Potent insecticidal activity of two Streptomyces species isolated from the soils of the western ghats of Agumbe, Karnataka

Abstract

Background: The Western ghats, the range of hills running along India’s west coast, are well known for their rich and unique assemblage of flora and fauna. The present study was performed to evaluate the insecticidal potential of Actinomycetes isolated from the soils of the Western ghats of Agumbe, Karnataka. Methods: For isolation, the serially diluted soil sample was plated on Starch casein agar and incubated aerobically. The actinomycete isolates were identified by various parameters such as colony morphology, spore arrangement, staining, and biochemical reactions. The isolates were grown in Starch casein broth for seven days, the culture broth was extracted with butanol solvent and concentrated to get crude extract. Insecticidal activity of different concentrations of butanol extract of the isolates was determined against the second instar larvae of Aedes aegypti. The larvicidal effect, in terms of mortality of larvae, of the extracts was determined by counting the number of dead larvae after 24 hours. Results: Two actinomycete isolates were recovered from the soil sample and were identified as the species of Streptomyces on the basis of phenotypic, microscopic, biochemical, and staining characteristics. The colonies of the Streptomyces isolate 1 were creamish-white with yellow pigmentation and the spore arrangement was straight, whereas colonies of the Streptomyces isolate 2 were light grey with dark green pigmentation. The spore arrangement in isolate 2 was of the open loop type. Both the isolates were Gram-positive, non-acid fast, and caused hydrolysis of starch and casein. The insecticidal activity of different concentrations, namely 1.0, 2.5, and 5.0 mg/ml, of butanol extract of the Streptomyces isolates was tested against the second instar larvae of Aedes aegypti. The insecticidal potential of butanol extracts, in terms of larval mortality, was found to be dose dependent. Among the isolates, isolate 2 showed a marked insecticidal activity than isolate 1. At a concentration of 5 mg/ml, both the isolates caused 100% mortality of the larvae. At concentrations of 1 and 2.5 mg/ml, isolate 2 exhibited a stronger larvicidal activity than isolate 1. Conclusion: The insecticidal efficacy of the Streptomyces species might be due to the presence of active constituents in the extract. Isolation and characterization of active constituents from the butanol extract possessing insecticidal potential are to be investigated.