Host identity, nest quality, and parasitism strategy: influences on body size variation in parasitoid bees and wasps

Body size determines mobility and fitness across taxa in various ways. Yet, drivers of body size in higher trophic invertebrates, especially parasitoids, including intra‐ and interspecific variations, are poorly understood due to complex interactions between parasitoid behaviour, the environment and their hosts. We measured the body size of 393 individuals of four parasitoid species (collected from 2220 parasitized brood cells) sampled with trap nests for cavity‐nesting bees and wasps in the Southern Black Forest, Germany. We related parasitoid body size to the size of 15 host species and the diameters of their nests along five environmental gradients (proportion of conifers, canopy cover, structural complexity, herb cover and deadwood diameter). Host identity, nest diameter, and to a lesser extent, size differences within host species were primary drivers of parasitoid body size, albeit responses varied among parasitoid species. For instance, when the host black wood borer wasp Trypoxylon figulus doubled in size, the ichneumon wasp Nematopodius debilis (parasitising the host directly) increased by 37% in size, while the blue cuckoo wasp Trichrysis cyanea (parasitising food resources) increased by only 8%. Across host–parasitoid species combinations, parasitoid size correlated weakly with host size, and environmental gradients did not significantly influence host or parasitoid body size. Our findings highlight the primary factors influencing body size, with host identity and nest diameter emerging as influential factors within and between parasitoid species, although not uniformly. In contrast, the relationship between the top trophic level (parasitoids), the lower trophic level (hosts), and host size with environmental gradients were less influential. Considering the environmental variables that directly affect body size, such as microhabitat conditions and biotic interactions, may further clarify the dynamics shaping the variation in body size at higher trophic levels and should be considered in future studies addressing how land management influences multitrophic interactions.