1.6 km from maize were identified in four counties of the Mississippi Delta. Tarnished plant bug adults were collected from the cotton fields weekly throughout the growing season and analyzed with stable-carbon and nitrogen isotopes to determine the natal host. The proportion of tarnished plant bug adults collected from cotton that developed as nymphs on plants that used the C4 photosynthetic pathway, primarily maize and pigweed, peaked from 79–92% during 2008–2009. Distance of cotton fields from maize did not influence the peak proportion of tarnished plant bug adults with a C4 signature from cotton fields, the peak time, or peak duration. Time of the C4 peaks occurred from 28 June to 8 July, and the duration was between 13 and 19 days. Stable-nitrogen isotope analyses indicated maize was the major contributor of C4 tarnished plant bugs infesting cotton compared to pigweed. Nevertheless, pigweed produced a significant proportion of tarnished plant bug adults infesting cotton. Improved control of these weed species could potentially reduce abundance of tarnished plant bugs in cotton. Because maize makes up a significant amount of the crop acreage in the Mississippi Delta, this information can be used to estimate the time of mass movement, thus allowing more efficient use of insecticide to control tarnished plant bugs in cotton. It may also allow future targeting of tarnished plant bugs in field corn before movement to cotton.

Temporal and Spatial Distribution of Helicoverpa zea and Heliothis virescens (Lepidoptera: Noctuidae) Moths in Pheromone Traps across Agricultural Landscapes in Arkansas

Pheromone trap captures of Helicoverpa zea (Boddie) and Heliothis virescens (F.) from 2002 - 2005 were used to examine the distribution of these species across a multiple crop environment in southeast Arkansas. The greatest number of H. virescens moths was collected during 2003, but densities were low compared with H. zea throughout the study. Overall, fewer H. virescens moths were captured next to Bt corn and Bt cotton than next to nonBt cotton and early-maturing soybean. A significant relationship was not detected between pheromone trap captures of H. virescens and the percentage of these moths that fed on cotton as larvae. More H. zea moths were captured in traps located next to Bt corn and nonBt corn during the month of June than in traps located next to cotton, sorghum, or soybean, whereas traps located adjacent to early- and late-maturing varieties of soybean, Bt cotton, nonBt cotton, and grain sorghum captured greater numbers of moths than those traps located next to corn during July. Therefore, the distribution of H. zea appeared to be influenced by corn acreage during the month of June. Later in the season, this species was widely distributed across the agricultural landscape. Compared with 2 other regions of AR, fewer H. zea moths were captured during the month of June and more H. virescens moths were captured during August in Mississippi Co., an area of intense cotton production.

Comparison of Three Bioassay Methods to Estimate Levels of Tarnished Plant Bug (Hemiptera: Miridae) Susceptibility to Acephate, Imidacloprid, Permethrin, Sulfoxaflor, and Thiamethoxam

A laboratory colony of tarnished plant bugs reared solely on a meridic diet was exposed to acephate, imidacloprid, permethrin, sulfoxaflor, and thiamethoxam in dose-response experiments using floral-foam, glass-vial, and dipped-leaf assays. Results indicated that different assay methods produced different relative results across the different insecticides. Dose- and time-response regression models also indicated that length of exposure of tarnished plant bugs to insecticide-treated plant tissue is important. Time of exposure required to reach an LC90 at estimated recommended field rates suggested that the recommended lower field rate of acephate (0.56 kg ai/ha) would reach an LC90 of exposed tarnished plant bugs between 48 and 96 h post initial exposure. An LC90 of tarnished plant bugs exposed to permethrin (0.11 kg ai/ha) was not predicted from the regression modes over the 168-h observation; lower recommended application rates of imidacloprid (0.053 kg ai/ha), sulfoxaflor (0.053 kg ai/ha), and thiamethoxam (0.042 kg ai/ha) reached projected LC90s between 96 and 168 h of exposure. Collectively, the results of this study corroborate current existing procedures for tracking tarnished plant bug resistance to insecticides, but also illustrate the importance of additional field studies that empirically associate assay results to projected field control.

Leaf Tissue Assay for Lepidopteran Pests of Bt Cotton

Abstract. Laboratory measurements of susceptibility to Bt toxins can be a poor indicator of the ability of an insect to survive on transgenic crops. We investigated the potential of using leaf tissue for evaluating heliothine susceptibilities to two dualgene Bt cottons, Gossypium hirsutum L. A preliminary study with different cotton leaf disk combinations was performed to determine the best procedure for using cotton tissue to assay bollworm, Helicoverpa zea Boddie, and tobacco budworm, Heliothis virescens F. Neonate larvae from a laboratory colony were assayed simultaneously on leaf disks of non-Bt and two dual-gene transgenic cottons in addition to meridic diet overlaid with discriminating doses of a commerciallyformulated Bt product. The bollworm colony in the laboratory was more tolerant than tobacco budworm to the commercially-formulated Bt product. When corrected for the non-Bt check, percentage of bollworm larval mortality was 42.1 and 71.9 on leaf disks of the two dual-gene Bt cottons. No tobacco budworm survived on dualgene Bt cotton leaf disks in the study. Assays using transgenic cotton leaf disks may compliment current meridic diet-based methods by providing a linkage to insect survival on Bt cotton plants.