| Chilo suppressalis (Walker) is one of the main pest in the rice field which passes thewinter by mature larvae's diapause. By using the super cooling point as the indicator ofcold-hardiness, taking natural wintering population as study subject, this researchconducts a preliminary study about the cold tolerance of Chilo suppressalis (Walker)overwintering larvae and its wintering ability. The results reveal the relationship betweenlarvae's cold-hardiness and the change trends of some major physiological andbiochemical indicators, such as larvae's weight during the winter, water and sugar, fats,glycerin, protein, amino acids. By taking laboratory group as studies, the author revealsthe ralationship between Chilo suppressalis' diapause features and its cold-hardiness. Theresults are as follows:1. Death rate of the over-wintering larvaeThe death rate of the over-wintering larvae will increase gradually from October tonext March, death rate in October is the lowest one which is 1.06%, and the highest is inthe March which is 5.75%, it is lower than 6% during the whole winter. By analysing therelationship between the larvae's death rate and the temperature, the author found that thedeath rate ties up with the enviroment temperature and the chilliness flow at the end of theover-wintering period affects the greatly.2. Relationship between the change of super cooling point and freezing point ofthe over-wintering larvae and the temperatureThe super cooling point and freezing point of larvae declines gradually before theover-wintering period and then rises smoothly after it. SCP in October and November issignificantly higer than it in September but lower than it in December and January. Thecooling point reaches its bottom -21.15℃in February and then rises to -16.26℃with therising of the temperature. The freezing point fell from -2.45℃to -8.73℃. SCP andfreezing point have the same trend with the variation of enviroment temperature, but SCPchanges greater than the freezing piont. The result of hypothermia test showed thatover-wintering larvae's cold-hardiness reached a higher level in December.3. The variation of the height and water content of the Chilo suppressalis over-wintering larvaeWater content of the larvae appeares a significant trend of reduction fromSeptember to the next February. From September to November, the body water contentrapidly declines, from 68.83% to 59.60%, drops a total of 8.23%, and it recovers in Marchgradually again. It shows that water content of the body has the same trend with thevariation of enviorment temperature, but fall behind the latter. During the over-winteringtime, the larvae weight changes showed the first increase after weight reduction trendwhich may help to point out that weight is an important factor of its cold tolerance ability.4. Sugar content changes of Chilo suppressalis over-wintering larvaeDuring the over-wintering period, the changes of total sugar, trehalose and somemain carbohydrate glycogen content are very significant. Glycogen, which is the energysubstances for the larvae, increases first and then drops in the November with the drop ofthe temperature, glycogen content is reduced to the minimum in February to 2.69μg/mg.However, the change of trehalose content behaviors exactly the contrary. During thewinter, as the temperature decreases, the content of trehalose significantly increased, andsome cold hardiness material, such as trehalose has been changed into energy materialwhen the temperature began to rise. This is further evidence of inscets' accumulation oftrehalose can improve its cold hardiness ability and Trehalose maybe is a key indicator todirecte Chilo suppressalis larvae's diapause state.5. The change of body fat and protein content of over-wintering larvae indifferent periodsDuring the whole winter, the body dissociated fat content decreses from 23.42% to20.72% and then increases to 21.6% in the later winter. Content of the dissociated fatshows a contraty trend with the change of larvae's cold hardiness. From analyzing amarked increase of fat content during the winter, we can deduce that the change of the fatcontent is correlated with the change of it's super cooling point and influences thelarvae's cold hardiness at the same time. Throughout the whole winter, the protein contentof the larvae shows up after the first downward trend and it has significant differencesduring different periods.6. Vivo glycerol content changes of over-wintering larvae in different periods Vivo glycerol content of over-wintering larvae increases from 3.13μg/mg inSeptember to 12.05μg/mg in January, an increase of 285%. After the over-winteringperiod, with the rise of temperature, the body glycerol content declines significantly, andshows a content of 6.34μg/mg in March. The same variation trends of glycerol contentand the larvae's cold hardiness notes that Chilo suppressalis accumulated Vivo glycerolas a kind of cold resistance material to improve their cold-hardiness.7. content of Amino acid in the over-wintering larvaeContent of Serine and Alanine in the over-wintering larvae rises substantially in theearly winter and decreases significantly in the late winter. However, the changingdynamics of Methionine is exactly the contrary. It can thus be considered preliminary thatAlanine and Serine are important cold hardiness materials for the larvae during thewinter.8. Diapause characteristics and cold-hardinessThe larvae's diapause rate gradually increases with the decreasing of the hours oflight and temperature. In the same light conditions, The larvae's diapause rate graduallyreduces with the rise of temperature, when the temperature is 22℃, 25℃and 28℃,the critical light cycle is 13h35min, 13h31min, 13h25min respectively. The cooling-offpoint of the experimental population of non-diapause larvae is -9.12℃and the freezingpoint is -2.48℃, for diapause larvae, they are -9.95℃-2.69℃respectively, but thedifference is not significant. Stocks and field diapause larvae with a cooling point of-13.43℃and a freezing point of-4.67 is significantly lower than the two experimentalpopulation (P＜0.05). Hemolymph weight, Protein and Sugar content in the Diapauselarvae are significantly higher than non-diapause one, it means substances such as proteinand small molecule carbohydrates can increase the cold tolerance of Chilo suppressalislarvae. However, trehalose and glycogen has been maintained at a certain level, thismaybe can tell why there was no significant differences of cooling and freezing pointbetween the experimental population diapause larvae and the non-diapause one. |
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