Thermal reaction norms for caterpillar growth depend on diet

Hypothesis: Interactions between diet and temperature affect reaction norms for growth rate in ectotherms. Organisms: Full-sib families of the small cabbage white butterfly, Pieris rapae, derived from field populations near Seattle, WA, USA. Methods: We used a split sib-family experimental design with two food treatments (collard leaves and artificial diet) and four test temperatures (11, 23, 35 and 40 C), and measured short-term growth rate (mass increase) of fourth-instar caterpillars. The data were analysed using mixed-model analysis of variance. Results: Temperature, food type, family, and twoand three-way interactions all had significant effects on growth rate. The thermal sensitivity of growth rate was greater on collards than on the artificial diet; mean growth rate was greater on collards than on the artificial diet for temperatures from 11 to 35 C, but this effect was reversed at 40 C. Estimated broad-sense genetic variances were greater on collards than on the artificial diet; the genetic covariance of growth rate at 35 and 40 C was strongly positive on the artificial diet, but weak or negative on collard leaves. Conclusions: Both the mean and genetic variation in thermal reaction norms for insect growth rate were influenced by food type in this system. Studies of the thermal sensitivity of growth and feeding that utilize artificial diets may not accurately reflect genetic variation or constraints on thermal reaction norms that may occur on natural food resources.