Efficacy of entomopathogenic nematodes (Nematoda: Rhabditida) against Culex gelidus (Diptera: Culicidae) larvae

Abstract

The breeding sites of Culex gelidus, a secondary vector of Japanese Encephalitis, are close to agricultural areas and homes, such as temporary and semi-permanent fresh ground water from pig farms and rice fields. Entomopathogenic nematodes (EPN) are an alternative bio-control for insects. Therefore, the application of EPN to control Cx. gelidus larvae was studied with the objectives of 1) determining the efficacy of EPN between 2 genera against 3^rd-4^th instars larvae of Cx. gelidus under laboratory conditions and 2) determining the dosages of EPN effective against Cx. gelidus larvae. The experiment was carried out in the laboratory under room temperature of 29 ± 2°C and relative humidity (RH) 70-80 %.

Results indicated that mortality rates of 3^rd-4^th instars Cx. gelidus larvae caused by Steinernema carpocapsae (Weiser) EPN were greater than Heterorhabditis indica (Local Thai strain) 63% and 13%, respectively. The mortality of both control groups was 5%. Infection rates between the 2 genera were 14.5% and 2%, respectively. The thorax of dead Cx. gelidus larvae were the site where EPN were mostly found, more than other parts of their bodies. Comparing mean difference for mortality rates of Cx. gelidus larvae between 2 genera at 48 and 72 hours post exposure found significant difference by T-Test (p-value < 0.05). S. carpocapsae (Weiser) kills more than 50% at dosage 2000 and 4000 IJs per larvae, but there was no significant difference in number of 3^rd-4^th instars larvae Cx. gelidus killed at either dosage. There was significant interaction between the 2 genera at the various dosages (p-value < 0.01, analysis by 2 way ANOVA).

The results showed that under laboratory conditions, S. carpocapsae (Weiser) EPN have potential as a bio-control against 3^rd-4^th instars Cx. gelidus larvae. Further study should involve water depth, temperature, pH of water and feeding behavior of target host prior to use in field trials.