甜菜夜蛾抗寒与越冬能力研究

A study on the cold hardiness of the beet armyworm,Spodoptera exigua

研究了甜菜夜蛾不同虫态的过冷却点、耐低温能力、可能的越冬虫态及其越冬能力 ,并结合气象资料的分析 ,初步确定了我国甜菜夜蛾的越冬北界。甜菜夜蛾 3～ 5龄幼虫的过冷却点较高 ,分别为 -1 1 .1 7℃、-1 1 .96℃和 -1 0 .5 0℃ (平均 -1 1 .2 1℃ ) ,预蛹期次之 ,为 -1 2 .47℃ ,蛹期最低 ,平均为 -1 7.1 6℃。在 5℃、0℃、-1 0℃和 -5℃条件下 ,甜菜夜蛾各种虫态的 LT50 、LT90 和 LT99.9均随温度的降低而缩短 ,尽管不同虫态的耐低温能力有很大的差别。其耐低温能力由弱至强的排列顺序依次为卵 <成虫 <幼虫 <蛹。这些结果表明 ,蛹是甜菜夜蛾的四个虫态中耐低温能力最强的 ,因而是最可能的越冬虫态。但由于蛹在 0℃条件下的 LT99.9为 3 8.0 6d,表明甜菜夜蛾在冬季 0℃以下的温度超过 3 8d的地区不能越冬。另外 ,连续两年冬春的田间试验结果表明 ,蛹在北京西郊越冬死亡率均为 1 0 0 % ,说明甜菜夜蛾在北京地区不能越冬。根据这些结果并结合我国气温的变化规律 ,初步将我国甜菜夜蛾的越冬北界定于北纬 3 8°左右 ,即 1月份 -4℃等温线左右。

The beet armyworm Spodoptera exigua(Hb.) is a long distance migratory insect.Although the species does not have diapause state during winter,its long distance migratory ability enable it to attack crops in temperate regions.In recent years the damage area for the species have broadened every year in China,however the overwintering conditions to survive during winter have been questioned.The study was undertaken to understand how well the beet armyworm survives cold temperature,to do this,we determined the surpercooling points,degree of cold tolerance,field overwintering test and analyzed the geographic map for overwintering of the beet armyworm in China.The beet armyworm was provided by Hubei academy of agricultural science and reared in mass in laboratory at temoperature 28±1℃,70%RH,and photoperiod L16:D8.Larvae were fed with fresh maize seedings from larvae hatching to 2 instar,then transferred to glass tube at the density of 3～4 each tube and fed with artificial diets up to prepupal period,the moth were reared in pair in glass bottle of 850ml and fed with 5% honey(v/v)solution after eclosion.Supercooling points were measured with thermocouple instrument.The specimen were fixed to the thermocouple with plastic jaw.A thermocouple with an attached specimen was placed in low temperature box,the cooling rate was measured as 1℃/min.The data reflected the specimen temperature change then were put into computer,the cure diagram of temperature change was analyzed with computer and the data were analyzed by Duncan's multiple range test. Supercooling points of the beet armyworm were significantly different as the developmental stages varied. Pupae were with the lowest supercooling point ( -17.16℃), followed by prepupae (-12.47℃) and the 3rd to 5th instar larvae (-11.12℃ in average) in an increasing order, suggesting that pupae is the most possible stage for hibernation of the beet armyworm. The viability of various developmental stages in beet armyworm was measured at low temperature of -10℃,-5℃,0℃ and 5℃,respectively.Larvae and pupae were placed in a 12cm petri dish,About 15 larvae were placed in each petri dish,the bottom and top of petri dish were provided with filter paper for absorbing liquid materials from the specimen to prevent the possibility of inoculative ice nucleation and larvae escape.Eggs and adults were placed in a 850ml glass bottle,the bottom and top of glass bottle were provided with filter paper and 120 cohort gauze, respectively.The specimens at four low temperature were picked up every different period interval.About 1 000 eggs,45 larvae,30 pupae and 25 adults were picked up each time in a total of times.Survival of larvae and adults was determined 24h after recovering at temperature 20±1℃,70%RH,and photoperiod L16:D8.If specimens were able to move when they were prodded with forceps in the head,thorax and abdomen,they were considered alive.Survival of eggs and pupae was determined by hatch and emergence,respectively.Specimens were pretreated at 5℃ for 1h before subzero temperature(-10℃ and -5℃)bioassays were performed.The relationship between probit % mortality and natural log of exposure time was linear,indicating that probit analysis was appropriate for low temperature tolerance study.LT 50 ,LT 90 ,LT 99.9 are the period in days to 50%,90% and 99.9% mortality for the developmental stage of beet armyworm at low temperature respectively.Capacity to survive low temperature also differed greatly as temperature and developmental stage of the beet armyworm varied.Mortality increased quickly as the tested temperature decreased from 5℃ to -10℃ with an interval of 5℃,regardless of developmental stages of the insect.Time needed to reach a given moratality of the tested individuals(i.e.LT 90 )at all the tested temperature was usually shortened for the egg,adult,larva and pupa in an increasing order.The LT 99.9 for pupal stage were 11.0, 24.8, 38.1 and 67.9 days respectively as the temperature raised from -10℃to 5℃ with an interval of 5℃, while for any other devel