Ovipositional Behavior and Host Discrimination in Three Scelionid Egg Parasitoids of Stink Bugs

Ovipositional behavior of the scelionid egg parasitoidsTrissolcus basalis(Wollaston),Psix tunetanus(Mineo & Szabo), and two color forms ofTrissolcus utahensis(Ashmead) was observed in laboratory experiments that employed a common host, the pentatomidEuschistus conspersusUhler. Brood sex ratios were female biased and less than 0.20;P. tunetanusdeposited a precise sequence of male and female offspring compared to the other species. In host discrimination experiments in which individual females were presented with an egg mass partially parasitized by a conspecific female,T. utahensisshowed considerable ovipositional restraint toward parasitized eggs. When presented with an egg mass partially parasitized by a female of another species—eitherT. basalisorP. tunetanus—T. utahensisdisplayed little discrimination and oviposited in most of the parasitized eggs that it contacted. When the experiment was reversed—T. basalisandP. tunetanusfemales presented individually with an egg mass partially parasitized byT. utahensis—both species showed little discrimination and oviposited in most of the parasitized eggs that they contacted. For eggs in which multiparasitism involved the combination ofT. utahensisandP. tunetanus,the latter emerged from over 70%, regardless of whether it was the first or second ovipositing female. When super- or multiparasitism involved the combination of the twoT. utahensiscolor forms or the combination ofT. utahensisandT. basalis,offspring of the first ovipositing female emerged from over 70% of the eggs. These results are consistent with the view that interspecific host discrimination in parasitoids is less common than conspecific host discrimination and provide some support for the hypothesis that brood sex ratio and competitive ability are inversely related in guilds of quasi-gregarious egg parasitoids. Implications for the selection of parasitoid species for classical and augmentative biological control are discussed.