The influences of host plant and genetic resistance to Bacillus thuringiensis on trade-offs between offspring number and growth rate in cabbage loopers, Trichoplusia ni

1. Variation in progeny size and quality is common among insects and this variation can strongly influence individual fitness. Larger progeny typically survive better and develop faster under adverse conditions and may have higher fecundity. Due to resource limitations, however, trade-offs may arise between having fewer large offspring or more smaller ones. 2. For cabbage loopers, Trichoplusia ni, pepper leaves are a poorer larval host than cucumber or tomato leaves as indicated by survival, development rate, and body size. Moths reared on cucumber produced more slower growing offspring than those that had been reared on pepper, which produced fewer, faster growing progeny. 3. Traits conferring resistance to Bacillus thuringiensis (Bt) generally are associated with strong deleterious effects that may influence resource allocation and reproductive trade-offs between progeny size and number. 4. Unlike the host-plant related trade-off between progeny size and fecundity observed among susceptible control moths, Bt-resistant parents had both the lowest fecundity and smallest progeny size on all host plants. This finding suggests that the progeny size–number relationship is constrained in resistant individuals.