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- Efficacy of entomopathogenic nematodes against the coconut rhinoceros...
Efficacy of entomopathogenic nematodes against the coconut rhinoceros beetle larva, (Coleoptera: Scarabaeidae) in Nakhon Si Thammarat province, Thailand
Dissertation Abstract:
The coconut rhinoceros beetle, Oryctes rhinoceros, is a major coconut insect pest in Thailand. This study was conducted to determine the efficacy of five species of entomopathogenic nematodes (EPN) namely, Steinernema carpocapsae All, Steinernema glaseri, Steinernema siamkayai, Heterorhabditis indica EPNKU80 and Heterorhabditis bacteriphora against first instar larvae of O. rhinoceros. Mortality of O. rhinoceros larvae varied significantly among the nematode concentrations used and times of observation in the bioefficacy experiments, indicating that as the concentration increases, so does the susceptibility of the insect. H. bacteriphora at 2,000 infective juveniles (IJs)/larva caused significantly higher mortality rates to when O. rhinoceros larvae as compared to other nematode species at 7, 14, and 21 days after application (DAA). Based on the LC50 value, H. bacteriphora was the most virulent with only 577.32 IJs required at 21 DAA; whereas 793.44, 799.39, 957.74 and 3567.99 IJs were required for H. indica EPNKU80, S. glaseri, S. carpocapsae All and S. siamkayai, respectively. Similarly, H. bacteriophora showed significantly (P<0.05) higher mortality rates compared to S. glaseri when the first instar larvae of O. rhinoceros were treated for 4 weeks during April to June 2021 under two field site conditions. This study suggests that H. bacteriophora has a better potential to control O. rhinoceros larvae under field conditions. In addition, the persistence and reproductive ability of EPN are important factors to consider in insect biological control agents. Therefore, the present study also compared the persistence and productivity of these EPN isolates in Galleria mellonella larvae. The productivity among the EPN significantly varied among the nematode species. Generally, Heterorhabditis species produced more infective juveniles (IJ) than Steinernema species in the cadaver. The IJ production was highest in H. bacteriophora but was not statistically different from H. indica EPNKU80. In contrast, the lowest number of IJ was produced by S. carpocapsae All. The persistence test showed that H. bacteriophora had the highest penetration ability and had the greatest number of EPN produce inside G. mellonella larvae at all exposure times than the other EPN species. In addition, the highest efficacy was observed in H. bacteriophora (84.40%) at 15 days after application, which was statistically different from S. glaseri (63.30%), S. siamkayai (59.10%), H. indica EPNKU80 (58.50%), and S. carpocapsae All (19.30%). Heterorhabditis species have higher soil persistence and higher IJ reproduction than Steinernema species, hence, can be used for insect pest control.