There is increasing evidence that the diversity and pattern of host-associations of phytophagous insects cannot be explained solely by the interactions between insects and their host plants, and that the tritrophic context of plant-insect-enemy interactions must be taken into account. One strategy for examining the role of tritrophic interactions in generating the diversity and determining the specificity of plant-insect interactions is to focus ecological studies on adaptive radiations of insect lineages across host plants. In such cases of rapid, selection driven diversification, the ecological forces shaping diversification should be most evident.One such apparent adaptive radiation can be seen in the gall midge Asteromyia carbonifera which feeds on a wide variety of N. American species of goldenrods (Solidago spp).

Results/Conclusions

We show using mtDNA and AFLP data that this nominal species consists of a large number of genetically distinct, yet morphologically cryptic, lineages differing in host plant use.  We then explore the relative likelihood of non-ecological, bitrophic, and tri-trophic hypotheses to explain the apparent rapid diversification, high degree of specialization, and ecological coexistence of these gall midge lineages, focusing on the divergent gall forms present on one goldenrod host (S. altissima). Broad overlap in distribution of many host-plant species as well as the existence of genetically and morphologically distinct gall morphs on a single host-plant species suggests that purely non-adaptive, neutral divergence is unlikely. Field surveys of gall phenology also indicate a lack of allochronic isolation.  Significant variation parasitism rates and parasitoid composition of galls both among host-plant species and among gall morphs suggest a potentially important role of enemy x plant interactions in driving ecological and reproductive isolation among gall midge populations.  In addition, associations of the gall midges with a fungus may provide an additional axis of niche differentiation and may facilitate host-plant shifts.