Integrating theory of clutch size and body size evolution for parasitoids

Abstract

The way that parents allocate resources amongst offspring has been a subject of interest since the beginnings of modern life history theory (Roff 1992, Stearns 1992). Many animals, such as altricial birds and many insects, deposit eggs together in discrete groups, called clutches, where the offspring develop together for some time. Variation in clutch size is typically then studied as a route to understanding the selective pressures and constraints on the resource allocation strategy of the organism. Here we propose a neglected but complementary route to understanding resource allocation in insect parasitoids involving offspring body size as a focal parameter. Our message is that consideration of both body size and clutch size variation simultaneously can enhance understanding or resource allocation strategies in this group of organisms, illustrating the value of an integrated life history approach. Parasitoids are insects (the vast majority wasps) which lay their eggs on or in the bodies of other arthropods. The young parasitoids feed on the still-living body of their host, eventually killing it. Parasitoids may lay from one to several hundred eggs on or in a host, variability which begs explanation. In the mid1980s, theory of clutch size evolution, which developed under an ornithological tradition, began to be applied to parasitoids and other insects, and many models were developed based around the trade-off between clutch size and offspring fitness (reviewed in Godfray 1994). In testing those models, it was realized that one of the major fitness related variables which trades off with clutch size is the body size of resulting offspring: because host insects represent a resource of limited size, larger clutches or broods tend to result in smaller-bodied offspring, especially in idiobiont species whose hosts do not grow after parasitoid oviposition. The optimal clutch size then depends upon how adult body size is related to fitness, a relationship which itself has spawned a small industry of research. Parasitoids can either develop solitarily or gregariously. In solitary parasitoids only one offspring develops successfully from each host, and the offspring frequently display contest competition for resources. In gregarious species several offspring may complete development from a single host and competition more closely approaches a scramble situation. A multitude of studies (reviewed in Godfray 1994), have demonstrated that body size in solitary parasitoids tends to be positively correlated with host size; if the host is fully consumed (see Harvey et al. 2000 for an exception) and only one offspring develops per host, then larger hosts providing more resources for growth should lead to larger offspring. Less intuitively though, a number of studies have shown that offspring body size increases with host size within and across gregarious species (Opp and Luck 1986, le Masurier 1987, Hardy et al. 1992, Ode et al. 1996, Mayhew 1998, Mayhew and Hardy 1998). This trend is surprising because it has been widely found that gregarious species will lay larger clutches of eggs on larger hosts (Godfray 1994), a fact which intuitively leads us to expect that they are compensating for the extra resources provided by larger hosts. Apparently though, gregarious species tend to undercompensate when adding eggs to larger hosts such that larger hosts still provide more per capita resources for offspring (Mayhew 1998). We do not know of models which have explicitly tried to explain this trend. Parasitoid clutch size models rarely explicitly consider body size. Several more gen-