Mechanism of agreggation in an ant-tended treehopper

Background/Question/Methods

Publilia concava is a univoltine phloem-feeding insect found primarily on tall goldenrod (Solidago altissima) for populations in northwestern Massachusetts and upstate New York, USA. Several genera of ants (e.g. Tapinoma, Formica, etc.) form mutualistic associations with treehoppers in which ants consume honeydew and protect treehoppers from predation. Treehopper densities are aggregated at the scale of goldenrod stems, and previous studies have suggested that this aggregation is an adaptive response that increases feeding performance or benefit from ant-tending. At a proximate level, previous studies have shown that treehoppers preferentially oviposit on plants with ants, but the hypothesis that treehoppers aggregate in response to conspecific is untested.

Results/Conclusions

We show that the probability of treehopper oviposition increases with ant-presence or abundance, but decreases with increasing treehopper abundance. Thus our results are inconsistent with the hypothesis that aggregation results from conspecific interactions and suggest that aggregation is a by-product of selection for ant-tending. These results are discussed in the context of the adaptive significance of aggregation and the evolution of life-history strategies in ant-tended herbivores.