Assessment of a diketopiperazine, cyclo(Trp-Phe) from Streptomyces griseoplanus SAI-25 against cotton bollworm, Helicoverpa armigera (Lepidoptera: Noctuidae)
Microorganisms produce a range of metabolites with varying pest control properties. With this concept, we earlier identified 15 Streptomyces spp. strains with insecticidal activity against Helicoverpa armigera (Hübner), Spodoptera litura (Fabricius), and Chilo partellus (Swinhoe). In recent studies, we evaluated an insecticidal compound purified from the extracellular extract of S. griseoplanus SAI-25 by bioactivity-guided fractionation against H. armigera. Spectral studies by infrared (IR), nuclear magnetic resonance (NMR), and electron spray ionization mass spectroscopy (ESI-MS) confirmed that the purified compound was cyclo(Trp-Phe) of the diketopiperazines class. Cyclo(Trp-Phe) exhibited antifeedant (70 %), larvicidal (67 %), and pupicidal (59 %) action against H. armigera in a dose-dependent manner. The lethal dose for 50 % of the group (LD50) and LD90 values for larvicidal effect were 619 and 2750 ppm, respectively. In addition, the purified compound prolonged larval (10.3–11.1 days) and pupal (10.9–11.8 days) periods compared with the untreated control (larval duration 9.8 days, pupal duration 10.6 days). This is the first report on the presence and biological activity of cyclo(Trp-Phe) isolated from the genus Streptomyces.
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