Effects Of Sublethal Doses Of Insecticides On The Important Melon Pests

Melon is an important economic crop in China.It has good quality and taste,and is popular in consumers.However,there are many kinds of pests and serious harm,as well as the abuse of chemical insecticides in the melon planting process,which not only affect the quality of melon directly,but also pose a great threat to the environment and human safety.Thus,seeking effective pest control strategies has become an urgent problem in the melon production.In this dissertation,field surveys and questionnaires were conducted to investigate the current melon planting pattern and insecticide application condition in Guangdong Province,as well as the annual dynamics of melon pests.Then,the membrane method was selected to determine the toxicity of 5 commercial insecticides which were commonly used in the field against the important melon pests-Bactrocera cucurbitae（Coquillett）and Bactrocera tau（Walker）.Finally,the sublethal effects of insecticides on reproduction and ovarian development of the 2 species of fruit flies were studied.Our results will provide scientific information for using insecticides reasonably in the chemical control process of fruit flies.The main results are summarized as follows:（1）Investigation of melon planting pattern and insecticide application condition in Guangdong ProvinceIn August 2018,a survey of melon planting pattern and insecticide application condition was carried out in Foshan,Qingyuan and Zhanjiang city,the main melon producing area in Guangdong Province.Results indicated that the melon in Foshan and Qingyuan were planted in open field and the main melon variety was Xueli melon;and the melon in Zhanjiang was planted in the greenhouse and the variety was Donghai melon.During the investigation,it was found that there were many problems in the current melon planting process,such as the large number of pest species and serious pest damage;farmers couldn’t identify the pest species accurately;insecticides were mixed and abused during pest control process,and the application dose of insecticide was often 2-3 times of the recommended dose in the field.（2）Annual dynamics investigation of melon pestsResults showed that the common pests during the growing period of melon included Bemisia tabaci（Gennadius）,Aphis gossypii Glover,Liriomyza sativae Blanchard,Diaphania indica（Saunders）,B.cucurbitae and B.tau.During the spring-summer planting season,B.tabaci,A.gossypii and L.sativae occurred throughout the entire melon growth period（seedling stage,spreading stage,prophase of fruit,mid fruiting stage,and late fruiting stage）,D.indica only infested during the period from spreading stage to late fruiting stage.During the summer-autumn planting season,B.tabaci,A.gossypii,L.sativae,and D.indica existed throughout the entire melon growth period.And the fruit fly infestation was only observed during the mid and late fruiting stages,regardless of the spring-summer or summer-autumn planting seasons.Compared with the other pests,the amount of aphids per plant was significant higher,with the highest value up to 104 per plant.The amount of aphids per plant showed a gradual decrease during the spring-summer planting season,and gradual increase during the summer-autumn planting season.It’s worth noting that the harm of fruit flies to melon is devastating.Once the melon is attacked by the fruit flies,the edible value will be significantly reduced.（3）Acute toxicity of field-recommended concentration of insecticides against the 2species of fruit flyThe toxicity of trichlorfon,pyriproxyfen,abamectin+β-cypermethrin,spinetoram,and lambda-cyhalothrin at recommended field concentrations against the adults of B.cucurbitae and B.tau were determined under laboratory conditions.Results indicated that all the insecticides were highly toxic to both fruit flies,and the mortality of adults was 100%when they were exposing to the toxic films for 72h.Both B.cucurbitae and B.tau were most susceptible to abamectin+β-cypermethrin,the LT₅₀values were1.736 and 3.644h,respectively;followed by trichlorfon,lambda-cyhalothrin,pyriproxyfen,and spinetoram.（4）Sublethal doses of insecticides on 2 species of fruit flyIn addition,the sublethal doses of insecticides on the adults of B.cucurbitae and B.tau were determined.The LC₃₀values of Abamectin+β-cypermethrin,trichlorfon,lambda-cyhalothrin,pyriproxyfen,and spinetoram to adults of B.cucurbitae were0.024,0.026,0.055,1.865,and 0.028g/l,respectively;and the LC₃₀values of Abamectin+β-cypermethrin,trichlorfon,lambda-cyhalothrin,pyriproxyfen,and spinetoram to adults of B.tau were 0.019,0.005,0.036,2.186,and 0.020g/l,respectively.（5）Effect of sublethal dose of insecticides on reproduction of fruit fliesSublethal dose(LC₃₀)of trichlorfon and abamectin+β-cypermethrin had significant effect on the survival and fecundity of both B.cucurbitae and B.tau,after treatment with these insecticides,the survival time of females（B.cucurbitae:5.5-8d;B.tau:4.3d）and the number of eggs laid per female（B.cucurbitae:90-98.7;B.tau:24-33.4）were significantly lower than those of control.However,there was no significant effect of pyriproxyfen,lambda-cyhalothrin,and spinetoram on the survival and fecundity of B.cucurbitae or B.tau.Moreover,insecticide treatment during the adult stage of fruit flies did not affect the hatching rate of their eggs.（6）Effect of sublethal dose of abamectin+β-cypermethrin on the ovarian development of fruit fliesSublethal dose(LC₃₀)of abamectin+β-cypermethrin can significantly inhibit ovarian development of both B.cucurbitae and B.tau.After exposure to abamectin+β-cypermethrin,the ovarian length（B.cucurbitae:1.58-2.22mm;B.tau:1.92-2.67mm）,ovarian width（B.cucurbitae:1.09-1.34mm;B.tau:1.24-1.66mm）,and ovarian index（B.cucurbitae:1.823-2.912mm²;B.tau:2.38-4.405mm²）of fruit fly females were significantly lower than those of control.Compared with control females,significant fewer eggs were observed in the ovaries of female B.cucurbitae（24.9-31.9）and B.tau（30.6-59.5）after exposing to sublethal dose of abamectin+β-cypermethrin.Moreover,abamectin+β-cypermethrin treatment caused the ovary to stop developing earlier in fruit fly;and the ovary length,ovary index,as well as egg holding volume in the ovary of treated females reached the maximum values 4 days earlier than control.Based on the results,the abuse of chemical insecticides in melon planting process in Guangdong Province was very serious.Bactrocera cucurbitae and B.tau show were the main pests during the melon growing period.Trichlorfon,pyriproxyfen,abamectin+β-cypermethrin,spinetoram,and lambda-cyhalothrin were toxic to both B.cucurbitae and B.tau.Among them,trichlorfon and abamectin+β-cypermethrin can play a good control effect on the 2 species of fruit fly even under a sublethal dose(LC₃₀)conditions;therefore,these 2 insecticides can be used in the chemical control program of fruit flies during the melon planting process.