Taste receptors for pyrrolizidine alkaloids in a monophagous caterpillar.

Caterpillars of Utetheisa ornatrix are monophagous on species of Crotalaria which they obtain pyrrolizidine alkaloids (PAs) for defense and which the males convert to a pheromone. We show that a taste receptor neuron in each of the lateral and medial galeal styloconic sensilla responds to PAs of three different types. Monocrotaline, commonly present in Crotalaria species, is the most strongly stimulating with thresholds of response below 10(-11) and 10(-9) M in the two sensilla. These are among the lowest known taste thresholds in any insect and are similar to the thresholds for PAs in a polyphagous arctiid caterpillar that also sequesters PAs and uses them as the source of male pheromone. The receptors also respond to heliotrine, a type of PA that is probably never encountered by the insects. Monocrotaline and senecionine N-oxide are shown in behavioral assays to be phagostimulants. The data show that there is no tight link between taste receptor sensitivity to specific PAs and hostplant selection in these caterpillars. Perhaps the adults are primarily responsible for hostplant selection.