1. Parasitoids can encounter patches of hosts of varying sizes. Tradeoffs may result in different developmental strategies and fitness characteristics in parasitoids.2. To explore developmental strategies and host suitability of a bethylid parasitoid, Sclerodermus pupariae, in relation to the size of oak long-horned beetle larvae, Massicus raddei , effects of host size on parasitoid fitness parameters were tested under laboratory conditions.3. Maternal parasitoids obtained lower rates of parasitism when inoculating small or large hosts due to early host death and parasitoid injury, respectively, indicating significant fitness costs associated with paralysing those host sizes. Host size was shown to have highly variable effects on selected fitness parameters. For maternal females, increasing initial host size led to increasing host handling time, and fertility exhibited a parabolic relationship with host size. The highest fertility was exhibited when parasitoids oviposited on medium-sized hosts.4. Host size effects were very apparent for parasitoid offspring, with the largest hosts producing later-emerging but larger females. Parasitoid offspring sex ratios in all host size classes were significantly female-biased, and exhibited a quadratic function with increasing host size. Assessment of host profitability revealed that medium-sized hosts presented the best fitness return for the parasitoids.5. The findings suggest that this bethylid parasitoid can achieve a compromise in optimisation of the two most important fitness functions when encountering an abundance of different-sized hosts.

The relationship between body size and fitness in parasitoid wasps has several effects on parasitic ability, reproductive behavior in female wasps, and progeny fitness. Female wasps with various body sizes were obtained by mass–rearing a gregarious ectoparasitoid, Sclerodermus pupariae, which is one of the excellent parasites to control the larvae and pupae of Buprestidae and Cerambycidae. We investigated the effects of body size of adult females introduced on Thyestilla gebleri (Coleoptera: Cerambycidae) larvae on their paralysis time, pre–oviposition period, oviposition period and fecundity, and the related fitness of their offspring. Results showed that small female wasps needed more time to paralyze a host and had a higher mortality rate than large female wasps. More offspring were produced by large female wasps than by small female wasps, and the percentage and body size of female offspring was not affected by maternal body size. The duration of the egg stage was not affected by foundress size, nor was that of the pupal stage, but the duration of the larval stage and generation time of small female wasps was longer than that of large females. Our findings suggest that the parasitic fitness and offspring performance are affected by maternal size, and there is need to choose reasonable body size of female wasps, to optimally utilize mass rearing and to control target pests with the lowest mortality cost.

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