Fireflies in the Snow: Observations on two early-season arboreal fireflies Ellychnia corrusca and Pyractomena borealis

Abstract

Field studies on two cold-hardy sympatric firefly species Ellychnia corrusca complex (Linnaeus) and Pyractomena borealis (Randall) were conducted from 2008 to 2011, in East Tennessee in the southeastern United States. Their behaviours, predation events, escape strategies, calendar and degree-day parameters for life events are presented and regional differences discussed. Adults of E. corrusca and last instar larvae of P. borealis become active in February when snow and below-freezing temperatures are common. They gather in the furrows on the warmer, sunlit south sides of still-leafless preferred trees, the majority of which are large diameter oaks (for E. corrusca), hickories (for P. borealis) and tulip poplars (for both firefly species), Quercus sp., Carya sp. and Liriodendron tulipifera, respectively. Ellychnia corrusca, lanternless and diurnal, abruptly appear low on colony trees in late winter (Jan-Feb), orienting to the sun and vertically to one another upon initial emergence. A 4–6 week quiescent period follows this emergence during which time they remain on their colony tree slowly patrolling or resting head down in the bark furrows. Flight, dispersal and mating occur in mid-March to April. Diurnal P. borealis larvae climb from cold winter ground up preferred trees seeking sunny, protected pupation sites. Frequent moving is common in Jan-Feb as larvae select ideal arboreal sites. Pupation takes place in March with protandrous males attaching, pupating and eclosing before most females. Adult males use two mating strategies. Initially day active, they seek out and tend both larval and pupal females for up to two weeks and attempt copulation as soon as the guarded virgin female ecloses, bypassing courtship and flashing. In the nocturnal courtship display, P. borealis males flash in flight every ca 2 s in April on fair-weather nights above 10-12°C, high in the forest canopy. Intense male competition, repeated mating and prolonged (1–8 day) copulation is common.