Biochemistry and physiology of overwintering in the mature larva of the sunflower stem weevil, Cylindrocopturus adspersus (Coleoptera: Curculionidae) in the Northern Great Plains

The sunflower stem weevil, Cylindrocopturus adspersus (LeConte), overwinters as a mature larva at the base of the stalk and in the root crown of cultivated sunflower plants. Sunflower stalks from fields known to be infested with C. adspersus larvae were collected in southeastern North Dakota in October 1991. Larvae from stalks kept outdoors accumulated a high whole-body concentration of trehalose (up to 69 μg/mg wet wt) at the expense of glycogen with the onset of winter followed by a partial reconversion of trehalose to glycogen with the onset of spring. Larvae from stalks acclimated to 0°C also accumulated a high level of trehalose (∼69 μg/mg wet wt) with a concomitant decrease in glycogen. Those larvae from stalks kept at 20°C showed an initial sharp increase in whole-body trehalose that then stabilized but at a concentration well below that of larvae acclimated to 0°C. This indicates that there exists in the larva an underlying developmental component to trehalose accumulation which is further enhanced by low temperature (0°C) exposure. The mean temperature of crystallization (Tc) of larvae exposed to outdoor conditions showed an abrupt drop from October (−25.0 ± 1.3°C) to November (−28.2 ± 0.6°C) with a minimum in February (−29.1 ± 0.3°C). The level of trehalose accumulated by the sunflower stem weevil larva is to our knowledge the highest reported in an overwintering insect.