Ant-mediated dispersal of the black willow aphid Pterocomma salicis L.; does the ant Lasius niger L. judge aphid-host quality?

Abstract

Many species of aphid have intimate associations with species of ant. These relationships, while variable in the level of their inter-dependence and conditionality, are often considered mutualistic, and the benefits to both parties are widely appreciated (Jones, 1929; Way, 1963; Breton & Addicott, 1992a; Dixon, 1998; Stadler & Dixon, 1999; Yao et al., 2000). The benefits to ants stem largely from the ready supply of carbohydrate available in aphid honeydew and, in certain cases, from the protein of the aphids themselves (Sakata, 1995), while aphids may benefit in terms of protection from natural enemies, improved colony hygiene, increased feeding rate, transport and dispersal (Way, 1963; Buckley. 1987; and references therein).
Given the importance of the ant partner to the success of tended aphid colonies, the presence, or potential for recruitment, of ants is likely to influence host selection by these aphids, as in myrmycophilous Iycaenid butterflies (Pierce & Elgar, 1985) and in the membracid homopteran Publilia concava (M. Morales, pers. comm.). A more direct contribution of the ant partner, made by dispersing the tended aphids themselves, has been observed in subterranean aphid specie,. where ant-aided dispersal may be obligate (Way, 1963), and in species in which eggs are sheltered in ant nests over winter (Pontin. 1960); however the suggestion that the black garden ant Lasius niger L. and other ant species may transport aphids above ground between host plants (Nixon, 1951) has not found support in later studies (EI-Ziady & Kennedy, 1956; Banks, 1958: Pontin, 1960). Although Lasius fuliginosus L. transports young fundatrices of Stomaphis quercus L. from their overwintering egg site on the base of the tree to suitable feeding sites in the upper canopy of oak trees (Goidanich, 1959), this subject has received little attention in recent years.
The investigation reported here resulted from the chance observation of a L. niger, carefully carrying a third-instar black willow aphid Pterocomma salicis L. in its jaws away from an established aphid colony. The ant deposited the aphid on another host plant, previously uncolonised by the aphid, and remained with it for some time until settling behaviour was observed. Pterocomma salicis does not have an obligate dependency on attendant ants and the first experiment was designed to assess rapidly whether attendance by L. niger influences P. salicis in terms of population growth and adult mass. The second experiment was designed to investigate whether ant attendance leads to increased dispersal of aphids from an established colony and whether there is an interaction between the presence of ants and the choice of new host tree.