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A search for novel insecticides has yielded the milbemycins, a new family of macrolide antibiotics with insecticidal and acaricidal activity. They are produced in submerged cultures of Streptomyces hygroscopicus subsp. aureolacrimosus. Fermentation studies on the strain were conducted in shaken flasks and 30-liter jar fermentors.From the culture broth 13 milbemycins were purified to homogeneity by column and thin-layer chromatography on silica gel and alumina. Physico-chemical data, such as mass spectra, UV and IR absorption spectra, optical rotations and melting points of the milbemycins are described.In a search for novel insecticides, we discovered that the culture broth of a Streptomyces was active against acarus, harmful agricultural and horticultural insects such as aphids and larvae of insects of the order Lepidoptera.1) The Streptomyces is a new subspecies and is designated S. hygroscopicus subsp.aureolacrimosus. Taxonomic studies will be reported elsewhere.From the culture broth thirteen milbemycins were isolated and purified to homogeneity by column and thin-layer chromatography on silica gel and alumina. As reported briefly,2,3) the milbemycins have the 16-membered macrolide structure shown in Fig. 1.Subsequently the avermectins, isolated from the culture broth of Streptomyces avermitilis, were found to have structures closely related to the milbemycin as, differing only in being substituted at the C-13 position by an oleandosyl oleandosyloxy moiety.4,5)In this paper we describe the fermentative production, isolation and physico-chemical properties of the milbemycins. Materials and Methods Fermentation studiesCultures were grown in Erlenmeyer flasks on a rotary shaker (220 rpm, 7 cm) at 28°C. Inocula were grown for 2 days in 500-m1 Erlenmeyer flasks containing 50 ml of seed medium with the following ingredients (in g/1,000 ml water): glucose 20, soybean meal 10, corn-steep liquor 5 and NaCl 3. The pH of the medium was adjusted to 6.8 before autoclaving at 120°C for 20 minutes.Production cultures in 100-ml Erlenmeyer flasks, each containing 30 ml of the medium, were inoculated with 1 ml of the seed culture.Fermentations with the Streptomyces were also conducted * The antibiotics were initially designated as B-41 .

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