Studies On Cold Hardiness Of Galeruca Daurica Joannis (Coleoptera Chrysomelidae) And Its Mechanisms

Galeruca daurica Joannis began to outbreak seriously since2009and has caused great losses in the Inner Mongolian grasslands. Its occurring region has expanded constantly and the damage has become more severe, which has greatly hindered the sustainable development of grassland animal husbandry and ecological safety in Inner Mongolia. Lower temperature is a key factor that affects the development and survival of the insect and its over-wintering survival mainly depends on its tolerance to low temperature in winter. Understanding of cold hardiness and its mechanisms is helpful to forecast the distribution and population dynamics. Therefore, we investigated the cold-hardness of G. daurica and its machenism. The major results as follows:1. The supercooling capability of different developmental stages and over-wintering eggs in different seasonWe used the thermocouple method to measure the supercooling points in various developmental stages in the laboratory. The result showed that the supercooling points had significant differences among different developmental periods and the order from low to high was egg (-29.8℃),1st instar (-14.6℃),2nd instar (-13.3℃), pupa (-12.1℃),3rd instar (-10.2℃) and adult (-9.0℃). The SCPs of eggs in December and January were the lowest, and the highest in February.2. Lethal temperature and lethal time of larvaeWe determined the survival rates of larvae exposed to different low temperatures (-6--14℃) for2h or exposed to-5℃for different periods (0.5-8d). The results showed that the survival rates of larvae declined with the decrease of treatment temperature and extend of treatment time. The half lethal time of individuals (Ltimeso) at-5℃was3.84d,3.80d and2.28d for1st instar,2nd instar and3rd instar, respectively, and the lower lethal temperature needed to kill50%of individuals (Ltempso) after2h exposure was-10.1℃、-9.1℃and-8.5℃, respectively, higher than their corresponding SCPs. It showed that with the developing of larvae, the cold hardness of G. daurica decreased and larvae was chill susceptible3. The effect of low temperature on supercooling point and developmental duration and mortality rate of larvae The SCPs of1st and2nd instar (the1st instar developed to2nd instar) were determined after the1st instar were exposed to-6℃and-10℃for2h. The results showed that the SCP of the1st instar after treated at-10℃for2h was significantly lower than those at-6℃for2h and control, but there was no differences when the1st instar developed as the2nd instar.When different cold exposures occurred at the1st larva, the lower the treatment temperature and the longer treatment time, the higher the mortality of larvae. The effect of low temperature treatments of1st instar for2nd instar,3rd instar and pupa on the mortality decreased gradually. When different cold exposures occurred at the1st instar, there were no differences in development time between the2nd,3rd instar and pupa. However, the development time of1st instar and total duration were greatly extended as compared to that of the non-treated beetle.4. The relationships of the content of water and fat in body with supercooling pointThe water contents of egg showed the trends of seasonal changes.The water content of eggs in over-wintering stage was significantly lower than that in the non-overwintering. The water content of eggs’was significantly lower than those of the other developmental stages except adult. There was no significant relationship between the water countent and supercooling point in different developmental stages. There was no obvious difference in the fat countents between different developmental stages, but the seasonal change was obvious in eggs and the content increased with the local temperature declining. The fat content in over-wintering eggs was obviously higher than those in the non-overwintering eggs. The highest fat content was in December. There was a significant negative correlation between the fat content and the SCP of eggs.5. The relationships of the contents of amino acid and low-molecular-weight polyols and sugars in body with supercooling pointContent variation of amino acid in the3rd instar, pupa and adult body were measured by an automatic amino acid analyzer. Relationships between these substances and SCP were analyzed. The results showed that the contents of GLY. HIS, TYR and GLU had a significant negative correlation with theSCPs, and therefore, there was a close relationship between the changes of these five kinds of amino acids and cold-hardiness. Experiments were carried out to examine the relationship between the contents of low-molecular-weight polyols and sugars and the SCPs in different developmental stages by High Performance Liquid Chromatography. The results showed that there was a negative correlation between the contents of Glycerin and Sorbitol and the SCPs. Glucose had a significant positive correlation with the SCP.In summary, the egg SCP of G. daurica was the lowest. Ltempso was higher than the SCP of larvae so that the larvae can be considered as chill-intolerant. The cold hardiness of larvae could be improved and the1st instar and total durations were significantly extended by low temperature treatment, but in creased the mortalities in larval development process. There was a close relationship between the contents of biochemical substances and the SCP:with the increase of content of water and Glucose, the SCP was raised; with the increase of content of fat GLY, HIS, TYR, GLU, Glycerin and Sorbitol, the SCP was reduced. Thus, cold hardiness was affected.