Pteromalus cassotis is a species of parasitic wasp in the family Pteromalidae that parasitizes the chrysalides of monarch butterflies. They are gregarious parasitoids, meaning a single female lays many eggs in a single host. Research into this species has documented that up to 425 adult wasps can emerge from a single chrysalis. The wasps have a heavy female bias, averaging 90% female.Stenoien, C., McCoshum, S., Caldwell, W., De Anda, A., & Oberhauser, K. S. (2015). "New Reports that Monarch Butterflies (Lepidoptera: Nymphalidae, Danaus plexippus Linnaeus) are Hosts for a Pupal Parasitoid (Hymenoptera: Chalcidoidae, Pteromalus cassotis Walker)". Journal of the Kansas Entomological Society. 88(1), 16-26. Maximum entropy models suggest that the natural habitat of this species encompasses the continental United States, southern Canada and parts of Mexico; areas inhabited by the caterpillars of monarch butterflies, which are the larvae's hosts.McCoshum, S. M., Andreoli, S. L., Stenoien, C. M., Oberhauser, K. S., & Baum, K. A. (2016). "Species distribution models for natural enemies of monarch butterfly (Danaus plexippus) larvae and pupae: distribution patterns and implications for conservation". Journal of Insect Conservation. 20(2), 223-237.
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