Only cool beetles reproduce: American burying beetles require a period of cold for reproduction

The federally threatened American burying beetle, Nicrophorus americanus Oliver (Coleoptera: Staph) is the largest burying beetle in North America. It has an annual lifecycle where new adults overwinter before breeding the following year utilizing small vertebrate carrion weighing up to 200 grams.  A laboratory colony at Oklahoma State University has not been able to be maintained perpetually as individuals from late summer broods fail to reproduce.  In its northern range, winter temperatures form a frostline and beetles likely stay underground from October until May. In its southern range, frozen ground rarely occurs but beetles are rarely found before April or later than early November.  We investigated if a period of chilling was necessary for late summer beetles to successfully breed. In November 2023, we placed plastic tubes with native soil in the ground near Stillwater, OK. Each tube contained one wild-caught Nicrophorus americanus with three separate tube lengths tested – 15 cm, 30 cm, and 55 cm. In February, tubes were retrieved, and survival was determined based on species, sex, and depth at which beetles were recovered. These beetles were fed for two weeks and then provided a carcass.  In a separate set of experiments, N. americanus from Oklahoma were held in a growth chamber where temperatures were stepped down by 2 degrees two-to-three times weekly from 25 to 5 Celsius before being increased at the same rate after January. In contrast to beetles maintained at constant temperatures, both field and laboratory beetles formed brood balls and reared offspring.