Ellychnia corrusca (Winter Firefly) is one of few winter-active insects. Exposed throughout the season, this beetle appears vulnerable to insectivorous predators, but was recently shown to possess lucibufagins (LBGs), potent chemical defenses. The Winter Firefly is closely related to summer-active fireflies. To provide an adaptive explanation for this apparent phenological shift, we hypothesized that winter activity may protect the Winter Firefly from summer-active fireflies in the genus Photuris, predators that specialize on LBG-containing prey. To test this hypothesis, we studied the Winter Firefly and Photuris that occur sympatrically, but asynchronously as adults, at our Connecticut field sites. Through 2 experiments, we demonstrated that Photuris selectively consumes the Winter Firefly and sequesters its LBGs. Our findings are consistent with a hypothesis that winter activity by the Winter Firefly, by enabling early spring reproduction, provides phenological escape from a specialist predator.