Host-resource utilization of parasitoid wasps on subcortical beetles

In forest ecosystems,complex interactions exist between many species of subcortical beetles and their natural enemies in declining and dead trees. Only a few studies have tried to measurue the role of natural enemies, especially parasitoid wasps,in regulating the population density of the beetles, because most of the biological and ecological traits and the interactions between hosts and parasitoids have not been clarified. This study investigated life histories and hostsearching behaviors of 2 solitary ectoparasitoids, Atanycolus initiator(FABRICIUS) and Spathius brevicaudis RATZEBURG, and improved the methods for use in oviposition experiments and rearing the wasps in the laboratory.The paper discusses criteria for evaluating beetle larvae as hosts and factors causing the differences in the parasitism pattern between the 2 parasitoids. \n(1) Life histories and host-searching behaviors in the 2 parasitoids 1.The 2 parasitoids are both polyphagous and parasitic on Cerambycidae, Curculionidae, and Scolytidae larvae infesting Japanese pines, Pinus densiflora and P.Thumbergii. Pine logs were collected in the field and kept in an outdoor cage. Adults of both species emerged from April to November. Emergence was not observed from July to September,probably because most of the host species had emerged by then. Most male adults of the overwintering generation of A.initiator emerged a few days before the females. 2.The duration from egg hatch to adult emergence of both species was almost identical under laboratory conditions.In both species, females required more time than males for development.The mean longevity of female adults was longer than males in both species when reared on dilute honey. Between 2% and 25% of the reared A.initiator larvae diapaused. 3. A.initiator adults were distinctly larger than S. brevicaudis adtults. A.initiator's ovipositor was 3.7 times longer than S.brevicaudis's. Clear allometric relationships were established between all combinations of body length, head width, body weight, and ovipositor length. 4. A new method was developed to observe the oviposition behavior of the parasitoids on subcortical beetles in the laboratory. A piece of pine bark was attached to the bottom of a clear styrene case by metal fittings, and living host larvae were placed under the bark. A female wasp was put in the case and induced to oviposit through the bark.Observation allowed the host searching behavior of A.initiator to be divided into 4 phases: random search,concentrated search,ovipositor penetration, and host paralysis and oviposition. (2) Sex allocation of the 2 parasitoids 1. Many solitary parasitoids manipulate the sex of their offspring in response to host size, as proposed by the host-size model of Charnov et al.(1981). Several experiments were conducted to test the sex allocation systems of the 2 parasitoids, which cannot see their hosts. The sex ratio of wasps emerging from Shirahoshizo spp.,the most abundant host species,was strongly male-biased in A. initiator, but female-biased in S. brevicaudis.The percentage parasitism of A.initiator on Niphades variegatus and Acanthocinus griseus, which are larger,was higher than on Shirahoshizo spp., and the sex ratio of emerging adults of both host species was almost 1:1. Only S.brevicaudis exploited 2 species of Scolytidae smaller than 10 mg fresh weight, and the sex ratio of emerging adults from 1, Tomicus piniperda, was female-biased. 2. The size of emerging wasps was positively correlated with host size in both parasitoid species. Regression coefficients of the size relationships were significantly different between males and females of both parasitoids for relatively larger host species.There was no significant difference in the regression coefficients between host species parasitized by each wasp species. Both parasitoids clearly responded differently to the size of their common host species, Shirahoshizo spp. For A.initiator,Shirahoshizo spp.were relatively small