Dietary self-selection and rules of compromise by 5th instar Vanessa cardui L. (Lepidoptera: Nymphalidae)

Using the geometric framework, I examined dietary self-selection and rules of compromise for the intake of protein (P) and digestible carbohydrate (C) by 5th instar Vanessa cardui L. (Lepidoptera: Nymphalidae). I presented fat-free diet pairs in a choice trial to determine the ‘intake target’ of the larvae and individual fat-free diets in two no-choice trials to determine the rules of compromise they exhibit when constrained to a sub-optimal dietary source. In choice trials caterpillars regulated nutrient intake to near a 1P:1C ratio. Larvae from different choice treatments did not differ in pupal mass or in the duration of the final stadium. In no-choice trials, larvae consumed less total diet on increasingly protein-biased diets and more on increasingly carbohydrate-biased diets, relative to a 1P:1C nutrient ratio. Differences in carbohydrate consumption were much greater between dietary nutrient ratios than differences in protein consumption. When accounting for initial larval mass and total consumption, dietary nutrient ratio affected pupal mass. Pupal mass decreased as nutrient ratio was shifted off of 1:P1C, but decreased to a greater extent when this was shifted toward carbohydrate than when shifted toward protein. Protein consumption significantly affected pupal mass while carbohydrate consumption did not. Dietary nutrient ratio significantly affected stadium duration, but this shift was more pronounced when nutrient ratio was shifted toward carbohydrate than toward protein. Regulation to near 1P:1C is consistent with results found for other Lepidoptera, and the rule of compromise exhibited by V. cardui is consistent with that expected for a generalist herbivore.