Are ant-aphid associations a tritrophic interaction? Oleander aphids and Argentine ants.

Oleander aphids, (Aphis nerii), which are sporadically tended by ants, were used as a moded system to examine whether host plant factors associated with feeding site influenced the formation of ant-aphid associations. Seasonal patterns of host plant utilization and association with attendant ants were examined through bi-weekly censuses of the aphid population feeding on thirty ornamental oleander plands (Nerium oleander) in northern California in 1985 and 1986. Colonies occurred on both developing and senescing plant terminals, including leaf tips, floral structures, and pods. Aphids preferentially colonized leaf terminals early in the season, but showed no preference for feeding site during later periods. Argentine ants (Iridomyrmex humilis) occasionally tended aphid colonies. Colonies on floral tips were three to four times more likely to attract ants than colonies on leaf tips, even though the latter frequently contained more aphids. Ants showed a positive recruitment response to colonies on floral tips, with a significant correlation between colony size and number of ants. There was no recruitment response to colonies on leaf tips. These patterns were reproducible over two years despite large fluctuations in both aphid population density and ant activity. In a laboratory bioassay of aphid palatability, the generalist predator,Hippodamia convergens, took significantly more aphids reared on floral tips compared to those reared on leaf tips. The patterns reported here support the hypothesis that tritrophic factors may be important in modifying higher level arthropod mutualisms.