The developmental strategies and related profitability of an idiobiont ectoparasitoid Sclerodermus pupariae vary with host size

Parasitoids can encounter patches of hosts of varying sizes. Trade‐offs may result in different developmental strategies and fitness characteristics in parasitoids. 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. 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. 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. 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.