HOST BEHAVIOR-MODIFICATION BY THE ENDOPARASITOID APHIDIUS-NIGRIPES - A STRATEGY TO REDUCE HYPERPARASITISM

1. One possible component of successful parasitism by insect parasitoids is the reduction of predation and hyperparasitism through the modification of host behaviour. 2. Just prior to death, potato aphids, Macrosiphum euphorbiae (Thomas), containing diapausing larvae of the parasitoid Aphidius nigripes Ashmead have been shown to leave the host plant to mummify in concealed sites, while those parasitized by non-diapausing individuals generally leave the aphid colony to mummify on the upper leaf surfaces. 3. The present study examines the within plant distribution of aphids and mummies containing non-diapausing A.nigripes, and compares parasitoid survival among microhabitats under field conditions. 4. On potato, Solanum tuberosum L., a significant proportion of aphids were found on lower leaf surfaces, whereas mummies were usually on the upper surfaces, particularly in the apical stratum of the plant canopy. 5. In both field surveys and experimental manipulations, parasitoid survival was higher on the upper surface of apical leaves than in other microhabitats, due to lower levels of hyperparasitism and, to a lesser extent, lower predation. This suggests that the pressure exerted by natural enemies has influenced the evolution of host behaviour modification.