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Patterns of host resource utilization and sex ratio manipulation in relation to host size were investigated for two solitary ectoparasitoid wasps, Atanycolus initiator and Spathius brevicaudis (Hymenoptera Braconidae). Both species parasitize subcortical beetles on the trunks of Japanese pine trees. A. initiator is on average 8 times larger in body weight and has an ovipositor that is 3.7 times longer than that of S. brevicaudis. In both parasitoids, the size of emerging wasps was positively correlated with host size, but the host/wasp size regressions were linear for all three major host species in A. initiator, whereas in S. brevicaudis the regression was logarithmic for a relatively large host species. The sex ratios (proportion of males) of both parasitoids emerging from different host species decreased with increasing host size, but the overall sex ratio on each host species was male‐biased in A. initiator, while female‐biased in S. brevicaudis. How the proportion of host consumed changed in response to host size, differed between the two parasitoids for the same host species. In the field survey, the size and sex ratio of the emerging two parasitoids from a dead tree were closely related to host size. However, the spatial distribution of the two parasitoids depended on the bark thickness of the trunk. The data suggest that differences in the relative evaluation of host size and in ovipositor length may enable the coexistence of the two parasitoid wasps.

Fitness elements (number and weight of emerging adults) and the success of parasitism (host mortality) in Dastarcus longulus were compared among three host stages (Monochamus alternatus larvae, pupae and adults). Different numbers of first-instar larvae were released onto single host individuals to determine the optimal host quality and rearing conditions needed to obtain large numbers of high-fitness individuals and to maximize the rearing efficiency of the parasitoid. Host mortality after the release of a single parasitoid larva was 88% on pupal hosts but Ͻ50% on larval and adult hosts. The numbers and weights of emerging adults per host were highest on pupal hosts. The estimated number of ovarioles in emerging adults was also the highest on pupal hosts. Pupae of M. alternatus thus provided the best host quality for rearing D. longulus. High host mortality was caused mainly by pupal immobility. A high feeding rate was associated with high parasitoid reproductive potential on pupal hosts. Although the release of 10 larvae onto a single pupal host resulted in the highest overall fitness of the rearing population of D. longulus, the release of two larvae onto each pupal host was the optimal procedure for maximizing the weight of individuals.

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