Biochemistry and physiology of overwintering in the mature larva of the red sunflower seed weevil, Smicronyx fulvus LeConte (Coleoptera: Curculionidae)

The red sunflower seed weevil, Smicronyx fulvus LeConte, overwinters in the soil as a mature fifth-instar larva. A strong correlation exists between minimum daily soil temperatures and trehalose accumulation. Larvae buried in the field in September 1988 showed a two-fold increase in whole-body trehalose levels by January (21.0±0.6 vs 40.1 ± 0.6 μg/mg wet wt). During this period of trehalose accumulation, glycogen levels fell from 17.9 ± 1.4 to 2.7 ± 0.6 μg/mg wet wt. There is a partial reconversion of trehalose to glycogen with the onset of spring; trehalose levels dropped while glycogen levels increased. Trehalose accumulation does not affect the supercooling capacity of the larvae. The mean temperature of crystallization of the larvae remained constant at about −24°C from September to June and was also unchanged with temperature acclimation. Respiratory rates fell from 0.65 μl/h/mg in September to 0.29 μl/h/mg in January. Larvae collected in September 1989 and acclimated to 0°C for 7 days also had increased trehalose levels with a concomitant reduction in glycogen. Early autumn larvae subjected to −5°C for 7 days had a survival of 96%. However, only 28% of the larvae collected from containers buried in the field in November survived 1 day at −15°C and none survived 1 day at −20°C. Under the conditions tested the larvae do not appear to be freeze-tolerant.