Ice nucleating effect of setae from caterpillars of the Isabella tiger moth (Pyrrharctia isabella)

Woolly bear caterpillars of the tiger moth Pyrrharctia isabella are freeze tolerant, which supports their winter survival throughout much of eastern North America. Woolly bears routinely undergo inoculative freezing of internal body fluids upon physical contact with ice; hence, the reliability of freeze episodes would benefit from features promoting crystallization of surface moisture at high subzero temperatures. We investigated the action of woolly bear setae on the supercooling points (SCPs) of aqueous solutions. SCPs of 100 microliter solutions rose modestly in proportion to the number of added setae, with SCP averaging nearly 1.8 C higher than the control solution when 5 setae were present. Nucleator potency was notably greater in small volumes of water, and the mean SCPs ranged between -5 C and -6 C for all solutions containing setae. The SCPs of 50 and 25 microliter samples containing 5 setae were elevated by 5.0 and 6.5 C, respectively, above their matching control solutions. The setae color morphs, red and black, did not differ in their nucleating potency, and heat treatment of isolated setae (100 C for 30 minutes) did not alter the nucleating action. Thus, setae of woolly bears may trigger freezing of water droplets on the body surface so that inoculative freezing occurs more reliably in these caterpillars.