Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

First Advisor

Roger Leonard


Studies were conducted to determine the effectiveness of Bacillus thuringiensis (Bt) insecticides and transgenic Bt cotton in controlling the bollworm (BW), Helicoverpa zea (Boddie) and tobacco budworm (TBW), Heliothis virescens (F.), complex. Laboratory tests indicated that rates $\ge$1.17 liters (form)/ha caused TBW mortality significantly higher than the untreated control 2 hours after treatment (HAT). Dipel ES$\sp\circler$ at 4.68 liters (form)/ha and Condor OF$\sp\circler \ge$ 0.58 liters (form)/ha caused significantly higher TBW mortality than the untreated control at 72 HAT. TBW mortality levels did not significantly increase as Dipel ES$\sp\circler$ or Condor OF$\sp\circler$ dosage rates increased. Results of a persistence study with Dipel ES$\sp\circler$ indicated that mortality decreased by ca. 50% within 24 HAT and mortality was negligible by 48 HAT. Field trials evaluating Dipel ES$\sp\circler$ and Condor OF$\sp\circler$ indicated that rates $\ge$1.17 liters (form)/ha significantly reduced BW and TBW damaged squares below that found in the untreated control. TBW larvae exposed to a single application of Dipel ES$\sp\circler$ at 0.29 or 1.17 liters (form)/ha required significantly more days to reach the pupal stage (ca. 2 and 4 days respectively) and had a longer developmental cycle (larva to adult) ($>$2 and 5 days respectively) than unexposed larvae. There was no significant effect of Dipel ES$\sp\circler$ on pupal weight or days to adult eclosion. TBW larvae exposed to Dipel ES$\sp\circler$ at 1.17 liters (form)/ha had significantly lower larval weights at 6 and 10 days after treatment and significantly lower adult survival than unexposed TBW. Results of field and laboratory trials with selected commercial Bt products indicated that all Bt insecticides provided control of TBW that was significantly better than untreated plots. Experiments with selected cotton cultivars and application volumes of Dipel ES$\sp\circler$ demonstrated that application volume did not have a significant effect on BW and TBW control. A Louisiana frego bract cotton line, LA 850082, was confirmed to exhibit some levels of resistance to BW and TBW and boll weevil, Anthonomus grandis grandis Boheman. Results of studies with transgenic Bt cotton lines showed that these constructs provided control of TBW comparable to that observed with recommended chemical control strategies.