RESUMO
Field results, using Bacillus thuringiensis delta-endotoxin in controlling spruce budworms, Choristoneura fumiferana, are far less satisfactory than those from the laboratoriesThe role of septicemia has also been shown to be important in mortality. However its relationship with insect diet has not been studied. Insect gut juice from spruce budworm fed on balsam foliage inhibited bacterial growth. Such activity has not been observed in the gut juice from insects fed on artificial diet. Moreover, when bacteria were given to the insects orally, and the insects then fed on balsam foliage overnight, little or no bacterial colonies could be detected in their gut juices in the tryptic soy agar plates. Such bacterial growth inhibition activity could also be found in the water extract of balsam foliage, and was suggested to be due to tannins. This result explains the role of allelochemicals affecting septicemia in spruce budworms, and hence the efficacy, when using Bacillus thuringinesis delta-endotoxin as a biologic control agent.
RESUMO
A genetically altered variant of Cry9Ca from Bacillus thuringiensis shows high potency against the spruce budworm, Choristoneura fumiferana Clemens. Its activity, as measured by feeding inhibition in frass-failure assays, is estimated to be four to seven times greater than B. thuringiensis subsp. kurstaki HD-1, the strain currently used in commercial products to control this insect. Bioassays against budworm of mixtures of the modified Cry9Ca and two of the Cry1A endotoxin proteins produced by HD-1 show neither synergism nor antagonism. Experiments with brush border membrane vesicles from budworm midgut revealed that Cry9Ca and the Cry1A toxins share a common binding site and that bound Cry9Ca can be displaced from the membrane to some extent by the Cry1A toxins. However, it is uncertain whether the binding site is actually the receptor molecule or a membrane protein associated with pore formation.
