A proposed new biological standard for bioassay of bacterial insecticides vs. Spodoptera spp.
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 Spodoptera. The high activity of B. thuringiensis subsp. entomocidus HD-635 against S. littoralis and S. exigua was found to fulfil these requirements. The spore-endotoxin 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, such as B. thuringiensis subsp. aizawai HD-133 and of a preparation with low activity, such as HD-1-S-1080, were carried out with reference to the newly proposed standard. Comparative studies revealed a possible reduction of the bioassay period by 4 days as time-saving and for a priori judgement on activity of certain microbial pesticides derived from B. thuringiensis.