Journal of Evolutionary Biology: An article asks whether autumnal leaf colors could act as a deterrent to insect herbivores [abstract free; subscription needed for more].
It may seem odd to think that trees could be interested in defending leaves that are about to drop off anyway; but the authors' idea is that trees with brighter red leaves are signaling a "commitment" to producing more defensive chemicals in next year's leaf crop. To test this hypothesis, the authors measured aphids' preference for leaf color in the fall, and whether fall leaf color predicted aphids' performance on the same trees in the spring.
The aphids showed a significant preference for green autumn leaves over red, but there was no correlation between fall color and aphid performance on the next spring's leaves. So, interesting idea, but no dice. The authors say, reasonably, that their results suggest aphids' color preferences have more to do with finding the most nutritious leaves in the fall than avoiding defensive chemicals in the spring.
It's important to note that this result is not necessarily coevolution, in the strict sense of reciprocal natural selection between the aphids and the trees. The aphids seem to have adapted to their host plant, but it's not clear (base on this study, anyway) that the aphids exert significant selection on the plant in return.
Ramirez, C. C., B. Lavandero, and M. Archetti. 2008. Coevolution and the adaptive value of autumn tree colours: colour preference and growth rates of a southern beech aphid. Journal of Evolutionary Biology 21:49-56.