The consequences of clustering by Aphis fabae foundresses on spring migrant production

Aphis fabae (Homoptera: Aphididae) foundresses were shown to have a cluster ed distribution on leaves of their primary host, Euonymus europaeus. Two he ld experiments were carried out to determine the casts or benefits of this clustering behaviour to aphid clones. The first experiment, in spring 1996, excluded predators from colonies formed by single fundatrices or clusters of three. No density-dependent effects on colony growth were observed early in the season, but the total production of migrants per foundress was sign ificantly lower for clustered foundresses. This demonstrated that there was no synergistic effect of group feeding on clonal performance. All colonies were initiated on randomly selected leaves and all grew successfully, ther efore clustering was not due to a shortage of feeding sites. The second exp eriment, in spring 1997, tested whether natural enemies cause selection for selfish herding by the foundress generation. Colonies were initiated by on e or three foundresses, but half were left unbagged. If natural enemy attac k is a selective force inducing clustering behaviour in foundresses, the co sts of clustering should have been reduced or negated in unbagged colonies. Colonies grown from clusters produced significantly fewer migrants per fou ndress, irrespective of bagging treatment. Other potential factors causing clustering behaviour are discussed.