The effects of the three components in sugar-acetic acid-ethanol mixture in trapping Grapholitha molesta (Busck)

Abstract: 【Objective】The bore-fruit pest, Grapholitha molesta (Busck), is widely distributed in the world.The larvae of this pest can attack fruits and tender shoots of the majority members in the family of Rosaceae including stone and pome fruits. Currently, the method based on insect pheromones has been widely applied to the bore-fruit pest control. However, there is a lack of monitoring and trapping for female individual as the pheromones only attract males. As a kind of eurytopic food bait, the mixture of sugar-acetic acid-ethanol can attract many insects and can be used for trapping and killing both the males and the females at the same time. For some pests, researchers studied the proportion of different components of the mixture of sugar-acetic acid-ethanol and its combination with other attractants. However, the roles of individual components including sugar, acetic acid, ethanol and water of the mixture has not been analyzed.Understanding the roles of each of the components and their ideal proportions is important to improve the trapping efficiency.【Methods】We utilized G. molesta to observe the effects of individual components in the mixture of sugar-acetic acid-ethanol and optimized their proportions. Sucrose, glacial acetic acid, eth-yl alcohol and running water were used as the reagents to prepare the mixture. Water-tray trap was selected as trapping device and random block design was used to locate the water-tray traps in the field. The experiments were conducted in a farm in the town of Beiwang in Taigu county(37°18'N, 112°29'E, 824 m),Shanxi province, China, in mid-August 2011 and mid-August 2012, when the third peak period of G. molesta adult moth occurred. In the first phase, the mixtures of sugar-acetic acid-ethanol were designed by orthogonal projects and a L9(34) orthogonal table was used. There were 9 proportions for treatment and water was used as the control. The catches of G. molesta moth were recorded daily. The results were analyzed with the method of range analysis to get the theoretically best proportion and the tested optimum proportion and to clarify which one was the main component. We made metric conversion of the proportions of sugar, acetic acid and ethanol with the bulk volume to explore the effect of different relative amount of the components on mean catches and peak trapping period of G. molesta moth. In the second phase, tested proportion was designed according to the theoretically best proportion and the tested optimum proportion obtained from the first phase result, and the coefficient of acetic acid/ethanol and dilution times were observed. There were 7 proportions and one control with only water in the water-tray traps. The catches of G.molesta moth were recorded daily in order to get the optimized the proportion of sugar-acetic acid-ethanol for trapping. We compared catch numbers of moth in the two phases of experiments, which used different water-tray traps. Traditional water-tray traps were used in the first phase and the mixture of sugar-acetic acid- ethanol was directly added in trays, and the improved water- tray traps were used in the second phase, where cotton balls were soaked the mixture of sugar-acetic acid-ethanol and hanged over the water in trays.【Results】In the first phase, the mixture of sugar-acetic acid-ethanol with proportions of 1∶2∶2∶160 and 1∶3∶1∶240 were better for trapping G. molesta with greater attraction compared with the other proportions. In the analysis of the catch results from orthogonal experiments with the method of range analysis, we observed the trapping capacity of the components in the mixture of sugar-acetic acid-ethanol in a declining order was sugar > acetic acid > ethanol > water according to the R range value and the theoretical best proportion of sugar : acetic acid: ethanol: water for trapping G. molesta was 1∶2∶2∶240 and the tested optimum proportion was 1∶2∶2∶160 according to the K value. We observed the effects of different relative amounts of the components on mean catches and the peak period of G. molesta and found that there was a close relationship between mean catches and sugar relative mass volume ratios(r=-0.561; F=3.946; P=0.087). The lower sugar mass/total volume was, the more catches were obtained, and the fitted model between them was a cubic curve(y=21.099-25.348x+10.628x2-1.38x3; R2=0.703). There was a close relationship between the peak attraction periods and ethanol relative volumewas, ratios(r=-0.625;F=6.624; P=0.034). The higher ethanol volume/total volume was the earlier the peak period occurred and the fitted model between them was also a cubic curve(y=19.251-24.549x+13.33x2-2.136x3; R2=0.799). In the second phase, the best proportion was 1∶2∶2∶80 for trapping G. molesta, which had the most effective attraction. The volume ratio of acetic acid /ethanol at 2:2(ethanol relative amount was higher) was more attractive to G. molesta than the volume ratio of 3:2(acetic acid relative amount was higher). When the other components had the same proportion, the more dilution times was, the fewer catches were obtained. We compared the catches of two different trapping devices and found that the device with a hanging cotton ball had more catches than the traditional device, which directly added the mixture of sugar-acetic acidethanol in the water-tray trap.【Conclusion】For luring G. molesta, sugar is the main component, acetic acid and ethanol are the secondary components, and water has the smallest role. Lepidoptera can forage forcarbohydrates in the adult stage. Sugar as a sweet bait can play a most important role in attraction and its relative mass volume ratio decides the catch number of G. molesta, which increased as the relative mass volume ratio of sugar decreased in a certain range. Sugar and other sweet bait can be converted to volatile products as acetic acid and ethanol through microbial glycolysis. Acetic acid may act as distant attractant and supplement of this component can increase moth catch. Ethanol serves as a carrier and helps odor release. Peak period can be advanced by increasing the percentage of ethanol. The effect of water is the smallest, and higher concentration of sugar, acetic acid and ethanol is propitious to trapping G. molesta.The trapping device of hanging cotton ball improves captures, saves chemicals and facilitates use compared with the traditional device.