Response Of Northern Populations Of Panonychus Citri To Low Temperature Stress

Panonychus citri is an important pest in citrus orchards in southern China,and it does serious harm to citrus.Generally,in China,it only occurs in southern or northern greenhouses.However,in recent years,it was found in the open-air orchards of Qufu,Shandong Province that there have been outbreaks of damage to the Pyrus spp.year after year,causing serious damage to Pyrus spp..It was identified as Panonychus citri.In addition to harming Pyrus spp.,the mite also occurs on open-air fruit trees such as Cerasus serrulata and Malus spectabilis.There are obvious differences in outdoor temperature in the four seasons between southern and northern China.The temperature in northern China is significantly lower than that in southern China.Panonychus citri will suffer serious low temperature stress in northern China.In order to clarify the response characteristics of Panonychus citri to low temperature stress,and to provide a basis for predicting its occurrence dynamic,diffusion and distribution range in the northern China,the following three aspects have been studied:（1）The effect of low temperature stress on the growth,development and reproduction of Panonychus citri;（2）The effect of low temperature stress on the activity of antioxidant enzymes of Panonychus citri;（3）The effect of low temperature stress on the cold resistance of small molecules of Panonychus citri.The results of the study are as follows:1.Effects of low temperature stress on the growth,development and reproduction of Panonychus citriLow temperature adversely affected the growth,development and reproduction of Panonychus citri.The lower the temperature and the longer the stress time,the greater the impact.At 25 ℃ control,the hatching rate of Panonychus citri eggs was 96.67 %.In the low temperature range from 1 ℃ to-7 ℃,the hatching rate decreased with the decrease of stress temperature.Under-7 ℃ stress for 1 h,the hatching rate decreased to 83.33 %.With the increase of stress time,the hatching rate decreased significantly,and the hatching rate was only 56.67 % at-5 ℃ for 3 h.The same was true for the effect of low temperature stress on the survival rate of female adult mites.The average survival rate at 25 ℃ was 98.89 %.When the temperature dropped to-7 ℃ for 1 h,the survival rate dropped significantly to 64.44 %,The survival rate did not change significantly after 1 h and 3 h stress at-3 ℃ and-5 ℃.When the temperature decreased,the average lifespan of female adult mites was shortened.The average lifespan was 18.3 days under the control condition at 25 ℃,and only 5.7 days at-7 ℃ stressed for 1 h.With the temperature decreased,the number of eggs laid by female adult mites decreased.The average number of eggs laid per female at 25 ℃ was 61.4 and that was 36.2 when the temperature was-5 ℃ for 1 h.The female adult mites died before laying eggs when stressed at 7 ℃ for 1 h.As time increased,the average number of eggs laid by a single female decreased.Under the low temperature stress from 1 ℃ to-5 ℃,the survival rate of female adult mites was higher than that of eggs;but when the temperature was lowered to-7 ℃,the survival rate of eggs was significantly higher than that of female adult mites.The median lethal temperature of P.citri under low temperature increased with the increase of stress time,which were-11.6 ℃,-9.7 ℃ and-6.8 ℃ at 40 min,60 min and80 min,respectively,and about-5.5 ℃ at 120 min.The lethal time of low temperature decreased with the decrease of temperature,which were 68.1 min,57.3 min,52.9 min and51.1 min at-7 ℃,-9 ℃,-11 ℃ and-13 ℃,respectively.2.Effects of low temperature stress on the activity of antioxidant enzymes of Panonychus citriWhen Panonychus citri was subjected to low temperature stress,the activity of antioxidant enzymes increased to play a protective role.Superoxide dismutase（SOD）activity,glutathione-S-transferases（GSTs）activity,total antioxidant capacity（T-AOC）,malondialdehyde（MAD）content increased first and then decreased with the decrease of temperature in the stress temperature range of 1 ℃ to-7 ℃ for 1 hour,reached the highest at-3 ℃,significantly higher than the control at 25 ℃,and decreased to the lowest at-7 ℃,having no significant difference compared with the control.The peroxidase（POD）activity increased with the decrease of temperature,and reached the highest-7 ℃,which was significantly higher than that of the control.Changes in catalase（CAT）activity were not particularly significant.Comparing the activities of antioxidant enzymes under the same stress temperature and different stress times（1 h,3 h）,it was found that the SOD activity,POD activity and T-AOC capacity increased significantly with the increase of the stress time,but at-5 ℃,the activity of CAT,GST and the content of MAD decreased at 3 h compared with 1 h.3.Effects of low temperature stress on the content of small molecule cold-resistant substances in Panonychus citriUnder low temperature stress,the citus spider mite accumulated small molecule coldresistant substances such as trehalose and sorbitol in the body to improve cold resistance.After being stressed by low temperature（1,-1,-3,-5,-7 ℃）for 1 hour,the trehalose content first increased,then decreased and then increased with the decrease of temperature.The content of trehalose was the highest at-1 ℃,which having no significant difference compared with the control at 25 ℃.That was the lowest at-5 ℃,decreasing significantly compared with the control,and increased to the control level at-7 ℃.The trehalose content increased with the increase of stress time.Compared with 1 h,when the mites were stressed for 3 h,the trehalose content increased significantly at stress temperature-5 ℃ and not significantly at-3 ℃.The content of sorbitol first decreased and then increased with the decrease of temperature,which decreased to the lowest at-5 ℃,significantly lower than the control,and increased at-7 ℃,but still significantly lower than the control.It was found that the changes in the activities of antioxidant enzymes were basically opposite to the changes in the contents of trehalose and sorbitol under low temperature stress.When the activity of antioxidant enzymes increased,the contents of trehalose and sorbitol decreased,and when the activity of antioxidant enzymes decreased,the contents of trehalose and sorbitol increased.