Supercooling capacity and cold hardiness of the eggs of the grasshopper Chorthippus fallax (Orthoptera : Acrididae)

Cold tolerance of the eggs of the grasshopper, Chorthippus fallax (Zubovsky), was examined in the laboratory. Egg supercooling points varied from -6degreesC to -32.4degreesC and could be divided into two groups. The supercooling points of the higher SCP group ranged from -6degreesC to -14degreesC and those of lower SCP group from -21.8degreesC to -32.4degreesC. Although low temperature acclimation could slightly decrease the supercooling points of eggs, the effect was not significant for all embryonic developmental stages or acclimation periods. The supercooling capacity was obviously different between pre-diapause, diapause and post-diapause embryonic stages. The mean supercooling points of pre-diapause and diapause eggs were similar; -28.8 +/- 1.6degreesC and -30.7 +/- 1.0degreesC for non-acclimated eggs and -29.5 +/- 1.3degreesC and -31.18 +/- 0.8degreesC for acclimated eggs respectively. However, the mean supercooling points of post-diapause eggs were significantly higher; -12.9 +/- 5.6degreesC for non-acclimated and -13.5 +/- 4.5degreesC for acclimated eggs respectively. The survival rates of diapause eggs at > -25degreesC were not significantly different from that at 25degreesC, but survival rates at < -30degreesC decreased significantly. The lethal temperature (Ltemp(50)) for a 12 hrs exposure was -30.1degreesC, and the lethal time (Ltime(50)) at -25degreesC was 44 days. Since the SCPs of diapause eggs was similar to their Ltemp(50), we may consider the supercooling capacity of such eggs is a good indicator of their cold hardiness and the species is a true freeze avoiding insect. Based on the analysis of local winter temperature data, pre-diapause and diapause, low SCP eggs can safely survive severe winters, but not the post-diapause, high SCP eggs. The importance of the overwintering strategy and the relationship between diapause and cold hardiness of this species is discussed.