Farmers, industry, governments and environmental groups agree that it would be useful to manage transgenic crops producing insecticidal proteins to delay the evolution of resistance in target pests. The main strategy proposed for delaying resistance to Bacillus thuringiensis ( Bt) toxins in transgenic crops is the high-dose/refuge strategy. This strategy is based on the unverified assumption that resistance alleles are initially rare (<10(-3)). We used an F(2) screen on >1,200 isofemale lines of Ostrinia nubilalis Hübner (Lepidoptera: Crambidae) collected in France and the US corn belt during 1999-2001. In none of the isofemale lines did we detect alleles conferring resistance to Bt maize producing the Cry1Ab toxin. A Bayesian analysis of the data indicates that the frequency of resistance alleles in France was <9.20 x 10(-4) with 95% probability, and a detection probability of >80%. In the northern US corn belt, the frequency of resistance to Bt maize was <4.23 x 10(-4) with 95% probability, and a detection probability of >90%. Only 95 lines have been screened from the southern US corn belt, so these data are still inconclusive. These results suggest that resistance is probably rare enough in France and the northern US corn belt for the high-dose plus refuge strategy to delay resistance to Bt maize.

A sand dilution assay was developed to study how composting affects the nutritional value of stored laying hen manure for larvae of the house fly, Musca domestica L. Equal numbers of eggs were inoculated into graded amounts of stock manure and incubated under standardized moisture conditions. Survival and mass per emerging adult diminished with progressively lower supplies of manure per larva, whether the manure was diluted into clean, white sand or placed on top of an equal volume of sand. Mass of adults per original egg was an increasing linear function of log, manure supply, with extrapolated lower supply threshold, S(L) = 0.06 g per egg. It is proposed that S(L) is a measure of a substrate's nutritional value--the greater the threshold, the lower its value. Dilution of the same stock manure in loam or sandy loam reduced the manure's apparent nutritional value, and dehydration of the stock manure to 20% water before rehydration to 70% also reduced nutritional value. Assays of bulk samples from replicated piles of laying hen manure mixed with sunflower hulls indicated the mixture was nutritionally equivalent to the stock manure, but that 3-4 wk of subsequent aerobic, thermophilic composting reduced it to approximately 10% of its initial value. These results suggest that composting may be a useful technique for reducing the fly breeding potential of laying hen manure and other substrates that must be stored before spreading and incorporation on crop land.

Two partial cleanout methods were compared with complete cleanouts in replicate caged-layer houses for effects on manure characteristics and subsequent dynamics in populations of manure-breeding house fly (Musca domestica L.), lesser mealworm (Alphitobius diaperinus [Panzer]), and associated natural enemies. Absolute densities of adult house flies increased by approximately two to four times within 3 wk after cleanouts of all kinds, and then remained stable over the next 3 mo. Increases were least in barns where residual pads were formed with manure that had been selectively retained from valleys between older piles. Compared with complete cleanouts, partial cleanouts reduced water content of subsequent manure piles. Partial cleanouts also conserved more pteromalid fly parasites, more predatory Xylocoris bugs, and more predatory Carcinops beetles, but not more macrochelid mites. Lesser mealworm populations were reduced by cleanouts of all kinds, but complete cleanouts reduced populations the most. Results suggest choice of cleanout method will depend on whether house flies or lesser mealworms are of primary concern.

The high-dose refuge resistance management strategy is the main approach used to delay resistance in targeted pests to Bacillus thuringiensis (Bt) toxins in transgenic crops. We used an F2 screen to test a critical assumption of the high-dose refuge strategy, which is that resistance allele (R) frequencies are initially rare (<10(-3)) in Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) from the southern Corn Belt. We expanded the methodological scope of the F2 screen so that both males and females may be used to initiate a screen and determined how the results from both sexes may be combined. In total, 62 female and 131 male O. nubilalis lines from Kansas and 39 female and four male lines from Texas were screened. No major resistance alleles were found and estimated R frequency for the southern Corn Belt was updated to between 0 and 0.0044 with 95% credibility. The experiment-wise detection probability was 98.7%. These results suggest the frequency of resistance alleles is low enough that the high-dose refuge resistance management strategy may be effective for delaying resistance evolution in O. nubilalis to Bt corn in the southern Corn Belt.

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