The effect of cold acclimation and deacclimation on cold tolerance, trehalose and free amino acid levels in Sitophilus granarius and Cryptolestes ferrugineus (Coleoptera)

Canadian and French laboratory strains of Sitophilus granarius (L.) and Cryptolestes ferrugineus (Stephens) were cold acclimated by placing adults at 15, 10 and 5 degrees C successively for 2 wk at each temperature before deacclimating them for 1 wk at 30 degrees C. Unacclimated S. granarius had an LT50 (lethal time for 50% of the population) of 12 days at 0 degrees C compared with 40 days after the full cold acclimation. At -10 degrees C, unacclimated C. ferrugineus had an LT50 of 1.4 days compared with 24 days after the full acclimation. Cold acclimation was lost within a week after returning insects to 30 degrees C. Trehalose, as well as the amino acids proline, asparagine, glutamic acid and lysine were higher in cold acclimated insects for both species. For S. granarius, glutamine was higher in cold acclimated insects and isoleucine, ethanolamine and phosphoethanolamine, a precursor of phospholipids, were lower in cold acclimated insects. For C. ferrugineus, alanine, aspartic acid, threonine, valine, isoleucine, leucine, phenylalanine and phosphoethanolamine were higher in cold acclimated insects. For both species tyrosine was lower in cold acclimated insects. There were small but significant differences between Canadian and French strains of S. granarius, with the Canadian strain being more cold hardy and having higher levels of trehalose. There were small but significant differences between male and female S. granarius, with males being more cold hardy and having higher levels of proline, asparagine and glutamic acid. In conclusion, high levels of trehalose and proline were correlated with cold tolerance, as seen in several other insects. However, correlation does not prove that these compounds are responsible for cold tolerance, and we outline further tests that could demonstrate a causal relationship between trehalose and proline and cold tolerance.