13.5. The evolutionary success of insect predation and parasitism
In Chapter 11 we saw how the development of angiosperms and their colonization by specific plant-eating insects explained a substantial diversification of phytophagous insects relative to their non-phytophagous sister taxa. Analogous diversification of Hymenoptera in relation to adoption of a parasitic lifestyle exists, because numerous small groups form a “chain” on the phylogenetic tree outside the primarily parasitic sister group, the suborder Apocrita. It is likely that Orussoidea (with only one family, Orussidae) is the sister group to Apocrita, and probably all are parasitic on wood-boring insect larvae. However, the next prospective sister group lying in the (paraphyletic) “Symphyta” is a small group of wood wasps. This sister group is non- parasitic (as are the remaining symphytans) and species-poor with respect to the speciose combined Apocrita plus Orussoidea. This phylogeny implies that, in this case, adoption of a parasitic lifestyle was associated with a major evolutionary radiation. An explanation may lie in the degree of host restriction: if each species of phytophagous insect were host to a more or less monophagous parasitoid, then we would expect to see a diversification (radiation) of insect parasitoids that corresponded to that of phytophagous insects. Two assumptions need examination in this context — the degree of host-specificity and the number of parasitoids harbored by each host.
The question of the degree of monophagy amongst parasites and parasitoids is not answered conclusively. For example, many parasitic hymenopterans are extremely small, and the basic taxonomy and host associations are yet to be fully worked out. However, there is no doubt that the parasitic hymenopterans are extremely speciose, and show a varying pattern of host-specificity from strict monophagy to oligophagy. Amongst parasitoids within the Diptera, the species-rich Tachinidae are relatively general feeders, specializing only in hosts belonging to families or even ordinal groups. Amongst the ectoparasites, lice are predominantly monoxenic, as are many fleas and flies. However, even if several species of ectoparasitic insects were borne by each host species, as the vertebrates are not numerous, ectoparasites contribute relatively little to biological diversification in comparison with the parasitoids of insect (and other diverse arthropod) hosts.
There is substantial evidence that many hosts sup- port multiple parasitoids (much of this evidence is acquired by the diligence of amateur entomologists). This phenomenon is well known to lepidopterists that endeavor to rear adult butterflies or moths from wild-caught larvae — the frequency and diversity of parasitization is very high. Suites of parasitoid and hyper- parasitoid species may attack the same species of host at different seasons, in different locations, and in different life-history stages. There are many records of more than 10 parasitoid species throughout the range of some widespread lepidopterans, and although this is true also for certain well-studied coleopterans, the situation is less clear for other orders of insects.
Finally, some evolutionary interactions between parasites and parasitoids and their hosts may be considered. Patchiness of potential host abundance throughout the host range seems to provide opportunity for increased specialization, perhaps leading to species formation within the guild of parasites/parasitoids. This can be seen as a form of niche differentiation, where the total range of a host provides a niche that is ecologically partitioned. Hosts may escape from parasitization within refuges within the range, or by modification of the life cycle, with the introduction of a phase that the parasitoid cannot track. Host diapause may be a mechanism for evading a parasite that is restricted to continuous generations, with an extreme example of escape perhaps seen in the periodic cicada. These species of Magicicada grow concealed for many years as nymphs beneath the ground, with the very visible adults appearing only every 13 or 17 years. This cycle of a prime number of years may allow avoidance of predators or parasitoids that are able only to adapt to a predictable cyclical life history. Life-cycle shifts as attempts to evade predators may be important in species formation.
Strategies of prey/hosts and predators/parasitoids have been envisaged as evolutionary arms races, with a stepwise sequence of prey/host escape by evolution of successful defenses, followed by radiation before the predator/parasitoid “catches-up”, in a form of prey/ host tracking. An alternative evolutionary model envisages both prey/host and predator/parasitoid evolving defenses and circumventing them in virtual synchrony, in an evolutionarily stable strategy termed the “Red Queen” hypothesis (after the description in Alice in Wonderland of Alice and the Red Queen running faster and faster to stand still). Tests of each can be devised and models for either can be justified, and it is unlikely that conclusive evidence will be found in the short term. What is clear is that parasitoids and predators do exert great selective pressure on their hosts or prey, and remarkable defenses have arisen, as we shall see in the next chapter.