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Abstract
The marked resistance of Spodoptera spp. to endotoxins of Bacillus thuringiensis including the well recognized standards HD‐1–1971 and HD‐1‐S‐1980 has necessitated the search for a new active strain that can be used as a standard reference in the bioassays of these lepidopterous insects. The high activity of B. thuringiensis var. entomocidus HD‐635 vs. Spodoptera littoralis and S. exigua was found to fulfill these requirements. The sporeendotoxin complex of this strain, designated as HD‐635‐S‐1987, is proposed as a reference standard for bioassays against Spodoptera spp. and is assigned arbitrarily the value of 10,000 iu/mg. Potencies of an active strain, i.e. B. thuringiensis var. aizawai HD‐133 and of a low‐active preparation, i.e. HD‐1‐S‐1980 were carried out in reference to the newly proposed standard. Comparative studies revealed the possible reduction of bioassay period to 4 days as time saving and for a priori judgement on the activity of certain bioinsecticide preparations derived from B. thuringiensis.