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Western corn rootworm, Diabrotica virgifera virgifera LeConte, is a serious pest of corn and is often managed with transgenic corn producing insecticidal toxins from the bacterium Bacillus thuringiensis (Bt). This pest has developed field-evolved resistance to all commercially available Bt traits, beginning with Cry3Bb1 in 2009. Fitness costs may accompany Bt resistance, where individuals with alleles for Bt resistance have reduced fitness on non-Bt corn compared to Bt-susceptible individuals. In conjunction with non-Bt refuges, fitness costs can delay the evolution of Bt resistance. Importantly, ecological factors may affect the presence and magnitude of fitness costs. For western corn rootworm, available data suggest that fitness costs of Bt resistance may be present in some cases. Using two Cry3Bb1-resistant western corn rootworm strains (Hopkinton and Cresco), a fitness-cost experiment was performed by rearing rootworm in the absence of Bt for six generations to test for fitness costs of Cry3Bb1 resistance and the effect of larval rearing density on fitness costs. Fitness costs were detected for both strains; however, strains were still resistant to Cry3Bb1 corn at the end of the experiment. Cresco experienced a greater loss of resistance at low versus high density, but no effect of density was detected in Hopkinton. Our study shows that fitness costs can accompany Bt resistance in western corn rootworm and may be more pronounced under low larval density. Even though fitness costs were present, it appears that rootworm populations may remain resistant to Cry3Bb1 corn for years after resistance has evolved.

Grasshoppers (Orthoptera: Acrididae) are important arthropods in the grassland ecosystem as sources of herbivory, nutrient recycling through frass production, and as prey items for other insects, mammals, and birds. It has been observed in previous research that common Acrididae subfamilies have dietary preferences: many species of Gomphocerinae are specialists on grass plants, many Melanoplinae species are polyphagous forb eaters. We characterized the community of Acrididae along a vegetation gradient in the Beaver River Wildlife Management Area, Beaver County, OK, and tested the hypothesis that these subfamilies would be in greater abundance in areas dominated by their preferred food resource. Vegetation types were characterized into four different functional groups: grass, forb, litter, and bare ground. The proportion of cover of functional groups was found to be correlated with relative abundance of Gomphocerinae and Melanoplinae grasshoppers. Gomphocerinae were in greater abundance in vegetation types consisting of a larger proportion of grass. Melanoplinae were in greater abundance in vegetation types consisting of larger proportions of forb cover. A factor analysis indicated that forb cover and grass cover contributed more than litter and bare ground cover to the relative abundance of these grasshopper subfamilies and it is our conclusion that this is evidence for dietary-based habitat partitioning as observed through subfamilial relative abundance.

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BACKGROUND: The western corn rootworm is an economically important pest of corn. Management tactics include pyrethroid and organophosphate insecticides, which may be applied as a mixture to protect corn roots. The goal of our study was to characterize the effects of pyrethroids and organophosphates alone and in combination on larval corn rootworm mortality and injury to corn roots. We evaluated two insecticide combinations: tebupirimphos with ⊎-cyfluthrin and chlorethoxyfos with bifenthrin. Using a soil-based, laboratory bioassay, we exposed larvae to five concentrations of the pyrethroid alone, the organophosphate alone, the combined formulation, and a water control. We calculated LC 50 values and co-toxicity factors to determine synergism or antagonism between organophosphates and pyrethroids. We also measured adult emergence and root injury in a field experiment that tested tebupirimphos alone, ⊎-cyfluthrin alone, the combined formulation, and an untreated control. RESULTS: Bioassay results indicated antagonism between the pyrethroid and organophosphate at most concentrations for both insecticide combinations. In the field experiment, tebupirimphos alone or in combination with ⊎-cyfluthrin significantly reduced adult emergence and root injury compared to the untreated controls, but ⊎-cyfluthrin alone did not differ from the untreated control for either metric. CONCLUSIONS: These results suggest that, at the concentrations tested, the pyrethroid component of pyrethroid-organophosphate mixtures may not contribute to a reduction of rootworm emergence or root injury. While these pyrethroids may confer a management benefit for other pests, such as seedcorn maggot, the concentrations of pyrethroids present in current formulations of these mixtures are likely too low for effective rootworm management.