H. ABOU-BAKR, F.F. SHALABY*, M.M. ASSAR* AND M.F. FAHMY
Biological Control Research Laboratory, Fac. Agric, Cairo Univ., Cairo, Egypt
* Plant Protection Department, Fac. Agric., Benha Branch, Zagazig Univ., Banha, Egypt

ABSTRACT

On the basis of their pathogenicity to the 1st instar larvae of Spodoptera littoralis (Boisd.), 35 varieties of Bacillus thuringiensis (Berliner) were classified into 4 groups: a) 11 highly effective strains (caused >75% mortality); b) 5 moderately effective (50-75%); c) 9 weak (20-50%) and d) 10 ineffective (caused <20% mortality). B. thuringiensis toiworthi (HD-301) seemed to be the most effective strain tested, causing 100% mortality after 3 days. Two other strains of toiworthi (HD-537 and 125) caused 100% mortality after 5 days of exposure. Three strains, namely darmstadiensis, wushansis and aizawal (HD122) caused also 100% mortality, but after 7 days of treatment.
Toxicity values were estimated for each of the most 10 virulent strains, the least values were associated with the 3 totworthi strains confirming their pathogenic superiority. Spore counts/nil in the 10 tested strains indicated that the pathogenicity is rather attributed to the virulence of the spore-crystals than the number of spores.

Key Words: Bacillus thuringiensis, Spodoptera littoralis