ALTHOUGH insects are known to defend nests, breeding sites and females 1, the defence of feeding sites is less well documented. Other than the defence of egg clutches and nymphs by female treehoppers2 and the existence of a soldier caste in wooly aphids3, territoriality has not been reported in the large insect order Homoptera which includes aphids, scale insects, hoppers, cicadas and whiteflies. As the evolution of territoriality is thought to be directly correlated with competition for resources in short supply4, territorial behaviour should only be exhibited when population densities approach the carrying capacity of the environment. Because parthenogenetic reproduction and the high population growth rates of aphids seem contradictory to the notion of limited resources, aphid territorial behaviour is not expected. I report here on the settling behaviour of the aphid, Pemphigus betae Doane, which forms galls of the leaf blade of narrowleaf cottonwood, Populus angustifolia. We have quantified the existence of a defended micro-territory, the production of a floater population of individuals displaced through competitive interactions, and the differential mortality of residents and floaters which favours the evolution of territorial behaviour.
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