Tangled web of the insect, plant and parasite arms race

Tommi Nyman of the University of Joensuu in Finland together with colleagues from Sweden and Germany uncovered a food web involving willow trees, sawflies, and the parasites feeding on sawfly larvae. Sawflies (a group of insects related to bees and wasps) lay their larvae in willows creating galls, which are protruding growths of plant tissue. Nyman's group used data from galls collected from willows over an 18-year period. They classified 43 sawfly species using genetic analysis and assessed the degree of parasitism to which these insects had been subjected by 72 different parasites.

Their statistical analysis showed that parasitism promotes the insects' ecological divergence in a co-evolutionary arms race. The sawflies develop new gall-types and, in doing so, can escape the attentions of the parasites without having to switch host plant. Some parasites, however, have in turn adapted to the divergence in gall-types, driving further speciation. These processes partly explain the unusual diversity of herbivorous insects and their parasitoids.

This study is the first time that key co-evolutionary hypotheses such as the classic Escape-And-Radiate (EAR) hypothesis have been tested in a phylogenetic framework. EAR suggests that hyper diverse communities result from co-evolutionary arms races featuring cycles of enemy escape, radiation, and colonization by new enemy lineages. The results show that the EAR hypothesis is too simplistic to be applied to complex species-rich food webs.

“Our results indicate that niche-dependent parasitism is a major force promoting ecological divergence in herbivorous insects,” says Nyman. “Diversification seems to be spurred by a continuous stepwise process, in which ecological and phenotypic shifts in prey lineages are followed by a lagged evolutionary response by some of the associated enemies.”