Variability in cold tolerance among regional populations of the Colorado potato beetle, Leptinotarsa decemlineata

Cold tolerance in insects is often characterized by supercooling points (SCPs). Supercooling points are the temperature at which ice-nucleation occurs within the cells of a freezing insect and is identified by a measureable exothermic release. Using supercooling points as a proxy for cold tolerance we compared three regional populations of Colorado potato beetle, Leptinotarsa decemlineata, from Mexico (Morelos, MX) and the USA (Vermont and Maryland) to determine the variability in winter adaptiveness. Both Vermont and Maryland diapausing individuals exhibited lower supercooling points as compared with Mexican beetles indicating a greater tolerance for freezing temperatures. The coefficient of variation for SCP was also greater in populations collected from warmer climates (MEX=30.4%, VT=23.5%, MLD=20.25%). Maryland beetles showed much lower variation in SCP, suggesting higher selection pressure for cold tolerant beetles within Maryland. HOBO thermocouples were also used to identify the minimum soil temperature ranges that would be experienced by overwintering beetles in Vermont. Thermocouples were placed at 10cm, 20cm, 40cm, and 100cm below the surface of the soil at UVM’s Horticultural Research Complex. Minimum soil temperatures fell within the range of SCPs for our sampled populations (10cm=-6°C, 20cm=-4°C, 40cm=-2°C, 100cm=1°C), indicating that soil temperature may be a significant selective force for overwintering beetles within Vermont.