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Beneficial alleles that spread rapidly as an adaptation to a new environment are often associated with costs that reduce the fitness of the population in the original environment. Several species of insect pests have evolved resistance to  Bacillus thuringiensis  ( Bt ) toxins in the field, jeopardizing its future use. This has most commonly occurred through the alteration of insect midgut binding sites specific for  Bt  toxins. While fitness costs related to  Bt  resistance alleles have often been recorded, the mechanisms behind them have remained obscure. We asked whether evolved resistance to  Bt  alters dietary nutrient intake, and if reduced efficiency of converting ingested nutrients to body growth are associated with fitness costs and variation in susceptibility to  Bt . We fed the cabbage looper  Trichoplusia ni  artificial diets differing in levels of dietary imbalance in two major macronutrients, protein and digestible carbohydrate. By comparing a  Bt -resistant  T. ni  strain with a susceptible strain we found that the mechanism behind reduced pupal weights and growth rates associated with  Bt -resistance in  T. ni  was reduced consumption rather than impaired conversion of ingested nutrients to growth. In fact,  Bt -resistant  T. ni  showed more efficient conversion of nutrients than the susceptible strain under certain dietary conditions. Although increasing levels of dietary protein prior to  Bt  challenge had a positive effect on larval survival, the LC 50  of the resistant strain decreased when fed high levels of excess protein, whereas the LC 50  of the susceptible strain continued to rise. Our study demonstrates that examining the nutritional basis of fitness costs may help elucidate the mechanisms underpinning them.

Dietary Mechanism behind the Costs Associated with Resistance to Bacillus thuringiensis in the Cabbage Looper, Trichoplusia ni