Fifth instar fall armyworms, Spodoptera frugiperda (J. E. Smith), and third instar beet armyworms, Spodoptera exigua (Hübner), were caged on conventional (cv. DP 5415) and transgenic Bacillus thuringiensis (Bt) cotton bolls (cv. NuCOTN 33B) of various ages to define the period of boll susceptibility to larval injury. Larval mortality, incidence of feeding, and boll penetration were examined for both cultivars. There was no significant linear relationship between incidence of feeding and boll age for either species caged on DP 5415 and NuCOTN 33B. However, a significant linear relationship between larval mortality and boll age was observed for both species caged on NuCOTN 33B (mortality increased as bolls matured), but no such relationship was found on DP 5415. A significant linear relationship between boll penetration and boll age was observed for both species caged on NuCOTN 33B (boll penetration decreased as bolls matured). Similarly, a significant linear relationship between boll penetration and boll age was observed for beet armyworms caged on DP 5415 (boll penetration decreased as bolls matured), while no relationship was found for fall armyworms caged on DP 5415. Fall armyworms penetrated ≥ 60% of DP 5415 bolls regardless of their age, while these bolls were tolerant (≤ 10% boll penetration) to beet armyworms at 390 heat units. NuCOTN 33B bolls were tolerant to fall armyworm damage at 864 heat units, while these bolls were tolerant to beet armyworm damage at 361 heat units. These data suggest that fall armyworms and beet armyworms are able to successfully penetrate bolls of 350 heat units at unacceptable (≥ 10%) levels.