The evolutionary ecology of parasitoids

Insect parasitoids - those insects that deposit their eggs on or in the eggs, larvae or adults of other insects and whose offspring use the resources of those hosts to fuel development - provide a rich area of study for theoretical and mathematical biology. They also provide a broad collection of examples of how the tools developed in the previous chapters can be used (and they are some of my personally favorite study species; the pictures shown in Figure 4.1 should help you see why).

There is also a rich body of experimental and theoretical work on parasitoids, some of which I will point you towards as we discuss different questions. The excellent books by Godfray (1994), Hassell (2000a), and Hochberg and Ives (2000) contain elaborations of some of the material that we consider. These are well worth owning. Hassell (2000b), which is available at JSTOR, should also be in everyone's library.

It is helpful to think about a dichotomous classification scheme for parasitoids using population, behavioral, and physiological criteria (Figure 4.2). First, parasitoids may have one generation (univoltine) or more than one generation (multivoltine) per calendar year. Second, females may lay one egg (solitary) or more than one egg (gregarious) in hosts. Third, females may be born with essentially all of their eggs (pro-ovigenic) or may mature eggs (synovigenic) throughout their lives (Flanders 1950, Heimpel and Rosenheim 1998, Jervis et al. 2001). Each dichotomous choice leads to a different kind of life history.

Figure 4.1. Some insect parasitoids and insects that have life histories that are similar to parasitoids. (a) Halticoptera rosae, parasitoid of the rose hip fly Rhagoletis basiola, (b) Aphytis lingannensis, parasitoid of scale insects, and (c) Leptopilinia heterotoma, parasitoid of Drosophila subobscura. (d, e) Tephritid (true) fruit flies have life styles that are parasitoid-like: adults are free living, but lay their eggs in healthy fruit. The larvae use the resources of the fruit for development, then drill a hole out of the fruit and burrow into the ground for pupation. Here I show a female rose hip fly R. basiola (d) ovipositing, and two males of the walnut husk fly R. compleata (e) fighting for an oviposition site (the successful male will then try to mate with females when they come to use the oviposition site). The black trail under the skin of the walnut is the result of a larva crawling about and creating damage between the husk and the shell as it uses the resource of the fruit.

Figure 4.1. Some insect parasitoids and insects that have life histories that are similar to parasitoids. (a) Halticoptera rosae, parasitoid of the rose hip fly Rhagoletis basiola, (b) Aphytis lingannensis, parasitoid of scale insects, and (c) Leptopilinia heterotoma, parasitoid of Drosophila subobscura. (d, e) Tephritid (true) fruit flies have life styles that are parasitoid-like: adults are free living, but lay their eggs in healthy fruit. The larvae use the resources of the fruit for development, then drill a hole out of the fruit and burrow into the ground for pupation. Here I show a female rose hip fly R. basiola (d) ovipositing, and two males of the walnut husk fly R. compleata (e) fighting for an oviposition site (the successful male will then try to mate with females when they come to use the oviposition site). The black trail under the skin of the walnut is the result of a larva crawling about and creating damage between the husk and the shell as it uses the resource of the fruit.

(a) enerations per year ultivoltine nivoltine

(b) Eggs per host

Solitary regarious

Figure 4.2. A method of classifying parasitoid life histories according to population, behavioral and physiological criteria.

(c) Egg production after emergence (d) Combining the characteristics ultivoltine nivoltine

Pro-ovigenic Synovigenic

Solitary Gregarious ultivoltine nivoltine

Pro-ovigenic Synovigenic

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