Abstract: 【Aim】 Sclerodermus spp. are unique in parasitoids for cooperative brood care by foundresses, an uncommon quasi-social behavior. This study aims to investigate how foundresses perform such cooperatively breeding behaviors on an unnatural yet larger host. 【Methods】 The larvae of the long-horned beetle Monochamus alternatus with the body weight ranging from 0.16 to 0.44 g were selected at random, individually weighed, and then parasitized by 1, 2 and 4 foundresses (mated and naive female adults) of S. sichuanensis, respectively. The foundress preoviposition time and offspring developmental parameters of S. sichuanensis were measured, and the effects of host body weight (as a continuous predictor variable) and foundress number (factorial predictor variable) on these parameters were analyzed. 【Results】 All of the observed variables were not influenced by an interaction between host body weight and foundress number, suggesting that the two factors are independent from each other in their effects on the measured performances. The pre-oviposition time of S. sichuanensis was prolonged with the increase in host body weight, but the pre-oviposition time under the 4-foundress treatment was shortened by 18.3% and 20.6%, respectively, as compared to those under the 1- and 2-foundress treatments. The average number of offspring parasitoids per foundress of S. sichuanensis under the 1-foundress treatment was 1.5 and 3.3 times as high as those under the 2- and 4-foundress treatments, respectively. The average number of offspring parasitoids per foundress did not vary with the host body weight. The offspring sex ratios (proportion of males) of S. sichuanensis under the 2- and 4-foundress treatments slightly increased as compared to that under the 1-foudress treatment and were ultra female-biased. The offspring sex ratio of S. sichuanensis did not vary with the host body weight. The egg-to-adult survival rate of S. sichuanensis offspring under the 4-foundress treatment decreased by 37.4% and 30.5%, respectively, as compared to those under the 1- and 2-foundress treatments. The egg-to-adult survival rate of S. sichuanensis offspring did not vary with the host body weight. The offspring body size (hind tibia length) of S. sichuanensis increased as an increasing function of host body weight, but was not influenced by foundress number. 【Conclusion】 The results of this study suggest that the cooperatively breeding performance of S. sichuanensis parasitoids on a large and unnatural host depends more on the foundress number than on the host body size.

Key words: Monochamus alternatus, Sclerodermus sichuanensis, quasi-social behavior, cooperation, brood tending, parasite natural enemies, biological control