Lespesia archippivora (Diptera: Tachinidae) Survival and Sex Ratios within Monarch Butterfly (Lepidoptera: Nymphalidae) Hosts

We studied factors associated with survival, sex ratio, and seasonality of the tachinid fly Lespesia archippivora parasitizing monarch butterflies (Danaus plexippus). A total of 424 L. archippivora flies were collected from 124 parasitized monarch larvae in 2016 and 2017. The majority of flies emerged during the monarch's 5th larval instar. Both fly survival and brood size were higher for those emerging during the 5th instar relative to flies emerging during the 4th instar or pupal stage. Brood size ranged from 1 to 10 flies, and the proportion of each brood that successfully pupated to emerge as adults increased with brood size. Adults emerging from larger broods were lighter and smaller, which likely impacts subsequent fecundity and survival. Overall sex ratio was skewed towards males, with 222 male and 129 female adults identified. L. archippivora were recovered from monarchs both early in the season and at the end of the season, with a gap from mid-June to mid-July; a pattern consistent with a bivoltine life cycle.