Cold Hardiness And Diapause Intensity Among The Overwintering Populations Of Chilo Suppressalis In Late-Rice Field

The rice stem borer, Chilo suppressalis (Lepidoptera, Pyralididae), is one of the most important rice pests in China. They are distributed in the paddy fields of North and South regions in China. The larvae can cause serious damage to the rice at different growth stages and cause serious impact on rice production, which results in heavy economic losses. The larvae of Chilo suppressalis overwinter by diapause, and overwintering is an important part of their life cycle. The numbers and the survival of the overwintering larvae in each year determine the extent of its occurrence in the following year. Also, the development of the overwintering larvae is inconsistent, and the adults occurrence with a feature of long period, multi-peak times after the winter. How about the cold hardiness and the diapause intensity among the overwintering larvae of Chilo suppressalis, and the impact on the development after winter? These problems are not yet clear, so it is necessary to study the problem from a new point of view.In this paper, based on the study of population structure, the overwintering larvae of Chilo suppressalis were divided into five groups according to the body weight:typeⅠ: body weight<35mg, typeⅡ:35mg≤weight≤57mg, typeⅢ:57mg≤weight<79mg, typeⅣ:79mg≤weight<100mg, and type V:body weight≥100mg. To reveal the cold hardiness, diapause intensity, the reproductive potential of adults, and the changes of the metabolism of different individuals, the supercooling points, days of pupation, numbers of impregnated eggs of adults, and the biochemical substances were measured. The main results are as follows:1. The population structure of the overwintering larvae of Chilo suppressalis in late-rice field. The results showed that, the 4～7th instar larvae can survive the winter, and the 5-6th instar larvae accounted for a major proportion, which were 40.4% and 40.0%, respectively. However, the proportion of the 7th-instar larvae was only 0.7%. The head capsule width of the overwintering larvae concentrated in the range of 1.1mm～1.5mm, accounting for 81.5% of the population. And the body weights were mainly in the range of 13mg to 77mg, accounting for 86.21%.2. The correlation of body size to supercooling points and water contents of the overwintering larvae of Chilo suppressalis in late-rice field. The results showed that, during the the winter, except on January 2009, the supercooling points of different individuals were significantly different. For example, on December 17 2008, the supercooling points ofⅠtoⅤgrade-larvae were-16.27℃,-13.34℃,-12.93℃,-11.36℃and-9.94℃, respectively. There was a significantly positive correlation between the supercooling points and body weight of larvae. In addition, during the winter, the water contents were maintained at about 50%, and which of the smaller individual was slightly higher than that of the larger individuals, but there was no correlation between supercooling points and water contents.3. The correlation of body size to diapause intensity of the overwintering larvae of Chilo suppressalis in late-rice field. The results showed that, at the temperatures of 15℃,20℃and 25℃(LD16:8), the larvae developed faster at higher temperature. At 15℃(transferred on November 21 2008), the average days before pupation of the larvae were about 140 d and that were only 42 d at 25℃. In the same month, the larger individuals pupated relatively faster than the samller ones. At 15℃(transferred on February 18 2008), the days before pupation of II to V grade-larvae were 81.38 d,82.06 d, 88.17 d and 101.00 d, respectively, and the differences is significant.4. The correlation of body size of the overwintering larvae of Chilo suppressalis in late-rice field to the numbers of impregnated eggs of adults. The results showed that, at the temperature of 15℃,20℃and 25℃, the weight of 3 day old pupae and the numbers of impregnated eggs of smaller individuals were significantly lower than that of the larger individuals, followed byⅡ<Ⅲ<Ⅳ<Ⅴ. At 15℃, the pupae weight of II to V grade-larvae was 31.8mg,46.1mg,64.5mg and 86.5mg, and the the average numbers of impregnated eggs of adults were 176.9,268.8,350.0, and 481.6, respectively.5. The changes and differences of the biochemical substances of the overwintering larvae of Chilo suppressalis with different body sizes in late-rice field. The results showed that, during the winter, the total lipids contents were relatively constant, maintained at 220μg/mg～280μg/mg fresh weight, and the contents of the larger individuals were relatively higher. The contents of carbohydrate (except on February 2009) and glycogen (except on December 2008 and March 2009) were significantly different among the larvae with different body sizes. In early winter, total sugar contents were 30μg/mg～55μg/mg, while only 10μg/mg～20μg/mg in later periods. However, the contents of trehalose were not significantly different among individuals, except on March 2009, which were also constant, maintained at 8μg/mg～20μg/mg. In addition, the levels of glycerol (except on January 2009) and protein were also significantly different among individuals. The levels of glycerol increased in the beginning of the winter utill January 2009, up to 30μg/mg, and then decreased. In early winter, the protein contens were 28μg/mg～55μg/mg, and reached to 40μg/mg～70μg/mg in late winter.The above research showed that, the cold hardiness, diapause intensity and reproductive potential of post-diapause female adults was related to body weight of the larvae of Chilo suppressalis. The larger individuals had higher supercooling points and deeper diapause intensity than smaller individuals in winter, and also the numbers of impregnated eggs of post-diapause female adults were higher. The contents of protein, trehalose, total lipids, glycerol, total carbohydrate and glycogen were also contributing to the above indexs.