Temporal modulation of pyrrolizidine alkaloid intake and genetic variation in performance of Utetheisa ornatrix caterpillars

Foraging on a variety of different food items allows individuals to balance nutrient intake, but it also may dilute toxins present in single items of the diet. The distinction between nutrient balancing and toxin dilution becomes less clear, however, for organisms that sequester potentially toxic compounds for their own benefit. Additionally, specific dietary components chosen may or may not affect metabolic rate, growth, and other measures of fitness. Using the arctiid moth Utetheisa ornatrix, we (1) evaluated the temporal pattern of larval feeding behavior when presented with diet enriched with the pyrrolizidine alkaloid (PA), monocrotaline, (2) estimated the cost of ingesting PAs by measuring larval nutritional performance on diets containing different amounts of usaramine, and (3) investigated the magnitude of genetic effects on the performance of families raised on diets of different usaramine concentrations. We demonstrate that an herbivorous insect can temporally vary its intake of sequestered allelochemical. Individuals combined a preference for intermediate amounts of monocrotaline with temporal modulation of the specific amount of PA ingested on a given day. Using usaramine, we found little evidence for a cost of ingesting PAs, except among some family groups. Measures of digestive and metabolic efficiency showed no consistent reduction in performance on higher alkaloid diets. Only when larvae in family groups were fed a single concentration of PA over more than one instar was there any evidence for a cost of ingestion. Additionally, there were large genetic components to both growth and an individual's growth response to dietary alkaloid. These results suggest that while genetic variation in performance on alkaloid-rich diets exists, modulation of intake rate by caterpillars may be an important mechanism to reduce effects on the growth and fitness of individuals.