Dennis D. Calvin

Effects on Monarch Butterfly Larvae (Lepidoptera: Danaidae) After Continuous Exposure to Cry1Ab-Expressing Corn During Anthesis

Abstract Effects on monarch butterfly, Danaus plexippus L., after continuous exposure of larvae to natural deposits of Bacillus thuringiensis (Bt) and non-Bt pollen on milkweed, were measured in five studies. First instars were exposed at 3–4 and 6–7 d after initial anthesis, either directly on milkweed plants in commercial cornfields or in the laboratory on leaves collected from milkweeds in corn plots. Pollen exposure levels ranging from 122 to 188 grains/cm2/d were similar to within-field levels that monarch butterfly populations might experience in the general population of cornfields. Results indicate that 23.7% fewer larvae exposed to these levels of Bt pollen during anthesis reached the adult stage. A risk assessment procedure used previously was updated with a simulation model estimating the proportion of second-generation monarch butterflies affected. When considered over the entire range of the Corn Belt, which represents only 50% of the breeding population, the risk to monarch butterfly larvae associated with long-term exposure to Bt corn pollen is 0.6% additional mortality. Exposure also prolonged the developmental time of larvae by 1.8 d and reduced the weights of both pupae and adults by 5.5%. The sex ratio and wing length of adults were unaffected. The ecological significance of these sublethal effects is discussed relative to generation mortality and adult performance.

Predicting weed emergence for eight annual species in the northeastern United States

Abstract A 2-yr experiment assessed the potential for using soil degree days (DD) to predict cumulative weed emergence. Emerged weeds, by species, were monitored every 2 wk in undisturbed plots. Soil DD were calculated at each location using a base temperature of 9 C. Weed emergence was fit with logistic regression for common ragweed, common lambsquarters, velvetleaf, giant foxtail, yellow foxtail, large crabgrass, smooth pigweed, and eastern black nightshade. Coefficients of determination for the logistic models fit to the field data ranged between 0.90 and 0.95 for the eight weed species. Common ragweed and common lambsquarters were among the earliest species to emerge, reaching 10% emergence before 150 DD. Velvetleaf, giant foxtail, and yellow foxtail were next, completing 10% emergence by 180 DD. The last weeds to emerge were large crabgrass, smooth pigweed, and eastern black nightshade, which emerged after 280 DD. The developed models were verified by predicting cumulative weed emergence in adjacent plots. The coefficients of determination for the model verification plots ranged from 0.66 to 0.99 and averaged 0.90 across all eight weed species. These results suggest that soil DD are good predictors for weed emergence. Forecasting weed emergence will help growers make better crop and weed management decisions. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; common ragweed, Ambrosia artemisiifolia L. AMBEL; eastern black nightshade, Solanum ptycanthum Dun. SOLPT; giant foxtail, Setaria faberi Herrm. SETFA; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; smooth pigweed, Amaranthus hybridus L. AMACH; velvetleaf, Abutilon theophrasti Medikus ABUTH; yellow foxtail, Setaria glauca (L.) Beauv. SETLU.

Evaluation of Noncorn Host Plants as a Refuge in a Resistance Management Program for European Corn Borer (Lepidoptera: Crambidae) on Bt-Corn

Abstract Studies in New York and Pennsylvania compared egg mass recruitment and larval survival on corn and other hosts of the European corn borer, Ostrinia nubilalis (H[umlaut]ubner), to assess the potential of these plants to act as a refuge in a resistance management program. Assessments were made on replicated plantings and natural plant stands in the field and under controlled conditions in the laboratory. Scouting of mixed field plantings revealed more egg masses on corn than any other crop or weed species. At least twice as many larvae per plant were recovered from naturally infested corn compared with the next best host plant across both years. Larval recovery from noncorn host plants varied widely. Fewer adults emerged from overwintering weed stubble than from corn stubble, and the parasitoid Macrocentris grandii (Goidanich) was found associated only with corn stubble. Survival on plants infested with corn borer larvae was consistently higher on corn than on other plants. In a laboratory study, the number of corn borer tunnels in corn was double the next best host, ragweed. Noncorn hosts appear unlikely to provide a substantial number of corn borer individuals susceptible to B. thuringiensis (Berliner) in comparison with the number expected from the 20% planting refuge mandated by EPA registration of Bt-corn. Evidence from these studies do not support a recommendation of reduced refuge planting areas in the northeastern United States.

