Jarrad R. Prasifka

Assessing the Effects of Pest Management on Nontarget Arthropods: The Influence of Plot Size and Isolation

Abstract Evaluations of field research on the nontarget effects of pest management, particularly the production of transgenic crops with insecticidal properties, suggest the methods used are sometimes unlikely to detect real differences among treatments. Among potential problems, abundance estimates may be scale-dependent for many arthropods, which move among experimental plots and between experimental plots and the surrounding environment. Insecticide-disturbed plots of field corn in a range of sizes in 2003 (0.03–0.53 ha; 18–72 m wide) and 2004 (0.01–0.13 ha; 9–36 m wide) were used for testing effects of scale on nontarget arthropod abundance. Possible effects of artificially isolating plots by removal of vegetation around plot borders were also studied in 2003. Community and taxon-based analyses showed abundance of foliar (aboveground) arthropods depended on plot size and isolation. While abundance of foliar arthropods was generally greater in smaller plots, isolation treatments suggested some taxa may have been either repelled or attracted to isolated plots. Levels of some epigeal (ground-dwelling) taxa were also size or isolation-dependent, but community-based analysis did not indicate a strong collective response to treatments. Recommendation of a practical but rigorous minimum plot size for nontarget studies may not be appropriate because responses to plot size varied among taxa. However, because arthropod movement into and out of plots can reduce differences between treatments, results suggest the use of small plots (width <9 m) for nontarget studies on transgenic crops generally should be avoided. Similarly, the taxon-specific effects of isolating plots should be considered when planning studies or interpreting results.

Effects of Living Mulches on Predator Abundance and Sentinel Prey in a Corn–Soybean–Forage Rotation

Abstract Living mulches are cover crops grown concurrently within main crops for their benefits to weed control and soil quality. Because living mulches increase the diversity of vegetation in agricultural systems, they may impact the abundance or effectiveness of natural enemies. To examine the effects of living mulches on epigeal predators, arthropod abundance and feeding were examined in a rotation of corn (Zea mays L.), soybean (Glycine max L. Merr.), and forage crops in 2004 and 2005. Compared with a no-mulch control, the presence of alfalfa (Medicago sativa L.) and kura clover (Trifolium ambiguum M. Bieb.) mulches in corn and soybean increased predator abundance and consumption of European corn borer (Ostrinia nubilalis Hübner; Lepidoptera: Crambidae) pupae used as sentinel prey. A complementary effect of increased predation caused by corn and soybean was found when both annual crops with living mulches were compared with mulches grown alone. Positive correlations between the percentage of O. nubilalis pupae preyed on and ground beetle (Coleoptera: Carabidae) abundance suggest carabids contributed to the differences in predation. In 2005, fewer beetles correlated with consumption of O. nubilalis pupae [Poecilus chalcites (Say) and Scarites quadriceps Chaudior] were collected, and living mulch impacts on predation were less frequent. Both changes could be related to delayed herbicidal suppression of the living mulches in 2005, which seemed to homogenize predator community composition among the corn, soybean, and forage plots. Although living mulches alone may not provide sufficient pest suppression, their potential to enhance biological control should be considered along with their other agronomic benefits.

Effects of Cry1Ab-expressing corn anthers on the movement of monarch butterfly larvae.

Abstract Decreased larval feeding and weight of the monarch butterfly, Danaus plexippus L., have been detected after 4 d of exposure in the laboratory to a high density of Bacillus thuringiensis (Bt)-expressing anthers. One hypothesis is that larvae exposed to Bt anthers exhibit increased wandering, resulting in less feeding and lower weight gain. To test this hypothesis, 2-d-old monarch butterfly larvae exposed to milkweed leaf disks with no anthers, anthers that express Bt (Cry1Ab, event MON810), or other non-Bt anthers were observed using a video-tracking system. As had been shown in previous studies, larvae exposed to Bt anthers fed less and gained less weight than larvae exposed to non-Bt or no anthers, yet there was no evidence of feeding on anthers. Total distance moved, maximum displacement from release point, percentage of time spent moving or near anthers, or mean turn angle did not differ across treatments. However, larvae exposed to Bt anthers spent more time off milkweed leaf disks than those exposed to no anthers and were more likely to move off the leaf than larvae exposed to non-Bt anthers. Results suggest that larvae exposed to Bt anthers behave differently and that ingestion may not be the only way Bt can affect nontarget insects like the monarch butterfly.

