Velvet ants are larval ectoparasitoids on a variety of insect taxa, and like many insects show sexual dimorphism in color patterns and size. For many mutillids, host size appears to be an important criterion in sex allocation. The criteria that female mutillids use when determining the sex of an egg are unclear and it is unknown how this size dependent sex allocation system is manifested. We conducted an experiment to test if size-based sex allocation in Sphaeropthalma pensylvanica is a product of host species specificity, differential mortality, or if it is facultatively size dependent. We show that male S. pensylvanica commonly develop in large host species, females in small host species, and both in intermediately sized host species, supporting facultative size-dependent sex allocation. We also report new observations on the natural history of five hosts of S. pensylvanica.