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Sociality is not common among parasitoid hymenopterans, but members of the ectoparasitoid genus Sclerodermus (in the aculeate family Bethylidae) may form multi‐foundress groups, producing and tending a single brood (of combined parentage) on a large host until offspring maturity. While individual foundresses can benefit more from reproducing within groups rather than alone, recent studies have indicated that there are conflicts of interest between co‐foundresses at the time of host suppression and during brood production, some of which may be mediated by intrinsic or acquired dominance. Evidence for overtly agonistic interactions has, however, been lacking.Here, we report that resident Sclerodermus guani Xiao et Wu females that have suppressed a host, can either accept introduced intruder females as co‐foundresses without aggression or repel, and even kill, intruders by biting and stinging them.Residents were more likely to exhibit aggression when they had commenced oviposition, when more days had passed since oviposition, when they were larger than the intruder, when the intruder was a non‐sibling, or when they were the sole resident rather than being in a group of two residents.We conclude that cooperatively brooding in Sclerodermus can be conditional on the evolutionary interests of foundresses with temporal priority on a host and their ability to repel potential co‐foundresses. [ABSTRACT FROM AUTHOR]