Freeze tolerance of a beneficial lady beetle, Hippodamia convergens

The convergent lady beetle Hippodamia convergens is widespread in the Americas and considered an important beneficial insect due to its use in pest control. Early work on this species characterized the beetles as freeze-avoidant (freeze-intolerant), suggesting they survive low winter temperatures by physiologically preventing ice formation to temperatures as low as −15°C. Here, we show that H. convergens can be freeze-tolerant if ice formation occurs at relatively high temperatures. There was 100% survival following inoculative freezing at −0.5°C and exposure to −3°C for 20 hours, as well as freezing that spontaneously occurred in fed beetles exposed to −4°C for 4 hours. Males exposed to 0°C or −4°C for 4 hours had similar mating behaviours (latency to first mating, copulation duration) as control beetles exposed to 4°C, although sample sizes were too small to determine whether freezing itself had an effect on these behaviours. Several putative cryoprotectants were detected in fat body tissue of H. convergens : glycerol, proline, trehalose, and myo -inositol – in order of abundance. Exposure to −3°C for 20 hours, whether frozen or unfrozen, did not statistically affect cryoprotectant accumulation, although there was a trend towards increased glycerol following freezing. This study is the first to describe inoculative freeze tolerance in a lady beetle and establishes a baseline for future studies that examine the mechanisms underlying this freeze tolerance and the effects of freezing on reproductive behaviour.