Casey W. Hoy

Influence of Aster Yellows Phytoplasma on the Fitness of Aster Leafhopper (Homoptera: Cicadellidae)

Abstract This study revealed that feral aster leafhoppers, Macrosteles quadrilineatus Forbes, exposed to aster yellows phytoplasma live longer and may lay more eggs than nonexposed leafhoppers. Aster leafhoppers were reared on asters infected with either of 2 strains of aster yellows phytoplasma or uninfected asters. After eclosion, adults were placed on uninfected healthy lettuce or oat plants and transferred periodically. The life span of test leafhoppers and the number of offspring they produced were compared. Females reared on noninfected aster plants lived for an average of 19 d, those reared on ‘severe’ and ‘bolt’ strain aster yellows phytoplasma-infected plants lived 26 and 28 d, respectively. The mean number of offspring produced by females reared on the bolt strain of aster yellows phytoplasma-infected asters was almost twice the number produced by nonexposed leafhoppers. The life span of feral leafhoppers or the number of eggs laid did not differ for leafhoppers maintained on either oats or lettuce after exposure to aster yellows phytoplasma-infected asters. Female leafhoppers lived twice as long as males. Our results suggest that the aster leafhopper may have had a long association with aster yellows phytoplasma. The longer life and higher fecundity of phytoplasma-infected leafhoppers may influence disease dynamics of aster yellows in lettuce.

Colorado potato beetle resistance management strategies for transgenic potatoes

NewLeaf potatoes could provide substantial ecological and economic benefits to potato growers. A concern with NewLeaf potatoes, however, is that Colorado potato beetle may develop resistance to the Bt endotoxin because of the intense selection pressure imposed by its constant presence in the transgenic crop. Before these cultivars were released, however, entomologists throughout North America conducted biological research needed for a strategy to prevent or delay resistance in Colorado potato beetle. This paper describes the considerations and research behind the proposed strategy. Because NewLeaf potatoes express a relatively constant and high concentration of Bt endotoxin throughout the foliage of each plant, reducing selection for resistance must come from planting standard potatoes as a refuge for susceptible beetles. Avoiding resistance through random mixtures of NewLeaf and standard potatoes was evaluated in laboratory and field studies. The conclusion from this research was that the seed mixture would not provide an effective refuge. The most effective alternative refuge was judged to be a block of standard potatoes planted in the same field as the NewLeaf potatoes. By allowing susceptible beetles to survive in their fields, potato growers might sustain the benefits of very effective controls.

Plant Host Range and Leafhopper Transmission of Maize fine streak virus

Maize fine streak virus (MFSV), an emerging Rhabdovirus sp. in the genus Nucleorhabdovirus, is persistently transmitted by the black-faced leafhopper, Graminella nigrifrons (Forbes). MFSV was transmitted to maize, wheat, oat, rye, barley, foxtail, annual ryegrass, and quackgrass by G. nigrifrons. Parameters affecting efficiency of MFSV acquisition (infection) and transmission (inoculation) to maize were evaluated using single-leafhopper inoculations and enzyme-linked immunosorbent assay. MFSV was detected in ≈20% of leafhoppers that fed on infected plants but <10% of insects transmitted the virus. Nymphs became infected earlier and supported higher viral titers than adults but developmental stage at aquisition did not affect the rate of MFSV transmission. Viral titer and transmission also increased with longer post-first access to diseased periods (PADPs) (the sum of the intervals from the beginning of the acquisition access period to the end of the inoculation access period). Length of the acquisition access period was more important for virus accumulation in adults, whereas length of the interval between acquisition access and inoculation access was more important in nymphs. A threshold viral titer was needed for transmission but no transmission occurred, irrespective of titer, with a PADP of <4 weeks. MFSV was first detected by immunofluorescence confocal laser scanning microscopy at 2-week PADPs in midgut cells, hemocytes, and neural tissues; 3-week PADPs in tracheal cells; and 4-week PADPs in salivary glands, coinciding with the time of transmission to plants.

