Cold tolerance and trehalose accumulation in overwintering larvae of the codling moth, Cydia pomonella

Seasonal variations in the supercooling point, survival at low temperatures and sugar content were studied in fieldcollected codling moth larvae. The supercooling point of field-collected larvae decreased significantly from a mean value of –13.4°C in August 2004 (feeding larvae) to –22.0°C in December 2004 (overwintering larvae). Survival at –20°C/24 h was 0% during early autumn, whereas it increased to approximately 60% during winter. The survival at low temperature was well correlated with the supercooling point. The supercooling point of the diapause destined larvae decreased from –16.9 to –19.7°C between September and October as the larvae left the food source and spun a cocoon. For early-diapause larvae, exposure to 5°C/30 days has an additional effect and decreased the supercooling point from –19.7 to –21.3°C. One-month exposure of overwintering larvae to 5°C led to a mortality of 23% in early diapause larvae, while only 4% of diapause larvae died after acclimation. Overwintering larvae accumulated trehalose during winter. There was approximately a threefold increase in trehalose content between larvae at the onset of diapause (5.1 mg/g fresh weight) and larvae in a fully developed diapause (18.4 mg/g fresh weight) collected in January. Trehalose content was correlated with supercooling capacity, survival at low temperatures and chilling tolerance, suggesting that trehalose may play some role in the development of cold tolerance in this species. 385 * Corresponding author; e-mail: [email protected], [email protected] by codling moth was around –31°C (Shel’deshova, 1967). The lower lethal temperature of non-diapausing codling moth larvae was significantly higher than the whole body supercooling points, which indicated chill sensitivity in actively feeding larvae. However, lower lethal temperatures of diapausing larvae were similar to the average whole body supercooling point, which indicated that the diapausing larvae were freezing intolerant (Neven, 1999). Accumulation of glycerol as a cryoprotectant was shown in overwintering codling moth larvae by Minder et al. (1984), but Neven (1999) demonstrated that diapausing larvae (diapause induction condition: 18 ± 2°C, 8L : 16D) did not accumulate appreciable levels of glycerol and trehalose and principally remove exogenous ice nucleators to avoid ice formation. The capacity for polyol accumulation may change seasonally. For example, many species of insects initiate polyol synthesis at low temperatures (Hayakawa & Chino, 1981; Storey & Storey, 1983; Nordin et al., 1984). The trigger temperature with the maximal rate of polyol synthesis in insects is typically in the 0 to 5°C range (Storey & Storey, 1988). Additionally a thermoperiod can accelerate this process (Pio & Baust, 1988; Koš ál et al., 2001). However, these trigger temperatures were not applied in research performed by Neven (1999). So, in this study, we used field collected diapausing larvae of the codling moth. The main purpose of the present study was to integrate the information obtained by simultaneously recording physiological and biochemical aspects of cold hardiness with the microclimate conditions in the overwintering habitat of a wild population of the codling moth. Such integration aims to provide a better understanding of overwintering strategy in its entirety. Field data obtained in this study are used to verify and extend earlier laboratory experiments. MATERIAL AND METHODS