In an attempt to increase the insecticidal effect of the ␦-endotoxin crystal protein CryIC on the relatively Cry-insensitive larvae of Spodoptera littoralis, a combination of CryIC and endochitinase was used. CryIC comprising the first 756 amino acids from Bacillus thuringiensis K26-21 and endochitinase ChiAII encoded by Serratia marcescens were separately produced in Escherichia coli carrying the genes in overexpression vectors. The endochitinase on its own, even at very low concentrations (0.1 g/ml), perforated the larval midgut peritrophic membrane. When applied together with low concentrations of CryIC, a synergistic toxic effect was obtained. In the absence of chitinase, about 20 g of CryIC per ml was required to obtain maximal reduction in larval weight, while only 3.0 g of CryIC per ml caused a similar toxic effect in the presence of endochitinase. Thus, a combination of the Cry protein and an endochitinase could result in effective insect control in transgenic systems in which the Cry protein is not expressed in a crystalline form.
We have studied whether the cooperative insecticidal e¡ect of certain scorpion toxin pairs, namely either a combination of excitatory and depressant, or alpha and depressant scorpion toxins, would improve the e⁄cacy of Autographa californica nucleopolyhedrovirus (AcMNPV) over a virus expressing only a single toxin, towards Heliothis virescens, Helicoverpa armigera, and Spodoptera littoralis larvae. The best result was achieved by combined expression of the excitatory toxin, LqhIT1, and the depressant toxin, LqhIT2, that provided an ET 50 value of 46.9 h on H. virescens neonates, an improvement of 40% over the e⁄cacy of wild-type AcMNPV, and of 18% and 22% over baculoviruses that express each of the toxins independently. These results demonstrate that signi¢cant improvement in e⁄cacy of recombinant baculoviruses is obtainable with toxins that exhibit a cooperative e¡ect, and may contribute to employ baculoviruses to replace hazardous chemicals in insect control. ß
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