| The grey tiger longicorn beetle, Xylotrechus rusticus L., is one of the most severe wood bores in northeast China and is listed as one of the national forestry quarantine pests in recent10years. The beetles mainly damaged the healthy and mature trees of Populus, Salix and Ulmus. Woods growth were seriously affected after larvae fed on the trunk, for which reason the damaged trees can be easily blown down and the safety of local populace were critically threatened. At present, this pest mainly occurred in high latitudes areas in China, however, the geographic distribution of X. rusticus had expanded to higher latitudes because of global warming. Generally speaking, temperature is a vital factor that limit the geographic distribution of insect. So it’s important to study the extreme temperature tolerance of X. rusticus for early warnning about potential distribution and its integrated control. Thus, based on the detailed observation of damage features, the present study aimed to determine the tolerance to extreme temperatures and the relevant physiological variations, as well as the change regulations of larvae microhabitat temperature during overwintering, thereby characterizing its limiting of tolerance to extreme low and high temperatures and the tolerant strategies, then to assess the suitable geographic distribution and its potential adaptive area in China. The main results are as follows:1. The detailed damage characteristics of X. rusticus in different development stage were systematically observed for the first time. After eggs hatched, the newly larvae would bore the trunk in cluster and leave a damaging feature of a chopped scar with mean depth of3.5cm; till the middle stage, larvae started to feed on phloem; the mature larvae would damage the xylem again in autumn and overwinter in the trunk mean depth of4.5cm until to the emergence of adults. Based on all above, a three dimensional gallery features internal the trunk were drawn refered to different damage stages, and the photos of corresponding damage features on the bark surface were also presented. 2. Thermal tolerance of X. rusticus adults and larvae were defined, and the ability for physiological adjustment that larvae reponse to high temperature stress were also revealed. The results showed that the critical thermal maximum (CTMax) ranged from38℃to39.5℃, during which most adults would lost the ability for of action and then went into shock state. All the values of CTMax were lower than the high lethal temperature of HLT50(44.87℃) after adults under thermal stress for2h, which implied that CTMax was not the upper lethal temperature for the beetles. However, the highest value of CTMax (41℃) was approximately the same with the HLT50(41.90℃) that adults were exposed to high temperature for4h, which indicated that CTMax would be the lethal damage temperature resulting50%mortality of adults if the time of exposure to the CTMax prolonged to4h. In addition, the HLT50(43.0℃)of larvae exposed to high temperature for24h was higher than that of adults (HLT50,37.79℃), which indicated that larva had stronger thermal tolerance than adults. It showed that larva of X. rusticus was the only developed stage over summer because of the long-term natural selection that the species in habitat, so the lethal temperatures of larvae in different time duration was more meaningful for assessing the potential distribution of X. rusticus. In addition, the body protein content of the two groups were both increased significantly when oversummering larvae stressed under35℃for4h and24h, respectively. It implied that there was much heat shock protein synthesized in larvae bodies, in order to progress physiological adjustment response to high temperature stress at35℃. While the body water content and lipid content did not change significantly, in other words, the two physiological substances could metabolize regularly at high temperature of35℃. Therefore,35℃is the endurable high temperature that larva can be tolerant through physiological adjustment. However, when larvae were exposed to high temperature of40℃, all the three physiological substances changed significantly, and maintained the similar level as temperature rising to45℃, though the body protein content and lipid rate decreased whereas the water rate increased. It indicate that oversummering larvae would loss the ability of physiological adjustment when the temperature rise to40℃or more.3. The cold hardiness of overwintering X. rusticus larva was demonstrated, and the physiological and biochemical indices were preliminarily revealed. The lower lethal temperature (LLT50) was respectively-33.64℃and-30.08℃under24h (short-term stress) and768h (long-term stress) whenX. rusticus overwintering larvae were exposed to different low temperatures. And the two values were far lower than the average (-11℃) and minimum (-14.7℃) of SCP, which indicated that the larva could survive after their body fluid freeze for a long time. It implied that X. rusticus belong to freezing-tolerant insect according to the traditional cold hardiness strategy classification. In addition, the similar variation tendency were showed between SCP and the environment temperature during the overwintering period, which implied that SCP could be used to assess the cold hardiness variation tendency during a sampling period and the cold hardiness differences between individuals in a population. However, SCP was not considered as the lower limit of lethal temperature of this species. The determination of physiological and biochemical indices showed:a. glycerol play an important role as major cryoprotectant for the accumulation in mid-overwintering stage and showed a significantly negative correlation with SCP (P=0.033, R=0.907); b. glycogen and lipid were considered as the important energy substances, which decreased obviously in mid-overwintering period, but only glycogen correlated positively with (P=0.006R=0.971); c. transformations between glycogen and glycerol(.P=0.046, R=0.885), as well as glycogen and mannitol (P=0.012, R=0.954) were showing negative correlations; d. water content of larva body changed little during the whole overwintering period, which implied that "Water-saving mechanism" may have been conducted to improve the cold tolerance; e. the protein content of larva body increased significantly in mid-overwintering period. It seemed that functional protein about cold resistance could be synthesized in larva body, which may be triggered by the low temperature.4. The change regulation of temperature in larva gallery during overwintering period was determined and clarified. The internal gallery temperature of poplar during January2012was detected by a micro-detector Auto-temperature recorder (1time per30min). The results showed that temperature changed inside gallery during January presented a significant feature of circadian periodicity and time delay; the minimum temperature of internal gallery (-39.58℃) was only0.3℃higher than that of the external gallery, and the average temperature of internal gallery in January was only0.227℃higher than that of the external, differences of both were not significant; in addition, the temperature difference between northern and southern internal gallery was0.15℃-1.85℃, averagely0.24℃, so the protection provided by the gallery to the larva was depended on the larger specific heat capacity of wood than air rather than the temperature of internal gallery higher than that of the external, this features of gallery enabled a lower change ratio of temperature of internal gallery, providing a protective buffer role for larva, and through this nature accommodation, the larva could improve their cold hardiness gradually.5. Highly and moderately adaptive distribution of X. rusticus were determined. The lowest and highest monthly mean temperature, within5years from2007to2012, were collected from January and July respectively, which were analyzed by meteorological data from43meteorological stations among China. The results showed that the highly adaptive area is northern, northwestern, southwestern China, and most part of northeastern China. The moderately adaptive area is Fujian, Zhejiang and Jiangxi province, most part of Jiangsu, Anhui, Hubei and Guangdong province, north part of northeastern China, southeast part of Henan province, and central part of Ejina County. There is no disadaptive area in China if take temperature as the only population restrict factor. |
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