| Harmonia axyridis (Pallas, 1773) (Coleoptera: Coccinellidae), native to a large part of Asia, is an important predatory lady beetle as natural enemies attacking agricultural and forestry pests such as aphids, scale insects, whiteflies, spider mites and so forth. During the last century, the lady beetle was introduced on several occasions into North America and Europe and had successfully colonized in several areas of both continents. At present, H. axyridis is already imported to many other major agricultural regions as an efficacious biological control agent. In natural habitats, H. axyridis overwinters as diapausing adults. A great deal of mortality could occur in overwintering time. Therefore, cold hardiness is a crucial biological factor effecting their population distribution, abundance and dynamics. In this paper, the response to cold stress and overwintering strategy of H. axyridis were investigated to reveal the internal adaptation mechanism to under low temperature on physiological and biochemical level. The main results were summarized as follows.1. In laboratory conditions, two developmental temperatures (18°C and 25°C) were used to induce a phenotypic variation in H. axyridis size and the relationships were analyzed among the adult size, fat reserves and SCP. Moreover, the effects of cold stress on survival of adults and the physiological mechanisms underlying chill injury were also investigated. The results showed that:(1) The adults grown at the lower temperature (18°C) were significantly larger than those reared at 25°C. The fat content was positively related to the dry mass of adults, indicating that larger individuals contained a higher proportion of fat. The SCP frequency distributions showed that the lower SCPs (﹣10°C～﹣9°C) were found in H. axyridis adults from the higher developmental temperature (25°C), whereas the higher SCPs (﹣8°C～﹣6°C) from the lower developmental temperature (18°C). The negative linear relation was observed between supercooling capacity and the adult volume.(2) A great deal of death of H. axyridis adults could occur at temperatures above the mean SCP. Otherwise, as temperature was lowered or duration of exposure was prolonged, cold injury became severer. Moreover, the cold survival of adults reared at 18°C was clearly higher than that reared at 25°C with the same duration of cold-exposure. These results indicated that energy storage was an important factor, but the vulnerability to chill-injury was supposed to be the primary factor regulating the survival of H. axyridis adults at low temperature.(3) The superoxide dismutase (SOD) and catalase (CAT) activities significantly increased after cold stress, indicating oxygen and hydroxide free radicals were probably efficiently detoxified at low temperature. It was a physiological adaptability under cold stress. The lactate dehydrogenase (LDH) and Na+, K+-ATPase activities markedly decreased underlying chill injury. It was suggested that the nature of chill injury might be a complex of metabolic disorder and a non-proportional decreased in enzymatic reaction and transports.2. The adults were interrupted daily for 2 h by placing the lady beetle at 5°C, 15°C, 25°C and 30°C when they were exposed at constant low temperature﹣5°C and the survival varied depending on the interruption temperature. There was a positive effect of temperature during the interruption interval on survival, increasing from 5°C to 25°C. However, the lady beetles were exposed at higher exposure temperature (30°C) had less survival, between the values found at 5 and 15°C. Otherwise, when the lady beetles were exposed at﹣5°C and then interrupted daily for 0.5 h, 1 h, 2 h and 4 h by placing the lady beetle at 25°C, the effect of high-temperature interruptions was increased significantly with increasing interruption duration (0.5﹣2 h), but there was no further increase with a 4 h interruption interval. When the interruptions were applied every second day, the effect on survival was slightly less than when they were applied daily, but were still strongly positive. When the interruptions were applied every third day, the survival was virtually the same as at﹣5°C. The effect of high-temperature interruptions on Lt50 was the same as the survival. It was clear that the cold survival of adults markedly increased when exposed to fluctuating thermal regimes (FTRs, cold-exposure interrupted by periodic short pulses at high temperature), compared with those maintained under constant low temperatures (CLT). The phenomenon, exposed to FTRs improving cold survival, may have an application in long-term cold storage of natural enemy insects.3. Complexity of the cold acclimation response in H. axyridis was investigated in the laboratory condition. The results showed that:(1) The adults increased their resistance to cold stress when they were successively acclimated at 5°C. The survival was improved from 46% (without cold acclimation) to 60% and 67% when adults were acclimated for 3 d and 5 d, but the effect of cold acclimation was lost within 7 days after returning lady beetles to 25°C. The SCP, water content and fat content decreased as the duration of cold acclimation prolonged. Short-term (5, 10 d) exposure to 5°C did not markedly decrease the SCP even though it significantly decreased water content and fat content in the body.(2) Both costs and benefits could be associated with the process of acclimation, the pre-oviposition period of females was delayed and the fecundity decreased after cold acclimation. The effects of acclimation were not just realized within the parental generation but also extend across generations. The fecundity and life span of offspring from acclimated parents were significantly decreased, but the acclimation did not affect development durations of offspring. In addition, based on age-stage, two-sex life tables, the intrinsic rate of increase (r), the finite rate of increase (λ), the net reproduction rate (R0) and the age-specific survival rate (lx) of offspring significantly decreased whereas the proportion of females increased.(3) Cold acclimation is a complex physiological and biochemical process, which is very important to the survival and reproduction in H. axyridis. The superoxide dismutase (SOD) and catalase (CAT) activities increased whereas the lactate dehydrogenase (LDH) and Na+, K+-ATPase activities decreased after cold acclimation. Glucose and mannitol were highly significantly accumulated whereas sorbitol and trehalose exhibited the lower amounts.4. The cold tolerance of the natural population and effects of overwintering adult aggregation on energy metabolism were investigated. The results showed that:(1) The cold tolerance of natural population of H. axyridis adults varied obviously with seasons. The overwintering adults had higher tolerance to lower temperature than the summer and post-winter adults. Summer adults with cold acclimation (5°C / 5 d) before the occurrence of low temperatures showed higher cold tolerance. The SCP and water content of overwintering adults were significantly lower than those in other seasons. However, the total fat contents of adults were highest at the beginning of overwintering.(2) The adults of H. axyridis aggregate significantly during overwintering. We have directly measured the respiration with different aggregation degrees and calculated the rates of respiration (Rr) and the rates of energy metabolism (Rm). There was a negative correlation relationship between the values of Rm/Rr and the aggregation degrees. As aggregation degrees were increased, the values of Rr and Rm became lowered. These results indicated that aggregation of H. axyridis can lower the metabolic rates, which may be advantageous for their successful overwintering. |
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