The evolution of developmental timing in natural enemy systems

Natural parasitoid systems exhibit considerable variation in their life history properties yet little is known about the effects of development time on parasitoid fitness or of the conditions that might select for rapid development at the expense of reduced parasitoid growth. In this study the techniques of adaptive dynamics are applied to a discrete time host–parasitoid model to examine the evolution of parasitoid life history strategies. In particular, we explore the conditions that select for variation in parasitoid traits, such as, the timing of parasitoid attack and emergence from the host. The process of evolutionary branching, leading to dimorphism, can occur when the benefits to reproduction of early parasitoid attack are bought at a cost in terms of mortality of late parasitoid emergence from the host. We also find that trends in parasitoid life history traits depend critically on the nature of the underlying population dynamics. Increases in the strength of host density-dependence acts to select for shorter parasitoid development time and lower searching efficiency when the underlying population dynamics are at equilibrium. This trend is reversed when the underlying population dynamics exhibit fluctuations. Here, fluctuations in host density driven by parasitism become more extreme as the strength of host density-dependence decreases and so the parasitoid selects early emergence to avoid the mortality experienced at outbreak host densities. Our results are consistent with the general principle that parasitoids facing high mortality risk favour short development times over size and high searching efficiency, whereas species facing low mortality risks favour size at the cost of increased development time.