15N -enrichment of plant tissue to mark phytophagous insects, associated parasitoids, and flower-visiting entomophaga

New techniques are presented on the use of 15 N to mark insects. 15 N, a stable isotope of nitrogen, was enriched above natural abundance in plant and insect tissues. Two laboratory studies demonstrated that enriched 15 N-concentrations could be tracked from plant to insect using mass spectrometry. In the first study, adult Cotesia plutellae (Kurdjimov) (Hymenoptera: Braconidae) and Hippodamia convergens Guerin-Meneville (Coleoptera: Coccinellidae) were allowed to feed at the flowers of rapid-cycling Chinese cabbage plants that had been fertilized with 15 N-enriched potassium nitrate (KNO 3 - 15 NO 3 ). Both insect groups were found to have significantly elevated 15 N levels after visiting the flowers of the 15 N-enriched plants for 48 hours. In the second study, 15 N-enriched bean plant (Phaseolus vulgaris L.) tissue was incorporated into an insect diet and fed to navel orangeworms, Amyelois transitella (Walker) (Lepidoptera: Pyralidae). When the navel orangeworm larvae were 4th instars, they were removed from the diet and exposed to the parasitoid, Goniozus legneri Gordh (Hymenoptera: Bethylidae). Results indicated that the enriched 15 N-concentration of the bean plants was transferred to the navel orangeworms and, subsequently, to the parasitoids. This work may provide useful techniques to help establish whether agriculturally important entomophaga visiting 15 N-enriched flowers or parasitizing enriched sentinel larvae in the field can be effectively marked with 15 N.