Insecticidal activity of Bacillus thuringiensis strains against the egyptian cotton leaf worm Spodoptera littoralis [Lep. Nocutidae]

B. Sneh*, Silvia Schuster, M. Broza

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

Among more than 50 isolates of Bacillus thuringiensisBerliner (B.t.) tested, 7 incited 100% mortality when 2nd instar larvae of Spodoptera littoralisBoisduval were fed on alfalfa leaves dipped in a spore-crystal suspension of 108 colony forming units/ml. Among those isolates, B.t. 24 demonstrated the highest activity. Larvae of instars 1 and 2 were the most susceptible to B.t. Susceptibility decreased with larval development. However, larvae of all instars were killed by isolate B.t. 24. Larvae that survived after feeding with B.t. 24 were retarded and fed less. Their weight relative to the controls was lower as the spore concentration on the leaves on which they fed was higher. Survival of the spores in the field dropped drastically to 2% after 4 days. Insecticidal activity of the sprayed suspension on those leaves, however, remained significant. B.t. 24 was also effective against larvae on cotton plants in the greenhouse and in a preliminary field experiment. Numbers of colony forming units recovered from leaves dipped in suspension of various spore concentrations showed significant correlation with the initial concentrations as did sprayed leaves. However, colony forming units recovered from sprayed leaves were 5-7.5 fold lower than from dipped leaves. Dipped cotton leaves showed 3.1×10-5 ml attached to 1 mm2 leaf surface whereas sprayed ones had 6×10-6 ml. Those data are important for the determination of spore concentrations in suspensions required for spraying. The isolate B.t. 24 was serotyped by H. de Barjac as H-6 B. thuringiensis entomocidus.

Original languageEnglish
Pages (from-to)179-190
Number of pages12
JournalEntomophaga
Volume26
Issue number2
DOIs
StatePublished - Jun 1981

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