探討台灣分離之蟲生線蟲於有害昆蟲之致病力與其相關細菌之特徵

Abstract

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Entomopathogenic nematodes (EPNs) are known to be great biological control agents in IPM strategies. EPNs parasitize and kill their insect host with the help of their symbiotic bacteria capable of phase switching. Because nematodes are very sensitive to the environment, the application efficacy of EPN products often relies on the biology of the nematodes and the bacteria. In 2019, the insect pest fall armyworm Spodoptera frugiperda (FAW) invaded Taiwan and caused severe damage to rice, maize, and sorghum. With the urgent need for an environmental-friendly management strategy for organic or sustainable growers, this study aimed to obtain local EPN populations, identify and characterize the EPNs and their symbiotic bacteria, and further evaluate their parasitism efficacy and pathogenicity against FAW. Between 2019 and 2020, four EPN strains were isolated in Taiwan. The nematodes were identified as Pristionchus pacificus (strain 6) and Oscheius myriophilus (strains 16, G1A1, and G1B1), and 17 bacteria strains were isolated. Under laboratory conditions, three concentrations (2000, 3000, and 4000 IJs/ml) of EPN suspension were examined for application efficacy. As a result of the laboratory assay, the mortality rate of FAW was between 18-23 %, 16-18%, 11-15%, and 17-21% for applying strains 6, 16, G1A1, and G1B1, respectively. The FAW damage on leaves was 54-60%, 56-59%, 53-56%, and 53-56% for each EPN strain, respectively. No significant difference was found between the three concentrations applied. For the OLW laboratory assay, the mortality rates caused by the four EPNs were between 26-41%, 46-55%, 43-46%, and 49-75% for each strain respectively. Those results were highly superior compared to the FAW results. Such mortality rates have contributed to reducing significantly the damages on tomato leaves compared to the control. Further, a greenhouse assay was conducted to investigate the EPN management efficacy of a hand sprayer application method. For the greenhouse experiment, 2000 IJs/ml was used. The lowest mortality rates of FAW in greenhouse conditions were obtained for strains 6 and G1A1 at 5 and 8%, respectively. Strains 16 and G1B1 caused higher rates with 15 and 21%. For the assay on OLW, the death caused by EPNs was lower compared to FAW results. EPN-6 caused 2.5%, EPN-16 6.25%, EPN-G1A1 7.5%, and EPN-G1B1 caused 8.75% of OLW mortality rate. None of the EPNs applied have allowed to reduce the impact of insect pests on plants. Finally, the symbiont bacteria strains' phase variation and specific activity were examined in vitro. Symbiotic bacteria of the EPN-6 were all able to produce a second variant except for strain 6d. However, in all the symbiotic bacteria of EPN-16, only strain 16e appeared to have a phase variation. As for EPN-G1A1, the bacteria strain G1A1-4 was the only bacterium of this EPN without phase variation. Finally, both bacteria strains symbiotic to EPN-G1B1 were able to vary. None of the strains or variants were able to produce an antibiosis activity. For the lecithinase test, only strains 16a, 16b have shown a clear positive result. Other strains seem always positive for the second phase, but results needed further confirmation. All second phases have demonstrated both proteolysis and lipase capacities and the behaviors of swimming and swarming. Finally, bacteria strains without phase variation were all able to proteolysis, lipase, and perform swimming behavior. Strains 16a, 16b, 16c, and 16d, bacteria cannot swarm. This study has demonstrated encouraging results to apply EPNs as biocontrol, but the application method still needs to be improved to obtain a higher reduction of the insect pest population. This is also the first phase variation report for many EPN-symbiotic bacteria. Finally, their characterization has shown that most of those bacteria have the necessary capacities to support the pathogenicity of EPNs.