Use of Spectral Vegetation Indices Derived from Airborne Hyperspectral Imagery for Detection of European Corn Borer Infestation in Iowa Corn Plots

Abstract Eleven spectral vegetation indices that emphasize foliar plant pigments were calculated using airborne hyperspectral imagery and evaluated in 2004 and 2005 for their ability to detect experimental plots of corn manually inoculated with Ostrinia nubilalis (Hübner) neonate larvae. Manual inoculations were timed to simulate infestation of corn, Zea mays L., by first and second flights of adult O. nubilalis. The ability of spectral vegetation indices to detect O. nubilalis-inoculated plots improved as the growing season progressed, with multiple spectral vegetation indices able to identify infested plots in late August and early September. Our findings also indicate that for detecting O. nubilalis-related plant stress in corn, spectral vegetation indices targeting carotenoid and anthocyanin pigments are not as effective as those targeting chlorophyll. Analysis of image data suggests that feeding and stem boring by O. nubilalis larvae may increase the rate of plant senescence causing detectable differences in plant biomass and vigor when compared with control plots. Further, we identified an approximate time frame of 5–6 wk postinoculation, when spectral differences of manually inoculated “second” generation O. nubilalis plots seem to peak.

The Effect of Weed Density and Application Timing on Weed Control and Corn Grain Yield1

A 2-yr experiment repeated at five locations across the northeastern United States evaluated the effect of weed density and time of glyphosate application on weed control and corn grain yield using a single postemergence (POST) application. Three weed densities, designed to reduce corn yields by 10, 25, and 50%, were established across the locations, using forage sorghum as a surrogate weed. At each weed density, a single application of glyphosate at 1.12 kg ai/ha was applied to glyphosate-resistant corn at the V2, V4, V6, and V8 growth stages. At low and medium weed densities, the V4 through V8 applications provided nearly complete weed control and yields equivalent to the weed-free treatment. Weed biomass and the potential for weed seed production from subsequent weed emergence made the V2 timing less effective. At high weed densities, the V4 followed by the V6 timing provided the most effective weed control, while maintaining corn yield. Weed competition from subsequent weed emergence in the V2 application and the duration of weed competition in the V8 timing reduced yield on average by 12 and 15%, respectively. This research shows that single POST applications can be successful but weed density and herbicide timing are key elements. Nomenclature: Glyphosate; corn, Zea mays L.; forage sorghum, Sorghum bicolor (L.) Moench. Additional index words: Glyphosate-resistant corn, herbicide application timing, postemergence weed control, weed competition, weed density. Abbreviations: POST, postemergence; WAP, weeks after planting.

Development and Validation of Models for Predicting the Seasonal Emergence of Corn Rootworm (Coleoptera: Chrysomelidae) Beetles in Iowa

Abstract Effective management of adult northern and western corn rootworms, Diabrotica barberi Smith & Lawrence and D. virgifera virgifera LeConte, respectively, requires knowledge of their emergence pattern so that scouting and adult insecticide applications can be accurately timed. The objective of this study was to develop and validate species- and sex-specific models that reliably predicted adult corn rootworm emergence in Iowa. Prediction began from a biofix defined as the date of first beetle emergence in a field. The models were fit with a 3-parameter Weibull function using emergence data collected in 57 Iowa cornfields over 5 yr. Models were validated with emergence data collected in 21 additional fields from a separate year. A single Pherocon CRW Trap per field was as effective as 13 emergence cages per field at detecting the biofix. Air temperature degree-days accumulated from the emergence cage biofix explained 85% of the variability in total corn rootworm emergence over 5 yr. This model explained 89% and 83% of the variability in total beetle emergence observed in the validation year from the emergence cage and Pherocon CRW Trap biofixes, respectively. These models do not eliminate scouting for adult corn rootworms but should improve the scouting efficiency by allowing growers to focus scouting to key periods, such as peak beetle emergence, when populations should be at their maximum abundance in the field.

Effect of Soil Disturbance on Annual Weed Emergence in the Northeastern United States

A 2-yr experiment evaluated the effect of spring soil disturbance on the periodicity of weed emergence. At four locations across the northeastern United States, emerged weeds, by species, were monitored every 2 wk in both undisturbed plots and plots tilled in the spring with a rotary cultivator. Eight weed species including large crabgrass, giant and yellow foxtail, common lambsquarters, smooth pigweed, eastern black nightshade, common ragweed, and velvetleaf occurred at three or more site-years. Spring soil disturbance either had no effect or reduced total seedling emergence compared with undisturbed soils. Total seedling emergence for large crabgrass, giant foxtail, smooth pigweed, and common ragweed were on average, 1.4 to 2.6 times less with spring soil disturbance, whereas eastern black nightshade and velvetleaf were mostly unaffected by the soil disturbance. The influence of soil disturbance on yellow foxtail and common lambsquarters emergence varied between seasons and locations. Although the total number of emerged seedlings was often affected by the soil disturbance, with the exception of yellow foxtail and common ragweed, the periodicity of emergence was similar across disturbed and undisturbed treatments. Nomenclature: Common lambsquarters, Chenopodium album L. #3 CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; eastern black nightshade, Solanum ptycanthum Dun. # SOLPT; giant foxtail, Setaria faberi Herrm. # SETFA; large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA; smooth pigweed, Amaranthus hybridus L. # AMACH; velvetleaf, Abutilon theophrasti Medicus # ABUTH; yellow foxtail, Setaria glauca (L.) Beauv. # SETLU. Additional index words: Integrated pest management, periodicity of weed emergence, spring soil disturbance.

