Insecticidal Activity of a Cry1Ca toxin of Bacillus thuringiensis Berliner

The Cry1C protein family of Bacillus thuringiensis form bipyramidal crystals, which are commonly associated with toxic activity against lepidopteran species; however, some members of this family may also be toxic to dipterans. In the present work, the Cry1Ca16 protein, synthesized by the B. thuringiensis LBIT-1217 strain, was analyzed. The gene coding for this protein was amplified, sequenced, and cloned into the pSTAB vector, which was electro-transferred into the acrystalliferous B. thuringiensis 4Q7 strain. The recombinant strain showed the expected bipyramidal crystal morphology, identical to the original LBIT-1217 strain and exhibited toxicity against larvae of Aedes aegypti (Diptera). Pure crystals from the recombinant strain were used in bioassays against Ae. aegypti larvae, estimating an [LC.sub.50] of 4.61 ug/ml. Further studies on Cry1Ca16 mosquitocidal potential included joint-action tests with the Cyt1Aa protein crystals from B. thuringiensis israelensis. An [LC.sub.50] using pure Cyt1Aa crystals was estimated at 0.73 ug/ml, whereas an [LC.sub.50] of 0.61 ug/ml was estimated when both toxins were tested together. Data from these bioassays was analyzed using joint-action tests such as the Tammes-Bakuniak graphical method and the formula proposed by Tabashnik (1992). Both tests clearly showed a synergistic effect between these two toxins. Graphical Key words: thuringiensis, synergism, Aedes aegypti, [delta]-endotoxins