Abstract: 【Aim】Sclerodermus foundresses can cooperate to parasitize hosts. Before oviposition, they need to sting the host and release venoms to immobilize it. It has been known that the cooperation enhances the efficacy of subduing and parasitizing strongly defensive hosts with a large body size. It is not yet known of fitness costs thereon incurred to the wasp for the cooperation. 【Methods】The experiment was conducted on the parasitoid S. guani with the flour beetle, Tenebrio moliter pupa as the substitutive host. Two mated female wasps (foundresses) were allowed to colonize together on a single host for different cooperation periods (0.5, 1.5, or 2.5 d) and then randomly retained one to continue the following parasitism on the host as the beneficiary foundress, while transferred another to a new host to make another parasitism as the altruistic foundress. The survival, oviposition, and offspring developmental performances were compared between the beneficiary and altruistic foundresses. 【Results】The odds of death of the altruistic foundress of S. guani were 5.4 times higher than those of the beneficiary foundress, regardless of the cooperation period. The pre-oviposition time was not different between the beneficiary and altruistic foundresses after the 0.5 d cooperation, while the altruistic foundress had 14.1% and 28.6% shorter pre-oviposition time than the beneficiary foundress after the cooperation of 1.5 d and 2.5 d, respectively. The two foundresses did not differ in the number of eggs laid, number of offspring emerged to adulthood, and offspring sex ratio (proportion of male offspring). Regardless of the cooperation period, the egg duration and larval duration of offspring of the beneficiary foundress were 6.8% and 8.4% longer than those of the altruistic foundress, respectively. The body weight of female offspring adults at emergence was similar between the two foundresses after the cooperation of 0.5 d and 1.5 d but the altruistic foundress had 25.4% less body weight than the beneficiary foundress after the 2.5 d cooperation. 【Conclusion】The results of this study suggest that S. guani foundresses incur fitness costs to their survival and some of their offspring development performances, especially for a longer period of cooperation before oviposition.

Key words: Bethylidae; Sclerodermus guani, altruistic behavior, cooperation, fitness, quasi-sociality