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No description available.

No description available.

No description available.

No description available.

Resistance to Bacillus thuringiensis (Bt) cotton, Gossypium hirsutum, is associated with fitness costs in pink bollworm, Pectinophora gossypiella. Previous work has demonstrated that costs of resistance are induced by environmental factors including insecticidal cotton phytochemicals such as gossypol. This study (1) tested the hypothesis that the presence of toxic phytochemicals would increase the dominance and magnitude of fitness costs associated with genes conferring resistance to the Bacillus thuringiensis (Bt) toxin, (2) coupled a selection experiment and simulation modeling to evaluate the fitness cost of resistance to Bt in pink bollworm and (3) used laboratory strains containing susceptible and resistant genotypes to test the hypothesis that gossypol accumulates more readily in pink bollworm with Bt resistance alleles, and that this gossypol accumulation reduces fitness. To test hypothesis (1), larval survival and weight were measured in two independent strains of pink bollworm, Pectinophora gossypiella, reared on diet containing the cotton phytochemicals gossypol and cyclopropenoid fatty acid, alone, or in combination. Cotton phytochemicals increased the dominance and magnitude of fitness costs associated with cadherin-based resistance to Bacillus thuringiensis (Bt) toxin. Gossypol and cyclopropenoid fatty acid combined had a more detrimental effect on fitness of pink bollworm than either compound alone. To test hypothesis (2), two pink bollworm strains fed synthetic diet were monitored over 30 generations to test the hypothesis that costs associated with Bt resistance would result in a decline in the frequency of resistance. A decrease in resistance allele frequency did occur in both strains and costs affecting each resistant genotypes were estimated. To test hypothesis (3), two strains of pink bollworm were fed on diet containing gossypol and on diet without gossypol, and gossypol accumulation in tissues of genotypes was measured. In both strains, significantly more gossypol accumulated in genotypes containing at least one resistance allele and gossypol accumulation was additive to dominant. In both strains, an increase in the dominance or magnitude of costs affecting larval weight was observed on gossypol diet, and the change in the magnitude of costs was positively associated with gossypol absorption. In one strain, the presence of gossypol increased survival costs but only in the genotype with the highest gossypol absorption. The mutation conferring resistance to the Bt toxin Cry1Ac is found in the cadherin-encoding region in three lepidopeteran pests (Helicoverpa armigera, Heliothis virescens and Pectinophora gossypiella) including the pink bollworm. Cadherin proteins had been hypothesized to play a role in maintaining the integrity of the insect midgut epithelial tissue. Taken together, these results support the hypothesis that cadherin proteins do contribute to gut integrity, toxic plant phytochemicals accumulate more readily in pink bollworm with resistance alleles, and that such accumulation increases the dominance and magnitude of fitness costs.

Helicoverpa armigera (cotton bollworm) is a major agricultural pest in Australia, Asia and Africa. Analysis of a midgut cDNA library revealed a surprising diversity of genes encoding lipases, enzymes that metabolise lipids. Prior to and during this project, 13 neural lipases and three acidic ones were discovered. These were classified and compared to other insect lipases using sequence alignments and phylogenetic analysis. Novel non-catalytic subfamilies were discovered, including one containing an H. armigera lipase. At least three other subfamilies were found to contain H. armigera lipases. Lipidopteran lipases were found to be almost entirely distinct from Dipteran ones.

Rainfastness of insecticides is an understudied aspect of agricultural research. Little is known about the residual of commonly used products for key pests of cotton, as well as their residual after a rainfall event. This project was designed to evaluate the impact of rainfall on the performance of commonly used insecticides for tobacco thrips, Frankliniella fusca (Hinds); tarnished plant bug, Lygus lineolaris (Palisot de Beauvois); and bollworm, Helicoverpa zea (Boddie), management in cotton. Laboratory and field experiments suggest that rainfall occurring within 16 hours after application had a negative impact on the performance of most insecticides. Chemical analyses of leaf tissue using a mass spectrometer confirmed what was observed with field and laboratory experiments. Although direct comparisons between insecticides cannot be made, results from this study suggest that spinosyns and insect growth regulators appeared to have the longest residual after a rainfall event.

The bollworm, Helicoverpa zea (Boddie), is an economically important pest of cotton and is becoming more difficult to control in the United States. This project was designed to develop a better understanding of the effects of resistance development and plant structures on the efficacy of Bt cotton and foliar insecticides. Experiments examined larval behavior and damage in current cotton varieties expressing proteins from Bacillus thuringiensis. Other experiments examined the effects of floral structures on the control of bollworm using foliar insecticides. Results suggest that larval feeding and avoidance behavior is dependent on point of eclosion. Fruiting form damage increased in two-gene cotton compared to earlier research but was low in cotton expressing the Vip3A protein. Increases in damage can lead to more frequent applications of foliar insecticides. Floral structures can hinder insecticide efficacy by reducing exposure. Results from these experiments will be important for refining management recommendations for bollworm in Bt cotton.

Helicoverpa zea, commonly referred to as corn earworm, has been a major pest of corn and cotton, along with many major crops grown in United States. Migration of this pest plays a major role in its distribution and successful survival. Part of the current resistance management strategy for transgenic crops, using non-Bt crops as refuges, is totally based on the movement of the adult populations between the Bt and non-Bt fields, and successful exchange of genetic material between the rare resistant and susceptible populations. To study the movement and migration patterns, and their implications in resistance management, suitable molecular genetic markers were comparatively selected, followed by a study of temporal variations in populations from north Mississippi. The dominant marker system Inter Simple Sequence Repeats (ISSR) was selected for the study based on higher polymorphism (5.0) and PIC (0.34) values compared to Simple Sequence Repeats (SSR) and Sequence Related Amplification Polymorphism (SRAP) marker systems. 53 ISSR loci were used in understanding the temporal variations in H. zea. Percent polymorphism and heterozygosity levels showed variation among the twelve collections tested. Early spring putative migrants were distinct from the rest of the generations. Population differentiation was higher in the beginning of the season and then declined by end of the season (pairwise FST = 0.341, early in March and 0.025, late in August). Average heterozygosity levels recorded were 0.11, coinciding with 1993 and 2002 data. The result of this study supports the existence of spring migration of adults and their contribution to the local gene pool. The extent of genetic exchange between the putative migrants and the local populations varied among three years. There is variation in population densities from migrants and local emergence in one of the three years. The results of this study indicate a need for continuous monitoring for genetic changes and their possible implications for resistance management in transgenic crops in Mississippi.