Increasing the effectiveness of spring trap crops for Leptinotarsa decemlineata

Insecticide resistance problems have increased interest in trap crops as a cultural control strategy for overwintered Colorado potato beetle adults, Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae). Growers in the north central United States have noticed and treated concentrations of adults at the edge of some of their potato fields each spring. Based on sampling in commercial potato fields over a 2‐year period, early planted fields that are adjacent to the previous years potato crop are most likely to have concentrations of adults at the field edge. Frequency of fields with significantly more adults at the edges than in the center sections as well as adult population density in the center sections of fields declined with both distance from the previous potato field and later planting date. The effects of both physical and chemical barriers to movement into potato fields from the field edges were studied in small plot trials and at the edges of commercial potato fields. In small plot trials, physical barriers had a greater impact than chemical barriers on adult beetle movement from a potato trap crop to the protected potatoes beyond the barrier. Barrier treatments reduced beetle numbers in and just beyond the barrier in commercial fields, but the effects were localized and no significant reduction of beetles was observed further into the field. Beetle flight was hypothesized to be responsible for the localized effects of barrier treatments and the lack of edge concentrations in later planted and more distantly rotated fields. In field studies, larger potato plants attracted more colonizing potato beetles than smaller plants. Attracting Colorado potato beetles to trap crops containing potato plants that were larger than those in the remainder of the field, however, provided no significant reduction of beetles in the remainder of the field. We found little opportunity to reduce beetle populations with trap crops at the edges of potato fields without controlling the adults in the trap crop itself. Growers can exploit naturally occurring concentrations of adults at the edges of early and adjacent potato plantings if they are prepared to monitor and regularly treat the field edges.

Naturally occurring biological controls in genetically engineered crops

Publisher Summary The chapter provides a framework to predict and measure the impact of crop genetic engineering on conservation of biological control agents, a framework that may help guide future development and use of genetically engineered crop cultivars. The number of times and extent to which insecticide use can be prevented by pest-resistant cultivars is very important in determining the prospects for conservation of biological control agents. The extent to which broad-spectrum insecticide use can be reduced also determines the extent to which diversity in the arthropod community can be enhanced with both new natural enemies and new prey contributing to biological control. In the near term, conservation is most likely to come from the reduction in insecticide use, upon which development and sales of these new cultivars largely depend. Future cultivar improvements, however, being added to a system with reduced reliance on insecticides, may increase natural enemy populations, and reduced pest outbreak potential may be able to focus on more subtle means of conserving natural enemies. By careful and strategic consideration of potential traits targeted for genetic engineering and likely management practices for the cultivars possessing those traits, developers of genetically engineered cultivars can contribute substantially to the conservation of biological control agents in crop systems.

Temporal Distribution of Aster Leafhopper Sex Ratios and Spatial Pattern of Aster Yellows Phytoplasma Disease in Lettuce

Abstract The aster leafhopper, Macrosteles quadrilineatus Forbes, is an important pest of fresh market vegetable crops as the primary vector of the aster yellows (AY) phytoplasma. Two sampling methods, sticky trapping and inverted cage trapping, were used to monitor male and female leafhopper populations over three growing seasons in leaf lettuce fields in Ohio. Captures by one sampling method could not be used to estimate captures by the other method, because sticky traps captured significantly more male leafhoppers and cage traps captured significantly more females. The proportion of females collected in cage traps decreased significantly in individual plantings as lettuce matured and also over the course of the season in one of 2 yr of sampling by using both techniques. Subsamples of captured leafhoppers were tested for AY phytoplasma infection by using a polymerase chain reaction (PCR) assay. In one year of our study, more male than female leafhoppers were infected with AY phytoplasma. Because the field distribution of insect-vectored diseases can reveal information about vector movement, as well as vector identity, spatial analysis was performed. Analyses indicated the distribution of symptomatic AY phytoplasma-infected lettuce plants was significantly clustered and followed a beta-binomial distribution. Our data suggest that females may be responsible for at least the early season inoculation of clustered symptomatic lettuce plants.

Indicative Value of Soil Nematode Food Web Indices and Trophic Group Abundance in Differentiating Habitats with a Gradient of Anthropogenic Impact

Indicative value of nematode food web indices and trophic group abundance was assessed for differentiating habitats in two soil types. A total of 604 soil samples were collected from croplands, grassy borders, turfgrass lawns, shrublands and forests in Huron County, Ohio characterized by muck soils. Also, a total of 242 soil samples were collected from two habitats, turfgrass lawns and croplands, in Wayne County, Ohio characterized by mineral soils. All nematodes extracted from soil samples were identified to genus level, categorized to trophic groups and various nematode community indices were calculated. Multivariate analysis of variance using composite of nematode measures revealed that croplands were significantly different from forests, shrublands, turfgrass lawns and grassy borders in the muck soils. Turfgrass lawns in muck soils were not different from forests but were different from shrublands. No differences were observed between forests and shrublands. Turfgrass lawns in mineral soils differed s...

