Abstract:The parasitic chalcid, Coccobius azumai Tachikawa, was introduced from Japan at the end of 1980′s. The parasitoid was an important biological agent used to control the pine armored scale, Hemiberlesia pitysophila Takagi in southern China. The parameters of supercooling point (SCP), mortality exposed to designated low temperature, and median lethal sum of low temperature (LST50) of the chalcid adults collected from different seasons, were measured and compared in Quanzhou, Fujian Province, China in 2007 and 2008.
Mean SCP value for female adults collected in spring, summer, fall and winter was -13.7559、-11.9700、-12.7936 and -13.6000 ℃, respectively, indicating females collected in spring and winter had a significantly lower SCP value than female adults collected in summer. Mean SCP value for male adults collected in spring was -15.3917℃, which is significantly lower than that for male adults collected in summer (-13.8400℃) and autumn (-13.2143 ℃). For adults collected in spring and summer, mean SCP value for females were considerably lower than those for males. However, this value was almost equal for both males and females collected in fall. When exposed to different constant temperatures, no female adults survived under -15 ℃. Mortality (under 0 ℃) was clearly lower for females collected in spring than females collected in summer. However, under -5 and -10 ℃ females collected in spring had lower mortality than that in summer and fall. The relationships between the sum of exposed low temperature and mortality of female adults collected in each season fits very well to a revised logistic model (equation 1).
Significant differences were observed between LST50s estimated by this model for female adults from different seasons (df=2,33, F=30.45, P＜0.01). Mean LST50 for the spring female adults was significantly lower than those for the summer and fall female adults. The SCP and LST50 for female adults decreased as the seasonal extreme low air temperature drops off and as the seasonal extreme difference in air temperature increases. The LST50 for the female adults also decreased with drop of SCP. These results suggested that, in terms of cold tolerance, C. azumai adults have obvious adaptability to seasonal variation of air temperature. One important strategy for the wasp to increase cold tolerance is to lower the SCP, which may be the results of its exposure to lower extreme low air temperature and high extreme difference in air temperature in fall.