Abstract
Sexual dimorphisms in adult size (SSD) and development time (SDTD) occur in many groups of organisms. In insects, some of the best examples occur in parasitoid wasps where most studies report that females are larger than males but take longer to develop. Sex-specific differences in the effects of size on reproductive success is generally regarded as the main factor responsible for SSD in parasitoids. Most studies also assume that development time must be extended in order to achieve larger size. Here, SSD and SDTD were compared in the solitary endoparasitoid, Microplitis mediator that develops in larvae of the moth Pseudoplusia includens. The relationship between male and female body size and development time were isometric in M. mediator, but contradict most predictions of parasitoid development models. Across first to fourth instars at parasitism, male wasps were consistently larger than females but completed their development significantly faster. The longer development time in female wasps was due primarily to an extended pupal phase, whereas the duration of larval development did not vary significantly with offspring sex. Secondary sex ratios (percentage male) also did not vary with host instar. We conclude that SSD in M. mediator is not only a function of extended development time, but also because of several possible factors: (i) selection favours an increase in male size (relative to female size); (ii) female size is constrained because of predation risk, (iii) as a cost of reducing the cost of inbreeding or (iv) constraints on egg maturation rate. We argue that failure to look carefully at how parasitoids grow can lead to incorrect conclusions about the basis or significance of SSD.
Highlights
Intraspecific differences in resource allocation to male aevnodlufetimonalaeryoffbspiorlionggishtass l(oRnegnsbcehe,n a19s5u9b;jeCctaoldfeirn,ter1e9s8t4t)o
Parasitoid develop ment time was recorded in days and was divided into three categories: overall egg-to-adult development time, which was further broken down to compare the duration between oviposition and larval egression, and the duration between cocoon production and adult eclosion
There was a remarkable congruity in the final size of hosts that produced male and female offspring (Fig. 4). The results of this investigation reveal that host quality, in terms of survival of M. mediator, is largely independent of host instar parasitized
Summary
Intraspecific differences in resource allocation to male aevnodlufetimonalaeryoffbspiorlionggishtass l(oRnegnsbcehe,n a19s5u9b;jeCctaoldfeirn,ter1e9s8t4t)o. Most parasitoid wasps are haplodiploid with unfertilized eggs developing into males and fertilized eggs developing into females This allows adult females to potentially control the sex of the offspring they lay on hosts of differing quality. In her survey of the literature, Hurlbutt (1987) found that females on average are larger than males in 18 of 21 families of parasitoid wasps. Presumably benefits both sexes, the trend of female parasitoids being larger than males is most often attributed to the idea that females benefit more from increased size than males (Charnov, 1982). To determine if each parasitoid sex exploits a given amount of host resources differently, we compared the development of male and female parasitoids in hosts of equivalent quality (e.g. the same stage or size at oviposition).
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