Mechanisms of behavioral alterations of parasitoids reared in artificial systems

A high quality of mass reared parasitoids is required for successful biological control of pest insects. Although the phenomenon of behavioral deterioration of parasitoids due to rearing in artificial conditions is well known, its significance is often underestimated, and the underlying mechanisms are poorly investigated. We quantified behavioral alterations of parasitoids reared in an artificial system vs. a natural system and elucidated some of the mechanisms involved. The model systems consisted of apple fruits (natural system) or an artificial diet devoid of apple (artificial system), the herbivore Cydia pomonella, and its larval parasitoid Hyssopus pallidus, a candidate biological control agent. Two parasitoid strains, one reared for 30 generations in the natural system and one in the artificial system, were compared by using the females' ability to respond to frass from codling moth caterpillars fed on apple fruits (apple-frass). The searching response of parasitoids reared in the artificial system compared to those reared in the natural system was reduced by an average of 53.2%. Gas chromatography-mass spectrometry (GC-MS) analyses of the two types of caterpillars' food and of the two corresponding types of frass showed that 15 compounds were present only in apple fruits and apple-frass, three compounds only in artificial diet and artificial-diet-frass, while four compounds were present in both frass types but not in the food sources. This suggests the presence of a food-derived and a host-derived component in the frass. Results from both bioassays and chemical analyses indicate that the kairomonal activity of the frass is due to both apple fruit and host components. The reduced response of parasitoids reared in artificial conditions might, therefore, be due to a lack of recognition of the apple fruit component. In a further experiment, the two parasitoid strains were reared in the opposite system for one generation. While the response to the host frass was significantly reduced in parasitoids that emerged from the artificial system, it was fully restored in parasitoids that emerged from the natural system. This indicates that the behavioral alteration was related to a learning process during ontogenesis rather than to a selection exhibited over generations.