Cold Hardiness And Overwintering Potential Of Different Temperature Strains Of Neoseiulus Barkeri

Neoseiulus barkeri（Hughes）is an indigenous phytoseiid mite commercially produced in China,which has the advantages of short developmental period and low mortality rate,and is important biological control agents.In order to improve its adaptation to high-temperature in the greenhouse or in summer,High Temperature Adapted Strain（HTAS）was obtained by repeatedly exposing the conventional strain to high temperature,to enhance its biological control efficiency in habitats with frequent high temperature stress.However,while it has been proved that the HTAS strain performed well under high temperature,little is known about how well it tolerates low temperature stress and successfully overwinter in Chongqing where high temperature events are frequent.Therefore,this study was conducted to investigate the supercooling point（SCP）,low temperature sensitivity and cold acclimation ability of the HTAS and Conventional Strain（CS）strains of N.barkeri.The water content,lipid,alginate and glycerol content,as well as SOD,POD and CAT antioxidant enzyme activities of the female adult mites of the two temperature strains were measured.The suitable distribution area of N.barkeri were predicted using the MaxEnt model.Outdoor investigation were performed to clarify the overwintering survival of the two strains of N.barkeri in Chongqing.The main results were as follows:1.Evaluation of the cold tolerance of HTAS and CS N.barkeriThis study showed that the SCP of the CS strain was significantly lower than that of HTAS.Both these two strains were cold sensitive,with the lowest SCP was found on diapause adult female mites（-24.72°C and-23.80°C for CS and HTAS,respectively）.The SCP varied significantly with the developmental stage of mites and whether the female was gravid or in diapause.The survival rate of female adult mites under low-temperature stress was significantly higher in the CS strain than in the HTAS strain.Both temperature and the duration under temperature had significant effects on the survival rate.The mortality of female adult mites increased significantly with the temperature became lower.Additionally,LT₅₀ and LT₉₅ in the HTAS strain were shorter than those in the CS strain,and Ltem₅₀ and Ltem₉₅ were higher than those in the CS strain.Moreover,cold acclimation improved the cold tolerance of adult female mites of both CS and HTAS strains.These results indicated that although the HTAS is more sensitive to low temperatures,it is assumed that both temperature strains of female adult mites can withstand low winter temperatures in Chongqing and have the potential to overwinter.2.Effect of low-temperature stress on the content of freeze-resistant protective substances and antioxidant enzyme activities of N.barkeriAfter experiencing low temperature stress,the water content of adult female N.barkeri of both temperature strains decreased with decreasing temperature,and the contents of fat,alginate and glycerol all increased significantly with decreasing temperature.It was speculated that water content,fat,alginate and glycerol are involved in the physiological response of N.barkeri to low-temperature stress.The results of antioxidant enzyme assay showed that low-temperature stress could induce the expression of antioxidant enzymes in N.barkeri in short term.The antioxidant enzyme activity of N.barkeri was inhibited by low temperature.3.Prediction of the suitability areas of N.barkeri based on MaxEnt modelThe MaxEnt model was used to predict the suitable areas of N.barkeri.The model AUC value was 0.916,indicating that the predictions were relatively accurate.The monthly mean diurnal temperature difference,driest month precipitation,warmest quarter mean temperature,wettest quarter precipitation and January mean minimum temperature had the greatest effect on the suitability of N.barkeri.The suitable areas of N.barkeri were mainly distributed in China’s central and southern regions.The total area of the suitable areas was about 2.59 million km²,accounting for 27%of the country’s total area.The predicted results showed that the northern and southeastern areas of Chongqing City were the low suitable areas for N.barkeri,and the rest of Chongqing is a less suitable area.4.Investigation on outdoor survival and overwintering of N.barkeri in Chongqing areaThe overwintering site of N.barkeri and its outdoor survival were investigated.The results showed that female adult mites of both temperature strains were found to overwinter successfully when food was adequate or food was offered regularly.However,they could not overwinter when there was no food available.In the control test,the survival rates of adult females of the two temperature strains of N.barkeri gradually decreased under the three food treatments.All adult female mites of the two temperature strains without food died on the 14th day of the survey,while those had access to food survived.The released N.barkeri on citrus seedlings was found to overwinter on the lower surface or axil of leaf,and the soil surface.The average number of surviving N.barkeri was 2.2 for the CS strain.The average number of surviving predatory mites was 0.2,for the HTAS strain.N.barkeri released on weed trays were found to overwinter on fallen leaves.The average number of surviving mites was 3.8 for the CS strain.In comparison,the average number of surviving mites was 0.8 for the HTAS strain.Both strains survived the winter by female mites.The results indicated that N.barkeri could successfully overwinter in Chongqing.In summary,this research evaluated the cold tolerance of two temperature strains of N.barkeri by determining physiological and biochemistry indexes such as supercooling point,low-temperature sensitivity,cold acclimation,anti-freeze protective substances,and antioxidant enzymes.Model prediction and simulation tests clarify that N.barkeri can overwinter in Chongqing.These results enriched our understanding of the adaptive mechanism of phytoseiid mites to low temperature,and also provided the scientific basis for improving the biological control efficiency of N.barkeri from the application level.