The effects of the alkaloid scopolamine on the performance and behavior of two caterpillar species

Plants have evolved many defenses against insect herbivores, including numerous chemicals that can reduce herbivore growth, performance, and fitness. One group of chemicals, the tropane alkaloids, is commonly found in the nightshade family (Solanaceae) and has been thought to reduce performance and fitness in insects. We examined the effects of the tropane alkaloid scopolamine, an alkaloid constituent of Datura wrightii, which is the most frequent host plant for the abundant and widespread insect herbivore Manduca sexta in the southwestern United States. We exposed caterpillars of two different species to scopolamine: M. sexta, which has a shared evolutionary history with Datura and other solanaceous plants, and Galleria mellonella, which does not. We showed that the addition of ecologically realistic levels of scopolamine to both the diet and the hemolymph of these two caterpillar species (M. sexta and G. mellonella) had no effect on the growth of either species. We also showed that M. sexta has no behavioral preference for or against scopolamine incorporated into an artificial diet. These results are contrary to other work showing marked differences in performance for other insect species when exposed to scopolamine, and provide evidence that scopolamine might not provide the broad-spectrum herbivore resistance typically attributed to it. It also helps to clarify the coevolutionary relationship between M. sexta and one of its main host plants, as well as the physiological mechanism of resistance against scopolamine.