Adaptive self superparasitism in a solitary parasitoid wasp: the influence of clutch size on offspring size

Abstract

Summary In solitary parasitoid Hymenoptera, normally only one offspring per host survives. Parasitism with more than one egg by the same female results in siblicide and hence is generally maladaptive. The braconid wasp Monoctonus paulensis Ashmead is a solitary parasitoid of aphids, including the Pea Aphid. Although all but one offspring are eliminated by competition between first‐instar larvae, females often produce a multiple‐egg clutch, even when searching alone. The hypothesis that the size and, presumably, the fitness of the surviving offspring in a clutch are greater than that of singly developing counterparts was tested in the laboratory. Adult size was measured as dry mass in parasitoids developing in Pea Aphids that differed in size (instar) and growth potential. Parasitoid size increased with host size, and females were larger than males. Parasitoids developing in aphids reared under crowded conditions (which had low growth potential) were smaller relative to initial host size. As predicted, the male offspring of virgin mothers developing in a clutch were 4–5% larger than counterparts developing alone. Clutch size had no consistent effect on the size of male or female offspring of mated mothers, however. A female of M. paulensis laying a multiple‐egg clutch can gain in fitness in terms of increased offspring size. The surviving offspring has access to greater nutritional resources because superparasitized aphids feed more than single‐parasitized aphids; the former also contain proportionately more teratocytes, which are egg‐derived cells aiding in parasitoid nutrition. Evidence that some species of solitary parasitoids do better when developing in a clutch suggests that the solitary lifestyle may not be an evolutionary absorbing state.