Comparison of pitfall traps and litter bags for sampling ground-dwelling arthropods

Abstract:  For their simplicity and effectiveness, pitfall traps have become a standard technique to measure the activity and relative abundance of ground‐dwelling arthropods. Permeable screen or mesh bags filled with plant material, referred to as litter bags, have also recently been employed as a complementary sampling technique for epigeal taxa. The anticipated need for increased field research on arthropod populations, particularly in transgenic crops with insecticidal properties, suggests that a relative assessment of both sampling methods could contribute to the design of future studies. Comparisons among pitfall traps, and litter bags placed above‐ or below‐ground indicated that above‐ground litter bags most frequently succeeded in collecting certain groups of arthropods associated with moisture and sheltered areas, including centipedes (Chilopoda) and beetle larvae (Carabidae, Staphylinidae). Conversely, pitfall traps most often captured taxa considered active at ground level, such as adult carabids or harvestmen (Opiliones). For taxa collected in >40% of all three trap types, bootstrap confidence intervals for the coefficient of variation (CV; used to assess precision or sampling efficiency) showed that above‐ground litter bags were significantly more precise than pitfall traps for sampling elongate springtails (Collembola) and adult rove beetles (Staphylinidae), but only during the first year of sampling. While below‐ground litter bags often appeared similar to one or both of the other trap types, in no case were below‐ground litter bags best based on frequency of collection or CV. Though differences were not always consistent between years, results suggest that the additional effort required to sample using litter bags may be justified for the collection of some ground‐dwelling taxa.

Effects on Fitness and Behavior of Monarch Butterfly Larvae Exposed to a Combination of Cry1Ab-Expressing Corn Anthers and Pollen

Abstract Anthers and pollen from corn, Zea mays L., expressing Bacillus thuringiensis (Bt)-derived protein frequently fall onto common milkweed, Asclepias syriaca L., growing in and near corn fields. Previous studies have shown that, alone, Bt anthers do not pose a significant risk to the monarch butterfly, Danaus plexippus L. To examine how exposure to a combination of Bt anthers and pollen affects larval fitness and behavior, three studies were conducted. A laboratory study using severed leaves in petri dishes and two studies with caged milkweed plants (tropical milkweed, Asclepias curassavica L., and common milkweed) in an environmentally controlled rearing room. In the petri dish bioassay, additive effects of Bt anthers and pollen were detected. The data suggest that the additive effects may be attributed to behavioral changes in larval feeding such as detecting and avoiding Bt anthers. An additive effect also was seen in both cage studies. In the common milkweed cage study, larvae exposed to Bt anthers and pollen took 1.8 d longer to develop and pupae weighed 6.4% less than those exposed to non-Bt anthers and pollen. These effects are similar to those found in a previous study with naturally deposited levels of Bt anthers and pollen, even though the anther levels we tested were two to three times greater. Despite these effects, when put into the context of risk, Bt corn is not likely to pose a significant risk to the monarch butterfly population in North America.

