Influence Of Distinct Simulated Overwintering Temperatures On The Population Of Laodelphax Atriatellus Resistant And Susceptible Strains To Insecticides

The small brown planthopper Laodelphax striatellus (Fallen) is one of the important pest insects of rice, by sucking nutrients from vascular tissue of rice, and transmitting rice stripe virus which results in huge economic losses. Winter becomes warmer, and the population of small brown planthopper bursts quite frequently in recent years. So in order to illustrate the relationships between overwintering temperature, insecticide resistance and the population outbreak in L. striatellus and improve the forecast precision, the effects of different overwintering temperatures:simulated warmer winter (4℃), simulated natural winter temperature (somewhat below0℃), constant28℃, and25/18℃respectively for day and nighttime on the development, reproduction and fitness of L. striatellus resistant and susceptible strains to different insecticides were systemically investigated and compared. The results were summarized as follows.After treated at different temperature, the chlorpyrifos resistant strain (ChR), buprofezin resistant strain (BuR) and the control susceptible strain (SS) all had low mortality (2%-14%) at each of their nymph instars, which was not significantly different from each other. But the development duration of ChR, BuR and SS was much different. After treated at constant28℃, three strains of L. striatellus at each nymph instar had significantly shorter development duration than treated at other temperatures, which indicated that it’s the most suitable temperature for the L. striatellus nymphs among the4groups of treated temperatures. After treated at the simulated natural winter temperature, the4th and5th instars of ChR nymphs had significantly shorter development duration than that after treated at25/18℃, and compared with SS, both the ChR and BuR showed significantly shorter nymphal development duration for whole nymphal stage or each instar after2nd.The results also showed that, after the3rd instar nymphs were treated at simulated natural winter temperature, the longevity (>48days) of developed female adults of ChR, BuR and SS was significantly longer than treated at other temperatures (<35days). Even after treated at same overwintering temperature, the fecundity profiles of both the resistant and SS adults were different from each other. After treated at simulated warmer winter temperature (4℃), the adult pre-oviposition duration and egg incubation duration of BuR were shorter than those of ChR and SS, but the oviposition duration of BuR was longest; and SS had the highest egg number. After treated at simulated natural winter temperature, the egg incubation duration of BuR was significantly longer than that of ChR and SS, but the oviposition duration, pre-oviposition duration and egg number had no significantly difference among ChR, BuR and SS. After treated at constant28℃, the pre-oviposition duration of ChR and BuR were shorter than that of SS, and the egg number of the both resistant strains was lower.For each of the susceptible strain and resistant strains after respectively treated at different overwintering temperatures, there were some significant differences in its fecundity. All the strains had shortest pre-oviposition duration and egg incubation duration after treated at constant28℃, but longest egg incubation duration after treated at simulated natural winter environment temperature. After treated at constant28℃or simulated warmer winter, the SS had highest egg number per female. The ChR showed lowest egg number per female after treated at every temperature treatment.Number of the female adults was less than the males out of each of the strains after treated at simulated warm winter or natural winter treatment, but more after treated at constant28℃or25/18℃. After overwintered at simulated nature winter temperature, the next generation of SS females had longer average longevity than that of other strain females, but there was no difference in survival nymph rate, adult emergence rate and copulation rate among the three strains. After treated at natural winter temperature, the relative fitness of both resistant strains (ChR,2.97; and BuR,9.58) was much higher than that of the SS, but the result was opposite after treated at other temperatures.For analysis of super-cooling points (SCP) and content water of susceptible strain and resistant strains of L. striatellus which reared in laboratory. The results showed that the SS had the lowest super-cooling point and freezing point (-22.19℃and-20.71℃), which was much lower than ChR’s (-19.83℃,-16.3℃) and BuR’s (-19.32℃,-16.86℃). But the natural population from fields had lower SCP than L. striatellus populations (SS, ChR, and BuR) reared in laboratory. And it also had lower freezing point (-22.54℃), which was significantly lower than two resistant strains but no difference from the SS. However, the SS, ChR, BuR, and the field population had no significant difference among their water contents in vivo. In summary, the field population and SS had stronger cold hardiness, which seemed somehow no connection with the water content of L. striatellus.