SUSCEPTIBILITY OF CONVENTIONAL AND TRANSGENIC COTTON BOLLS EXPRESSINGTHE BACILLUS-THURINGIENSIS CRYIA(C) PARTIAL-DERIVATIVE-ENDOTOXIN TO FALL ARMYWORM (LEPIDOPTERA, NOCTUIDAE) AND BEET ARMYWORM (LEPIDOPTERA, NOCTUIDAE) INJURY

Fifth-instar fall armyworms, Spodoptera. frugiperda (J. E. Smith), and third-instar beet armyworms, Spodoptera exigua (Hubner), were caged o n conventional 'DP 5415' and transgenic Bacillus thuringiensis (Bt) 'N uCOTN 33(B)' cotton bells of various ages to define the period of bell susceptibility to larval injury. Larval mortality, incidence of feedi ng, and bell penetration were examined on both cultivars. There was no significant linear relationship between incidence of feeding and boil age for either species caged on DP 5415 and NuCOTN 33(B). However, a significant linear relationship between larval mortality and bell age was observed for both species caged on NuCOTN 33(B) (mortality increas ed as bells matured), but no such linear relationship was found on DP 5415. A significant linear relationship between bell penetration and b oil age was observed for both species caged on NuCOTN 33(B) (boll pene tration decreased as bells matured). Similarly, a significant linear r elationship between boil penetration and boil age was observed for bee t armyworms caged on DP 5415 (boll penetration decreased as bells matu red), whereas no relationship was found for fall army worms caged on D P 5415. Fall armyworms penetrated greater than or equal to 60% of DP 5 415 bells regardless of their age, but these bells were tolerant (less than or equal to 10% bell penetration) to beet armyworms at 390 heat units. The NuCOTN 33(B) bells were tolerant to fall armyworm damage at 864 heat units, but these bells were tolerant to beet armyworm damage at 361 heat units. These data suggest that fall armyworms and beet ar myworms are able to successfully penetrate balls of 350 heat units at unacceptable (greater than or equal to 10%) levels.