Electrophysiological recordings from taste sensilla of the caterpillar Estigmene acrea with the pyrrolizidine alkaloid (PA) seneciphylline N-oxide demonstrated that extensive feeding on plants rich in PAs caused a loss in response of the PA-sensitive cell in the lateral styloconic

chemicals in insects (Blaney et al., 1986; Schoonhoven et al., 1998), responses falling to zero after experience are not known. For example, olfactory sensitivity of mosquitoes to lactic acid is reduced following a blood meal (Davis, 1984), and gustatory sensitivity to inositol and other plant chemicals was shown to be reduced by about 20% in caterpillars of the tobacco hornworm Manduca sexta if they were reared on diet containing those chemicals (Schoonhoven, 1969). Glendinning and co-workers demonstrated a more marked reduction in gustatory responses of M. sexta to salicin and caffeine that translated into reduced levels of deterrence (Glendinning et al., 1999, 2001). Reduced sensory input to sugars or amino acids, which occurs when the individuals are satiated with carbohydrates or proteins, respectively, is sometimes more extensive (Simpson et al., 1991). Estigmene acrea (Lepidoptera; Arctiidae) has highly polyphagous larvae that sequester pyrrolizidine alkaloids (PAs) as precursors of pheromone (Rothschild et al., 1979; Krasnoff and Roelofs, 1989; Weller et al., 1999) and presumably for defense. The importance of PAs to E. acrea is suggested by the caterpillar’s great gustatory sensitivity to them (Bernays et al., 2002a) and by the dedication of a high proportion of taste cells to their detection (Bernays et al., 2002b). We have previously shown that E. acrea caterpillars are extremely sensitive to PAs, with gustatory receptors responding to concentrations as low as 10–12·mol·l–1. Here, we show that a dramatic reduction in sensitivity to PAs can occur following ingestion of large amounts as a result of (1) recent experience of feeding on plants rich in PAs, (2) diets containing large proportions of powdered plants containing PAs or (3) diets containing pure specific PAs. The biological significance of these findings is discussed.