Host identity, nest quality, and parasitism strategy: influences on body
size variation in parasitoid bees and wasps.
Abstract
Body size determines mobility and fitness of insects in various ways.
Yet, especially in parasitoid species, drivers of body size are poorly
understood, as they can be influenced by complex interactions between
parasitoid behaviour, the environment, and their hosts. We measured the
body size of 393 individuals of four parasitoid species sampled with
trap nests for cavity-nesting bees and wasps in the Southern Black
Forest, Germany. We related the body size of the parasitoids to the size
of 15 host species and the diameters of their nests along four
environmental gradients (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 parasitoid species differed in their
responses. For instance, when the host Trypoxylon figulus doubled in
size, Nematopodius debilis (parasitizing the host directly) increased by
37% in size, while Trichrysis cyanea (parasitizing food resources)
increased by only 8%. Across host-parasitoid species combinations,
there was only a weak positive relationship between the size of
parasitoids and their hosts. In addition, we did not observe size
variations in hosts and parasitoids across environmental gradients. Our
findings highlight the primary factors influencing parasitoid size, with
host identity and nest diameter emerging as influential factors within
and between species, though not uniformly across all species. In
contrast, the relationship between parasitoid and host size and
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 variation in parasitoid size.