Nucleators and sites of nucleation in the freeze tolerant larvae of the gallfly Eurosta solidaginis (Fitch)

Abstract

The gallfly Eurosta solidaginis overwinters as freeze-tolerant, third-instar larvae in galls on goldenrod ( Solidago canadensis). The mean crystallisation temperatures of different samples of larvae collected in February 1988 were between −8.6 and −9.3°C. The mean crystallisation temperature of haemolymph extracted from the same groups of larvae was on average 7–10° lower at −18°C. Dilution of haemolymph to 10 6 had no effect on the crystallisation temperature. Unlike many freeze-tolerant insects the nucleator which induces freezing in the larva at −9°C does not appear to be in the haemolymph. Haemolymph extracted from unwashed larvae froze at a significantly higher temperature compared to that obtained from larvae which had been washed to remove the frass (faecal matter) which adheres to the larval cuticle. A film of surface moisture placed around the larvae froze at −6 to −7°C and raised the crystal-lisation temperature of the larvae to −3°C following ice penetration through cuticular apertures, but did not affect the ability of larvae to pupate or the success of adult emergence compared to larvae which were cooled dry and a control group. Suspensions of 10 and 1% frass in 0.9% saline froze at elevated temperatures similar to the crystallisation temperature of the larvae. Maintenance of galls containing larvae at 0 and 10°C for up to 10 days had no effect on the crystallisation temperature of the larvae. At 25°C some larvae supercooled to below −22°C after only 2 days, whereas other larvae at the same temperature were still freezing at −9°C after 8 days. Loss of nucleator activity in E. solidaginis is apparently closely related to the internal reorganisation of the larvae immediately prior to pupariation.