Overexpression of Drosophila juvenile hormone esterase binding protein results in anti-JH effects and reduced pheromone abundance

The titer of juvenile hormone (JH), which has wide ranging physiological effects in insects, is regulated in part by JH esterase (JHE). We show that overexpression in Drosophila melanogaster of the JHE binding protein, DmP29 results in a series of apparent anti-JH effects. We hypothesize that DmP29 functions in transport of JHE such that over- or under-expression of DmP29 results in increased or decreased JH degradation at specific sites respectively. Overexpression of DmP29 during the first or second instar was lethal, while overexpression during the third instar resulted in eclosion of small adults. Overexpression of DmP29 in newly eclosed flies reduced ovarian development and fecundity in addition to reducing the abundance of aggregation pheromone (cis-vaccenyl acetate) in males and courtship pheromone (cis,cis-7,11-heptacosadiene) in females. Both sexes also had lower levels of 23 and 25 carbon monoenes. Females exhibited reduced receptivity to mating, and males exhibited male-male courtship behavior, with both sexes being hyperactive: Male flies covered 2.7 times the distance of control flies at 2.9 times the maximum velocity. Application of the JH analog methoprene reversed impaired ovarian development, supporting a role for reduced JH in production of this phenotype. Rather than increasing lifespan as expected from a JH deficiency, overexpression of DmP29 reduced the life span of adult flies which may result from the hyperactivity of these flies. Underexpression of DmP29 resulted in reduced longevity, increased fecundity and reduced titers of pupal JHE. An alternative hypothesis, that mitochondrial dysfunction rather than JHE results in the JH-mediated phenotypes, is discussed.

Selection of nontarget arthropod taxa for field research on transgenic insecticidal crops: using empirical data and statistical power.

Abstract One of the possible adverse effects of transgenic insecticidal crops is the unintended decline in the abundance of nontarget arthropods. Field trials designed to evaluate potential nontarget effects can be more complex than expected because decisions to conduct field trials and the selection of taxa to include are not always guided by the results of laboratory tests. Also, recent studies emphasize the potential for indirect effects (adverse impacts to nontarget arthropods without feeding directly on plant tissues), which are difficult to predict because of interactions among nontarget arthropods, target pests, and transgenic crops. As a consequence, field studies may attempt to monitor expansive lists of arthropod taxa, making the design of such broad studies more difficult and reducing the likelihood of detecting any negative effects that might be present. To improve the taxonomic focus and statistical rigor of future studies, existing field data and corresponding power analysis may provide useful guidance. Analysis of control data from several nontarget field trials using repeated-measures designs suggests that while detection of small effects may require considerable increases in replication, there are taxa from different ecological roles that are sampled effectively using standard methods. The use of statistical power to guide selection of taxa for nontarget trials reflects scientists’ inability to predict the complex interactions among arthropod taxa, particularly when laboratory trials fail to provide guidance on which groups are more likely to be affected. However, scientists still may exercise judgment, including taxa that are not included in or supported by power analyses.

Establishment and Host Effects of Cereal Aphids on Switchgrass (Panicum virgatum L.) Cultivars1

Abstract. Potential for widespread plantings of switchgrass, Panicum virgatum L., as a biomass feedstock prompted its evaluation as a host for economically important cereal aphids. Seedlings of four cultivars of switchgrass, ‘Kanlow’, ‘BlackweN’, ’Cave-In-Rock’, and ’Sunburst’, were evaluated as hosts for greenbug, Schizaphis graminum (Rondani), biotypes E, I, and Florida; Russian wheat aphid, Diuraphis noxia (Kurdj.), biotype 2; bird-cherry oat aphid, Rhopalosiphum padi (L.); corn leaf aphid, R. maidis (Fitch); English grain aphid, Sitobion avenae (F.); and yellow sugarcane aphid, Sipha flava (Forbes). Greenbug biotype E and Russian wheat aphid biotype 2, both economically important cereal aphids in the United States, and English grain aphid were unable to establish on any of the switchgrass cultivars. However, greenbug biotypes I and Florida, bird-cherry oat aphid, corn leaf aphid, and yellow sugarcane aphid established on all cultivars tested. The aphids which successfully established on switchgrass also reduced plant growth (number of leaves added and leaf biomass), usually by 50% or more. Interactions between greenbug biotypes and cultivars, along with observations of other aphids on switchgrass cultivars, suggested potential for developing cultivars resistant to specific aphids and biotypes.