Evaluation of the biocontrol capacity of slow-release methyl salicylate by mediating abundance of predatory ladybirds in orchards

Abstract: 【Objective】Utilizing semiochemicals to mannipulite the foraging behaviors of natural enemy is avital measure to control pests. As a synergistic means, semiochemical solw-release is gradually popularized and applied in biological control management in the modern agricultural green protection system. Methyl salicylate (MeSA), is a kind of semiochemical, which is widerly released in the crops infected by herbivorous insects, and it can also attract ladybirds and other natural enemies from a long distance. As an effective compound, the MeSA is commonly used as an attractant for predatory ladybirds. The study aimed to clarify the regulation effect of the MeSA released by new sustained-release materials on predatory ladybirds and the control effect on the aphid, Aphis citricola in apple orchards. Compared with traditional pesticides, slow- releasing semiochemicals are efficient, non-toxic, non- resistant and harmless to the natural enemies. The slow releasing material is the major factor that affects the pest control effienicy of semiochemical in the field. At present, the main slow- releasing materials include gels, photosensitive compounds and lures, and some products have been commercialized. In our presentwork, we selected a commercial slow-release lures material, PE tubes, to test its stabilization for longterm slow- release, and whether it can maximize the effect of the MeSA on attracting the natural enemies to control the aphids.【Methods】We consecutively investigated the population dynamics of A. citricola in the apple orchards for two years to determine the use time of the MeSA. In the treatment area, the slow-releasing MeSA PE tubes were suspended evenly in 5 × 5 design. While the orchards without pest control and the orchards with chemical control were used as controls. The population dynamics of the predatory ladybirds and A. citricola were investigated and detected for the release rate of the MeSA. And the effects of the MeSA on the spatial distribution characteristics of the predatory ladybirds and A. citricola were analyzed by five commonly used aggregation index parameters. For ensuring the field application time of the MeSA PE tubes, a five points method was used to investigate the occurrence dynamics of the A. citricola and natural enemis in the orchard from May to July in 2020 and 2021. Two fruit trees were selected at each point, each tree had two branches fixed in five directions: east, south, west, north and middle, in an orchard in Changping District, Beijing (116°02′ E, 40°10′ N). The number of the A. citricola and predatory ladybird were investigated once a week. For investigating the effects of the slow-releasing MeSA on the dynamics and spatial distribution of the predatory ladybird and A. citricola, three apple orchards were selected as methyl salicylate slow-release area, chemical pesticide area and blank control area in 2022. The orchard with MeSA PE tubes; the orchard with imidacloprid insecticides; the orchard without chemical insecticide and the Methyl salicylate PE tubes were used as control. In each orchard, 25 fruit trees were investigated, the number of the A. citricola and predatory ladybird were investigated once a week. The slow releasing rate of the MeSA PE tubes in the field was monitored. The temperature was automatically recorded every two hours, and the weights of the PE tubes weighed every week. The release of the MeSA PE tubes at the initial stage and 5 weeks after the field test was detected, respectively. The 100 µm PDMS extraction head was inserted into the sampling bag containing the MeSA PE tubes to extract for 30 min, and then GC-MS detection was performed. The data were processed by the statistical software SPSS 23 and Excel. The investigation results of the A. citricola and predatory ladybirds were used to caculate the aggregation index and spatial correlation analysis.【Results】The occurrence peak of the A. citricola in the apple orchards was found from the end of May to mid-June. The number of predatory ladybirds in the treatment area was significantly higher than that in the chemical control area, and the number of the A. citricola was significantly reduced by applying the slow- releasing MeSA PE tubes one week before the occurrence peak. The spatial distribution showed that the A. citricola was in marginal aggregation distribution in each apple orchard. In addition, the release rate of the slow- releasing MeSA PE tubes was about 0.68 g per week. The results for two consecutive years in the apple orchard showed that the large-scale occurrence or peak occurrence of the A. citricola and predatory ladybirds was found between the end of May and the middle of June, and the appropriate time point of setting the MeSA PE rubber was May 30. The release rate of the MeSA was about 0.68 g per week. After 5 weeks of application, the remained effective MeSA could still be detected by GC-MS. After two weeks of utilization of the MeSA PE tubes, the number of apple aphids began to decrease, in the third week, the number of predatory ladybirds were significantly higher than theose of the others treatment areas. The aggregation index analysis result of the A. citricola showed that the aphids were clustered in the treatment area and control area. The spatial correlation analysis result of the predatory ladybirds and A. citricola showed the much higher distribution density of the ladybirds in the MeSA PE tube treatment area, the much lower occurrence density of the A. citricola than those in the others areas, and the marginal distribution pattern was more obviously.【Conclusion】The slow- re-leasing MeSA PE tubes could last for more than 4 weeks in the orchard, and increase the population of the predatory ladybirds to reduce the density of the A. citricola in the apple orchard. The application of slow-releasing MeSA PE tubes resulted in the highest density of the A. citricola on the edge in the apple orchard. The pest control effectness on the A. citricola was formed by regulating the spatial density of the predatory ladybirds, leading to the number reduction and the marginalization distribution range of the A. citricola. The application of green prevention and control technologies, such as semiochemicals slow- releasing technology, would reduce the population density or damage degree of the pests and increase the number of the natural enemies. The MeSA PE tube treatment had the equivalent control effect as imidacloprid treatment on the A. citricola, and more predatory ladybirds were attracted and conserved, which is conducive to improving the ecological stability of the orchard. Considering the slow release and storage capacity of materials, as well as the requirements for temperature, humidity and other external conditions, our results confirmed that PE tubes could be suitablly used as slow-releasing carriers of the MeSA, with long-term, economic and excellent control effects in the orchards in northern China. In addition, we also discussed the impact of the MeSA on the distribution of the pests and natural enemies, which would enrich the theoretical basis for the field application of semiochemicals and promote the application of related products. In our future work, we will expand the application area of the MeSA tubes and, test its release efficiency and control effect under different temperature and humidity environments, and also pay attention to the slow-releasing effect of PE tubes on other semiochemicals.