Using Sex Ratios to Estimate What Limits Reproduction in Parasitoids

The reproductive success of a foraging parasitoid may be limited by the number of eggs that she produces and/or the number of hosts that she can locate. Despite the significance for population dynamics and numerous areas of behavioural ecology, the relative importance of these factors remains an issue of contention. Attempts to resolve this controversy have been hindered because estimating the importance of factors limiting reproduction in the field can be extremely laborious and time consuming. We show how sex ratio data can be used as a relatively easy method to indirectly estimate the relative importance of the factors limiting reproduction. Sex ratio data from 48 samples of eight species suggest that: (a) the extent of host or egg limitation in a species varies between site collected and time of year; and (b) the majority of populations, and overall species means, are at an intermediate position on the egg/host limitation continuum, with a bias towards host limitation. Some of the most qualitatively and quantitatively successful research in evolutionary ecology has come from studies of parasitoid wasps (Godfray 1994). Female wasps search the environment for hosts. Upon finding a host a female wasp must make a series of behavioural decisions such as whether to feed or oviposit upon the host, how many eggs to lay (clutch size) and what sex should the offspring be (sex allocation). The optimum strategy for each of these behaviours depends upon the degree to which a female's reproductive success is limited by the number of hosts she can locate and oviposit on (termed time or host limitation) or the number of eggs she is carrying (termed egg limitation) (reviewed by Godfray 1994). However, despite the enormous amount of research on parasitoid oviposition behaviour, the relative importance of the factors limiting reproduction in natural populations remains unresolved and is a subject of recent controversy Several theoretical models have been constructed in order to predict which type of limitation is most likely. These models are based on the assumption that limited resources must be divided between egg production and maintenance , resulting in a trade-off between reproduction and survival. The models predict how egg load should evolve in response to variation in the ecological conditions (host encounter rate). This result is then used to predict the proportion of individuals that exhaust their egg supply before dying, and are therefore egg-limited. Rosenheim (1996, 1999) used this type of model …