Gustatory perception of phytoecdysteroids in Plodia interpunctella larvae

Phytoecdysteroids are steroidal compounds produced by various plants that disrupt growth and development of insects eating them. They exhibit an insecticidal activity on a number of insect pests such as Plodia interpunctella (Hubner) (Lepidoptera: Pyralidae). In this study, we further evaluated whether phytoecdysteroids deter larvae of this species from feeding, by using four phytoecdysteroid molecules, commonly occurring in plants: 20-hydroxyecdysone (20E), ponasterone A (PonA), polypodine B (PolB), and makisterone A (MakA). Fourth instar P. interpunctella avoided contact with food pellets treated with these phytoecdysteroids in a dose-dependent way (2-30 mM). In order to test whether this avoidance was mediated by taste sensitivity, we recorded the responses of taste neurons located in the lateral and medial sensilla styloconica of the galea. At least one neuron responded to each of these compounds in both sensilla. The neuron located in the medial sensillum had a detection threshold of 10 -6 M for PonA, 10 -4 M for 20E and PolB, and 10 -3 M for MakA. The lateral sensillum neuron responded with less intensity and its detection threshold was 10 times higher than that of the medial sensillum neuron. These results indicate that phytoecdysteroids are detected as deterrent stimuli by P. interpunctella larvae and that small structural differences significantly affect their biological activity.