Insects are the most species-rich group of life on earth, and plant-associated insects account for a majority of those. We’re interested in the processes that have brought about that diversity and the patterns that they have created. Aphids are tightly associated with their host plants, most found on only one or a few species of host. There are many plausible explanations for the great diversification of plant-associated insects. For example, they may have diversified while tied to their host. That is, through a process of co-speciation, when the host species split into two, the insect did as well. We’ve shown that, at least in one case, the diversification of aphids was caused not by sticking with the host, but by switching hosts. That is, a chance migration of one aphid from one host to another led to the genetic isolation of two populations and then, eventually, to their becoming distinct species. We showed this to be the case in Cinara species associated with the pinyon pines of the American Southwest: the aphid species on the same part of different host species (e.g., the branches or shoots) are more closely related to each other than they are to species found on different parts of the same host species (Favret & Voegtlin 2004). By switching hosts, all the while remaining specialized for feeding on particular parts of the hosts, means that Cinara has become more speciose than the conifers it feeds on.
More recently, we broadened our focus to the subfamily to which Cinara belongs, the Lachninae. This subfamily is unusual among aphids because its various members feed on the bark as well as the green parts of their hosts. They are also most diverse on conifers; most other aphid groups are diverse on angiosperms. We showed that the ancestor to all Lachninae fed on an angiosperm host, and more specifically, on a woody part of that host (Chen et al. 2016). It became most diverse when it switched to a conifer host, but it stuck to the woody parts of those hosts.
Another recent avenue of research focuses on a group closely related to aphids, the phylloxerids. In North America, they are particularly diverse on hickories. Like Cinara, hickory-feeding phylloxerids are more diverse than hickories. This time, they have specialized to form different kinds of galls. We’re trying to understand the evolutionary relationships between the insects, their hosts, and the type of galls they inhabit.