The cold-hardiness of Ctenucha virginica (Lepidoptera: Arctiidae) larvae, a freezing-tolerant species

Abstract

Typical of a freezing-tolerant species, Ctenucha virginica (Esp.) larvae had supercooling points above −11°C during the winter. For several weeks after the snow melt in the spring, larvae could tolerate freezing at the supercooling point, but lost this ability prior to pupation. However, C. virginica was atypical in that 5 μl samples of haemolymph froze from −13 to −16°C, indicating that haemolymph lacked the potent nucleators seen in other freezing-tolerant insects. The high supercooling points (−5 to −9°C) of feeding larvae were probably due to the presence of nucleators in the gut, as samples of the gut contents froze at similar or higher temperatures than the whole insect. Furthermore, premoult larvae, a developmental stage with a predominantly empty digestive tract, froze at temperatures (−12°C) similar to those of haemolymph samples. In addition to the gut contents, moisture on the surface of larvae may be another important source of nucleators as wet larvae froze at −3°C. Sorbitol and fructose concentrations increased with the onset of winter, but the role of these compounds as cryoprotectants was unclear as their concentrations were not correlated with lower lethal temperatures. Larval mortality evaluated 4 days after the termination of cold stress frequently underestimated total larval mortality. Thus the 24 h period following cold stress used to evaluate damage by most researchers, underestimates cold induced mortality. Cold stress can also have sublethal effects, such as increased larval developmental time, lower pupal weight, reduced fecundity, or higher infertility in survivors. These variables are often neglected in insect cold-hardiness studies, but are of considerable biological importance.