Abstract: In order to research the relationship between fluctuant high temperature and population crash of Aphis gossypii in midsummer, the mortalities of Aphis gossypii under different fluctuant thermal regimes were investigated in laboratory, and Aphis gossypii population abundance in field, affected by climatic factors, were analyzed. The results showed that the impacts of high temperature (30 ℃, 34 ℃, 38 ℃ and 42 ℃), exposure hours (2 and 6 h/d), exposure days (1-5 days) and interactions among these parameters on the mortality were extremely significant. The mortality of Aphis gossypii was increased with the increase of high temperature (>30 ℃) under the same treatments of exposure hours and days and the same high temperature treatment. The longer the high temperature continued, the higher of mortality of Aphis gossypii was. The mortality of Aphis gossypii was not significantly affected by fluctuant high temperature when temperature was lower than 38 ℃ (2 h/d). The date of population crash of Aphis gossypii in field coincided with the occurrence of high temperature (from July 31 to August 4). The diurnalnocturnal fluctuation of high temperature in field was consistent with the hightemperature thermal regimes which resulted in an increase of mortality of Aphis gossypii (24 ℃ 18 h/34 ℃ 6 h lasted for 4 days and 24 ℃ 18 h/38 ℃ 6 h lasted for 3 days). The results suggested that fluctuant high temperature played an important role in population crash of Aphis gossypii in midsummer. The results can be used to explain that the population crash of Aphis gossypii occurs usually in midsummer, but the mechanism should be further researched.

Key words: cotton field, Aphis gossypii, population, high temperature, mortality, population crash, Fukang, Xinjiang