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

<div><p>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 <i>Bacillus thuringiensis</i> (<i>Bt</i>) toxins in the field, jeopardizing its future use. This has most commonly occurred through the alteration of insect midgut binding sites specific for <i>Bt</i> toxins. While fitness costs related to <i>Bt</i> resistance alleles have often been recorded, the mechanisms behind them have remained obscure. We asked whether evolved resistance to <i>Bt</i> 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 <i>Bt</i>. We fed the cabbage looper <i>Trichoplusia ni</i> artificial diets differing in levels of dietary imbalance in two major macronutrients, protein and digestible carbohydrate. By comparing a <i>Bt</i>-resistant <i>T. ni</i> strain with a susceptible strain we found that the mechanism behind reduced pupal weights and growth rates associated with <i>Bt</i>-resistance in <i>T. ni</i> was reduced consumption rather than impaired conversion of ingested nutrients to growth. In fact, <i>Bt</i>-resistant <i>T. ni</i> showed more efficient conversion of nutrients than the susceptible strain under certain dietary conditions. Although increasing levels of dietary protein prior to <i>Bt</i> challenge had a positive effect on larval survival, the LC₅₀ of the resistant strain decreased when fed high levels of excess protein, whereas the LC₅₀ 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.</p>