A survey of soil microorganisms,with particular reference to the actinomycetes as sources of substances toxic to Heliothis virescens

Cultures of microorganisms isolated from cultivated soils were grown in submerged fermentation in test tubes kept on a rotary shaker at 28°C for 5 days using a fermentation medium composed primarily of soybean flour and dextrose. The beers were harvested and centrifuged, and the supernatants were incorporated at a dilution of 1:10 into artificial diets on which neonate larvae of Heliothis virescens were allowed to feed ad libitum for 7 days. If the beer killed or markedly retarded the development of at least 50% of the test larvae, the culture was regrown in shaken cultures in 500-ml Erlenmeyer flasks and assayed again. If a culture produced toxic beers in both fermentations, it was considered active. Over 2100 soil isolates were tested, of which an estimated 95% were actinomycetes. Slightly over 0.4% of the isolates were active, all of which, probably due to our selection procedure, were actinomycetes.