Testing method of pesticide on Grapholitha molesta (Busck) neonate larvae

Abstract:【Objective】Grapholitha molesta is a destructive pest of many fruits. Although difficult, chemi⁃cal pesticide spray is still the most common way to control the pests. There are serious problems in the current testing methods which are aimed at the adults and eggs, for instance, more dosage and longer timesare needed to effectively control the pests. We had proved that G. molesta neonate larvae were more sensi⁃tive to chemical pesticides than the adults and eggs. Neonate larvae should be one of the key methods ofchemical control. But currently there is no toxicity testing method to measure the efficiency to G. molestaneonate larvae because of the high mortality of neonate larvae crawling and gnawing on apples after hours.This paper aims to study a way to measure the pesticide efficiency with fewer errors.【Methods】In order tostudy the control efficiency determination method of pesticide to G. molesta neonate larvae, the survivalrate and damage rate of G. molesta neonate larvae was measured by using different developmental states ofthe pests and the apple fruit. The pests included ready-to-hatch eggs and neonate larvae, the apples in⁃cluded fruitlet (d=3.5-4.0 cm), ripening fruit (d≥7.0 cm) and cut-ripening fruit. The neonate larvae andlarvae were fed in a artificial intelligence incubator with (25±1) ℃, 3 000-4 000 lx light, D∶L=15∶9. Thenumber of eggs laid on the apples was 13 or 14 eggs per fruitlet, 40 eggs per ripening fruit or 20 eggs percut-ripening fruit. The number of neonate larvae laid on the apples was 10 neonate larvae per fruitlet, 15neonate larvae per ripening fruit or 30 neonate larvae per cut- ripening fruit. The survival rate was sur⁃veyed in 14 h after laying the eggs or 8 h after laying the neonate larvae. The damage rate was surveyed 5d after laying the eggs or neonate larvae. Damage symptoms and the growing development of the G. moles⁃ta was observed with an Olympus stereoscope in 10 h, 14 h, 18 h, 30 h, 54 h, 78 h and 5 d. The surveymethod was determined by carefully observing pest morphological characteristics and damage symptomsof G. molesta on apples and the damage rate at different times. The pesticides included λ-Cyhalothrin andChlorantraniliprole. 96.0% λ-Cyhalothrin and 95.3% Chlorantraniliprole were dissolved in propanoneand 5 different series concentrations were diluted with water. That was done as follows: washing and dry⁃ing apples were dipped into a solution for 5 min, pulled out and the solution was removed with filter paper,then laying eggs or neonate larvae were applied. The borers were investigated in 30, 54, 78 h and 5 d. Thenumber of borers was the same as the number of holes 30 h and the number of borers was the same as thenumber of traces at 54 h and 78 h. Experimental data were analyzed with Duncan’s new multiple rangetest statistical methods.【Results】The survival rate of G. molesta neonate larvae was＞90% and its dam⁃age rate was 76.81%-77.18% using ready-to-hatch eggs and apple fruitlets. The borer rate was 76.80%,77.18% laid eggs on fruitlets and cut-ripening fruit and greater than 62.60% laid eggs on ripening fruit.The borer rate dropped to 65.56%, 53.32% and 56.68% from 76.80%, 62.60% and 77.18% when G. mo⁃lesta neonate larvae was laid on the fruitlets, cut-ripening fruit and ripening fruit, respectively. The control efficacy of λ-cyhalothrin to G. molesta neonate larvae using cut-ripening fruit was higher than ripen⁃ing fruit and fruitlets. The control efficiency of 3.00 and 4.00 mg·L^-1 λ-cyhalothrin to G. molesta were21.88%, 22.38%, 26.04% and 31.25%, 31.33%, 35.42% for the laid eggs on fruitlets, cut-ripening fruitand ripening fruit respectively, and there were no significant differences among similar concentrations.The control efficiency of 5.00 and above λ-cyhalothrin to G. molesta were 48.98%-98.96% on cut-ripen⁃ing fruit which was higher than the 38.80%-97.01% on fruitlets and ripening fruit. Damage symptoms ofG. molesta larvae changed constantly with prolongation of time after laying ready-to-hatch eggs on the ap⁃ples. The number of traces of endangered pests was the same as the number of living G. molesta larvae in54 h and 78 h after laying ready-to-hatch eggs on the apples. G. molesta larvae was affirmed as dead aftershowing no action after being touched or not moving freely in 14 h after laying ready-to-hatch eggs on theapples or in 8 h after laying neonate larvae on the apples. The damage rate was investigated in 30 h and54 h after ready-to-hatch eggs laid on apple based damage symptoms. The holes which had adhering fra⁃ss and no traces of endangered pests in 54 h and 78 h after laying ready-to-hatch eggs on the applescould not be counted as borers. G. molesta larvae still died even if bored into the apples, particularly inpesticide treatments. The G. molesta larvae could continue to damage and grow when the individuals livedin 54 h after laying ready-to-hatch eggs on the apples. The control efficiency of 3.33 and 5.00 mg·L^-1 λ-cyhalothrin to G. molesta were 19.19% and 33.33% in 30 h and were significantly lower than 23.96% and40.63% in 54 h and after. So 54 h after laying ready-to-hatch eggs on the apples is reasonable surveyingtime of the toxicity test for G. molesta neonate larvae. The control effect of Chlorantraniliprole to G. moles⁃ta neonate larvae increased with the increase of the concentrations using the above methods and measuredparameters. Its corresponding linear equation was y=2.148 4x+1.367 8, R² was 0.937 1.【Conclusion】Us⁃ing the methods of immersion fruitlets and inoculation ready-to-hatch eggs, the survival rate of the controlled processing was no less than 90% and its damage rate was no less than 70%, the number of borerswere determined according to pest damage symptoms. The above methods and parameters could test thecontrol efficiency of G. molesta neonate larvae. It needs to be validated whether this control efficiencymethod was appropriate for measuring the toxicity test of G. molesta neonate larvae.