Coevolution Between Herbivorous Insects and Plants: Tempo and Orchestration

Abstract

For a herbivorous insect, a plant is more than just a meal — it is a way of life. For some species, just about every aspect of the life-cycle involves the plant host -eating, escape from predators, overwintering and mating, to name a few. Particularly for insects with narrow host ranges, life-cycles must coordinate or the insect misses out not only on a meal but on all those other things that make insect life worth while; thus, there must be tremendous selective pressure on insects to adapt to the peculiarities of their hosts. Walsh (1864, 1865) was among the earliest to recognize the fact that specialization by insect species on different host species is heritable and results from natural selection. A passionate early advocate of Darwinism, Walsh (1867) observed in his studies on Rhagoletis pomonella, the apple-maggot fly, that several populations of the native North American species fed not on the ancestral and typical hawthorn host (Crataegus) but rather on the introduced novel host, apple (Malus). He attributed the existence of intraspecific groups with different host usage patterns to evolution resulting from isolation due to “attachment” to different food plants. The apple-maggot fly is a species that does perform many vital functions — including mating, oviposition, and larval development — in or around its hosts. Walsh offered no explanation, however, for the motive forces behind host shifts.