Digging below the ice: Overwintering burying beetles (Coleoptera: Silphidae) survive when they are beneath the frost line

Winter survival in many animals involves physiological and behavioral changes in order to keep metabolic processes viable. Insects are usually grouped into three categories regarding cold weather survival – freeze tolerant, freeze avoidant, or migrant which avoid the colder weather altogether.  In northern climates, most burying beetle species, Nicrophorus, overwinter as adults.  However, little research has been done regarding Nicrophorus species and their overwintering strategies and survival. PVC tubes (10 cm x 1.3 m long) were cut in half and then sliced to allow water exchange with the surrounding soil while preventing beetle escapes. The tubes were placed in the ground, dirt was added and two Nicrophorus orbicollis or Nicrophorus americanus beetles were placed inside each tube.  Weather conditions and soil temperatures at 10 cm depths were recorded hourly with a HOBO weather station unit.  Beetles were added October 8, 2012 and the tubes were checked once a week during the fall, winter, and spring months.  At approximately 6 week intervals, subsets of tubes were extracted and beetles were assessed for survival and burial depth.  Sixty beetles were found with 26 alive and 24 dead either on top of the soil or buried at a shallow depth (<7.5 cm).  The mean burial depth of living beetles was found to be 36.1 cm with one beetle found at 82 cm; the shallowest burial depth of a living beetle was 8 cm.  One of 6 N. americanus was found alive at 14 cm.  The results of this study indicate that burying beetles survive winter by burying below the frost line in Nebraska.