Marking Methods and Field Experiments to Estimate Aster Leafhopper (Macrosteles quadrilineaus) Dispersal Rates

Abstract Four mark-release-recapture experiments were conducted from May to September 2000, to construct a 3D dispersal model for aster leafhopper (Macrosteles quadrilineatus Forbes). In the laboratory, flight mills and an optical sensor were used to measure the effects of fluorescent dust and rabbit protein marking on flight activity of aster leafhopper. No significant differences in proportion of leafhoppers flying, distance flown, average flight speed, or wing-beat frequencies were observed among marked leafhoppers versus unmarked controls. Leafhoppers were sampled in a grid pattern around a central release point of marked leafhoppers to estimate the patterns of leafhopper abundance and the distribution of dispersal distances. Geostatistical analysis of numbers of aster leafhopper adults captured by vacuum sampling in the grid pattern around a central release point was used to examine differences in dispersal pattern among crops. The spatial correlation range was ≈200 m for lettuce but only ≈35 m for endive. These differences in spatial pattern suggest leafhoppers disperse more slowly from lettuce plants, preserving aggregations over longer distances. The proportion of leafhoppers recaptured at various distances from a release point was modeled using a normal distribution for dispersal perpendicular to the wind and a Gumbel distribution for dispersal parallel with the wind. Goodness-of-fit tests indicated that both distributions fit field observations well. For all recaptured leafhoppers, the average distance from the release point was 53.6 m and the average distance for dispersal perpendicular to the wind was 17.6 m. The average distances moved parallel to the wind of those leafhoppers dispersing upwind and downwind were 12.1 and 43.9 m, respectively. The dispersal model will be used in spatially explicit simulation of aster yellows epidemiology.

Indirect selection for increased susceptibility to permethrin in diamondback moth (Lepidoptera Plutellidae)

Abstract We have been exploring the behavioral response of insect pests to heterogeneous distribution of toxins (low dose with refugia), and its genetic correlation with physiological tolerance to these toxins. A field-collected population of diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), from Celeryville, OH, was selected with permethrin to determine whether low heterogeneous doses could lead to increased susceptibility to permethrin by selecting indirectly on behavior. Two replicates of each of three selection regimes: uniform high concentration hypothesized to result in increased physiological tolerance, heterogeneous low concentration hypothesized to result in increased susceptibility through indirect selection on behavior, and a control with no exposure to permethrin, were maintained in 1-m3 cages in a greenhouse, for 33 generations. All life stages of the diamondback moth were exposed to the selection regimes, and new generations were started with a random selection of pupae from the previous generation. Lines selected with uniform high concentrations developed 76-fold levels of resistance to permethrin by the 17th generation, with little changes thereafter. For generations 1–20, lines selected with heterogeneous low concentrations remained slightly lower in LC50 but not significantly different from the unselected control lines. Based on confidence intervals from probit analyses, the LC50 of the lines selected with heterogeneous low concentration, however, were significantly lower than those of the control lines in generations 21–33. Our results demonstrate that selection on behavioral responses can result in greater susceptibility than no selection at all, despite exposure to the toxin and ample genetic variation and potential for increased physiological tolerance. The implications of our findings, which are based on selection scenarios that could take place in field situations, are that behavioral responses can prevent and even decrease the levels of resistance in insect populations, an important result with respect to resistance and resistance management.

Evidence for trade‐offs in detoxification and chemosensation gene signatures in Plutella xylostella

BACKGROUND Detoxification genes have been associated with insecticide adaptation in the diamondback moth, Plutella xylostella. The link between chemosensation genes and adaptation, however, remains unexplored. To gain a better understanding of the involvement of these genes in insecticide adaptation, the authors exposed lines of P. xylostella to either high uniform (HU) or low heterogeneous (LH) concentrations of permethrin, expecting primarily physiological or behavioral selection respectively. Initially, 454 pyrosequencing was applied, followed by an examination of expression profiles of candidate genes that responded to selection [cytochrome P450 (CYP), glutathione S-transferase (GST), carboxylesterase (CarE), chemosensory protein (CSP) and odorant-binding protein (OBP)] by quantitative PCR in the larvae. Toxicity and behavioral assays were also conducted to document the effects of the two forms of exposure. RESULTS Pyrosequencing of the P. xylostella transcriptome from adult heads and third instars produced 198,753 reads with 52,752,486 bases. Quantitative PCR revealed overexpression of CYP4M14, CYP305B1 and CSP8 in HU larvae. OBP13, however, was highest in LH. Larvae from LH and HU lines had up to five- and 752-fold resistance levels respectively, which could be due to overexpression of P450s. However, the behavioral responses of all lines to a series of permethrin concentrations did not vary significantly in any of the generations examined, in spite of the observed upregulation of CSP8 and OBP13. CONCLUSION Expression patterns from the target genes provide insights into behavioral and physiological responses to permethrin and suggest a new avenue of research on the role of chemosensation genes in insect adaptation to toxins.