Effects Of Temperature On The Survivorship And Development Of Overwintering Eggs Of Galeruca Daurica (Coleoptera:Chrysomelidae)

Galeruca daurica (Joannis), a new pest in the Inner Mongolia grasslands of China, is univoltine and overwinters in the egg stage, feeding as larvae and adults on the foliage of Allium plants. Temperature is the main environmental factor that effects insect growth and survival. In this paper, we studied the effects of temperature on the survival and development of G. daurica overwintering eggs and the cold hardiness of the overwintering eggs in order to make a necessary foundation for its forecasting and integrated control. The main results were as follows:1. Effects of temperature on the termination of egg diapause and post-diasause embryonic development of Galeruca daurica.Experiments were conducted to determine chilling requirements for diapause termination and post-diapause development rate of eggs at different temperatures in the laboratory. The results showed that temperature and chilling time affected significantly the hatching and post-diapause embryonic development rates. No eggs hatched at 15℃ in al experiments. Mean time to hatch decreased sharply as temperature increased from 17 t 29℃. The longer chilling, two months vs one month, shortened the post-diapaus developmental durations at different temperatures. From linear regression betwee temperature and developmental rate, the post-diapause developmental threshold a thermal requirement of Galeruca daurica eggs were calculated to be 16.2℃ an 103.1-140.9 day-degrees, respectively. In conclusion, low temperature was a prerequisit for termination of egg diapause in Galeruca daurica, which required at least one ment under natural condition whereas one-month chilling at 5℃ or two-month natural chilin, were enough. The longer chilling both at 5℃ and natural chilling was more efficient f completion and breaking of egg diapause, and accelerated the post-diapause embryoni development.2. Effects of alternating and constant temperatures on the developmental rate an hatching rate of Galeruca daurica eggsWe compared the influence of five combinations of alternating temperatures 8/20*C (average15℃),10/22℃ (17℃),14/26℃ (21 ℃),18/30℃ (25℃) and 22/34℃ (29℃) and six constant temperatures (15℃,17℃,21℃,25℃, and 29℃) on the developmental rate and hatching rate of G. daurica eggs. The results showed that both alternating and constant temperatures affected significantly the developmental rate and hatching rate of G. daurica overwintering eggs. The developmental rates under alternating temperatures (8/20℃,10/22℃,14/26℃,18/30℃and 22/34℃) were 44.11,47.43,40.17,33.68 and 19.28d, and the hatching rates were 60.09%,89.05%,91.15%,95.40% and 62.43%, respectively. The eggs could not develop and died at constant temperature of 13℃. The developmental rates of eggs at constant temperature of 17℃,21℃,25 ℃ and 29℃ were 67.74%,89.24%, 97.02% and 97.94%, respectively. At the alternating temperatures and the constant temperatures, the developmental threshold temperatures of eggs was 9.47℃ and 18.58℃, respectively. The effective accumulated temperatures was 397.23 degree-days and 72.94 degree-days. The alternating temperature could promote the development of G. daurica eggs at lower temperature (17℃) whereas constant temperature could promote the development of G. daurica eggs at a higher temperature (29 ℃).3. Effects of low temperature on the survivorship and development of overwintering eggs of Galeruca dauricaWe measured the survival rates and developmental durations of overwintering eggs of G daurica exposed to different low temperatures (-18℃-3℃) for 12 and 24 h or exposed to -30℃ for different periods (0-60 d) in the laboratory. The results showed that, low temperature intensity and exposure time affected significantly the survival rates of overwintering eggs of G. daurica. The survival rates declined with the decrease of exposure temperature and the extension of exposure time. After the overwintering eggs were exposed to the temperatures≤-33℃ for 12 h or the temperatures≤-30℃ for 24 h, their survival rates were significantly lower than the controls (25 ℃). The lower half lethal temperatures (LT50) after 12 and 24 h exposure were-33.08 and -32.13℃, respectively, and the half lethal time (Ltso) at -30℃ was 33.33 d. The developmental duration of surviving eggs prolonged significantly after exposure to -36 ℃ for 12 h or the temperatures ≤-33℃ for 24 h, but had no significant change after exposure to -30℃ for 30 days. The overwintering eggs of G. daurica have strong cold hardiness, and low temperature in winter may usually not cause mass mortality of overwintering eggs.