The discrepancy between food plant preferance and suitability in the moth Dysauxes ancilla

Abstract. Growth responses to and preference for different food plants were studied in larvae of the geographically isolated Swedish population of the moth Dysauxes ancilla. Laboratory rearing of D. ancilla larvae showed that, besides a mixed diet, four species from different plant families supported development to the adult moth. There was a significant suitability order among these species according to higher female adult weight and shorter development time; mixed diet and Calluna vulgaris > Hieracium pilosella > Thymus serpyllum > Brachytecium sp. However, these species were not top ranked in preference trials by the larvae. Instead larvae preferred Rumex acetosella, a plant that did not support development to adult moth as a single food source. This discrepancy between larval performance and preference may be explained by advantages from food mixing by the polyphagous larvae; an improved nutrient balance, a possibility of diluting toxic secondary substances and of switching foods to fit changing physiological needs. In Nature other factors such as microclimatic conditions, predators and parasitoids probably also influence the foraging behaviour of D. ancilla larvae.