Influence of Sweet Corn Growth Stages on European Corn Borer (Lepidoptera: Crambidae) Oviposition

Abstract Oviposition rates of the European corn borer, Ostrinia nubilalis (Hübner), were examined on corn plants from the 3-leaf through postharvest growth stages during 1994–1996. Per-plant rates in the field during vegetative stages showed a gradual increase as new leaves appeared, and a sharp increase when reproductive plant parts emerged. Laboratory studies indicated a similar relationship between oviposition rate and plant growth stage: significantly greater rates occurred on reproductive plants. Laboratory experiments showed that when only 4-leaf to early green tassel (preanthesis) plants were present (simulating conditions during early-season European corn borer activity), per-plant oviposition was significantly greater on the oldest (11-leaf to green tassel) plants. When the oviposition rate was based on leaf area rather than on a per-plant basis, however, statistical differences between vegetative stages were lost. When European corn borer females were given only reproductive plant stages (simulating conditions during late-season European corn borer oviposition), plants in the green tassel, anthesis, green silk, and brown silk stages had generally equal oviposition rates, and these were significantly greater than on postharvest plants. This European corn borer oviposition simulation, when based on leaf area, showed the same pattern as the per-plant oviposition. Thus, leaf area appears to influence oviposition during the vegetative stages, when leaf area is rapidly expanding, but is of little influence after tassels emerge and leaf area is expanding at a slower rate or declining. Moth alighting in the laboratory indicate a behavioral difference between sexes. Females alighted on plants identical to those favored for oviposition. In contrast, males showed no preference for any growth stage of corn.

Vertical Distribution of European Corn Borer (Lepidoptera: Crambidae) Egg Masses on Sweet Corn

Abstract Within-plant height (leaf position) of European corn borer, Ostrinia nubilalis (Hübner) egg masses were analyzed in four sweet corn hybrids planted across four planting dates from 1994 to 1996. From the five-leaf to 15-leaf vegetative stages, the mean leaf position of an egg mass was typically at the midpoint of available (nonsenescent) leaves. The mean leaf position of egg masses gradually increased from early to late leaf stages as new leaves were added, and continued to increase during the first reproductive stage (green tassel). The increase in vertical position during vegetative stages was modeled as y = −0.766 + 0.653(x), where y = nodal leaf position, and x = total leaves expanded (r2 = 0.94). However, when the mean egg mass position was expressed as the mean position within available (expanded and nonsenescent) leaves, no relationship was found. These data indicate that during vegetative stages the mean egg mass position increases relative to ground level, but remains near the midpoint of available (nonsenescent) leaves. Mean egg position during the next four reproductive stages (anthesis, green silk, brown silk, and postharvest) stabilized at ear height, or within the immediate two leaf positions above the ear. The distributions of egg masses among leaf positions during the vegetative and reproductive stages generally exhibited normal distributions: where sample sizes were sufficient, 80% (17/21) of the comparisons (having sufficient sample sizes) did not deviate from normality. This study represents the most detailed examination to date on vertical distribution of European corn borer egg masses in corn. Implications of the study in ovipositional ecology and pest management are discussed.

Female European Corn Borer (Lepidoptera: Crambidae) Ovarian Developmental Stages: Their Association with Oviposition and Use in a Classification System

Abstract Reproductive development of female European corn borer, Ostrinia nubilalis (Hübner), was investigated and a classification system proposed. Females collected in a blacklight trap during 1982 and 1983 were dissected and their reproductive system examined. Female reproductive systems were divided into six stages based on ovum development within the ovarioles, ovum depletion, ovariole appearance, and fat body color and shape. The female reproductive systems were also staged on the basis of spermatophore appearance. The time necessary to classify a female is also reported. Based on the classification system, the relationship between female age and stage of ovarian development was quantified under three temperature regimes. Females were found to experience a 3- to 5-d preoviposition period before initiation of egg deposition under optimal temperature conditions. This delay between adult emergence and initiation of egg laying corresponded with more advanced ovarian developmental stages collected in blacklight traps and indicates that actively ovipositing females are primarily being collected in blacklight traps.