Isolation, identification, and nematicidal activity of two milbemycin D isomers from Streptomyces bingchenggensis HU176

To explore novel milbemycin-type compounds with insecticidal activity, two isomers (1 and 2) of milbemycin D with nematicidal activity were isolated and purified from the secondary metabolites of Streptomyces bingchenggensis HU176 by gel column chromatography and high-performance liquid chromatography (HPLC). On the basis of spectroscopic analysis, including ¹H NMR, ¹³C NMR, and HRESI-MS, as well as comparison with literature data, their structures were identified as 12-desmethyl-12-ethyl milbemycin A₄ and 24-desmethyl-24-ethyl milbemycin A₄, respectively. Bioassay results showed that compounds 1 and 2 exhibited moderate nematicidal activity against Bursaphelenchus xylophilus, with LC₅₀ values of 14.53 and 16.89 μg/mL, respectively, which were comparable to that of the positive control avermectin B1a (LC₅₀ = 12.42 μg/mL). Compound 1 is a new milbemycin analog, and compound 2, a known compound, is reported as a natural product for the first time. The results of this study provide a valuable reference for the further development and utilization of such compounds.