Winter weight: Caterpillars change their diet choices to fatten for winter

For many insects, cold temperatures and food scarcity during winter pose a serious threat. Diapausing insects mitigate this threat by becoming dormant during the winter, thus synchronizing their growth or reproduction with food availability. Insects often diapause for months without feeding, so incur a substantial nutrient cost. To cover the nutrient cost of diapause, insects store fat and protein prior to diapause. Field and laboratory studies generally find that diapause-destined insects carry greater nutrient stores than their non-diapausing counterparts. However, insects may struggle to store nutrients because food availability dwindles as winter approaches. If an insect cannot store enough nutrients to cover the cost of diapause, will it avert diapause? Will an insect planning to diapause choose foods that allow it to store nutrients more quickly? We are examining these questions in the corn earworm, Helicoverpa zea, using the geometric framework of nutrition to vary food protein and carbohydrate content. We find that diapause-destined caterpillars choose to eat less protein, thus becoming fatter than non-diapausing caterpillars. Caterpillars will avert their diapause program if forced to eat a high protein, low carbohydrate diet. Caterpillars prevented from storing fat with the drug 5-tetradecyloxy-2-furanocarboxylic acid do not avert diapause, suggesting that nutrient intake, not nutrient stores, regulates a caterpillar’s decision of whether to avert diapause. Our results can be practically useful in IPM programs for pests on novel hosts. Protein and carbohydrate content of novel foods may predict whether new pests will diapause and pose a threat the following spring.