Resource pulses, occasional events of ephemeral resource superabundance, represent a fundamental mechanism by which energy, nutrients, and biomass are transported across ecotones. They are widespread in extant ecosystems; however, little is known about their deep-time record. We report the earliest-known mayfly swarm from the Early Jurassic Xiwan biota of southern China. Our taphonomic and sedimentological analyses show that these mayflies were buried on the bottom of a calm lake after post-mating death. Our suite of analyses suggests that the complex mating-swarm behavior was already well established in mayflies by the Early Jurassic. More importantly, our find represents the earliest-known resource pulse of insects, a mechanism that can play a substantial role in nutrient transport from aquatic ecosystems to surrounding terrestrial ecosystems. Such an aquatic-terrestrial ecosystem linkage may be a key novelty in Mesozoic lacustrine ecosystems. Our results high-light the underappreciated ecological significance of insects in deep-time terrestrial ecology.