PEST RESISTANCE TO BACILLUS-THURINGIENSIS - CASE-STUDIES OF ECOLOGICAL CROP ASSESSMENT FOR BT GENE INCORPORATION AND STRATEGIES OF MANAGEMENT

An ecological assessment is made of the suitability of apples and kiwi fruit in New Zealand for transformation with genes from Bacillus thur ingiensis (Bt). The assessment focuses on the risk of pests of these c rops developing resistance to the Bt toxins expressed in the plants or in Bt used as a biopesticide. The key target pests which were assesse d were tortricids. The most important ecological features of the crops and their pests which influenced the assessment were the host range o f the pests, the availability of refugia of susceptible insects on non -transformed host plants within and around the crop, the mobility of t he pests enabling the flow of genes for susceptibility into the select ed pest populations, the ratio of susceptible to selected pest individ uals, and insect behaviour in relation to Bt gene expression in the cr op. Strategies are proposed for the management of transformed apple an d kiwi fruit to maintain pest susceptibility to Bt, and if these are f ollowed, the crops are considered suitable for transformation. It is e ssential that these Bt crops are deployed within an integrated pest ma nagement programme. The greatest risk of resistance to Bt is associate d with polyphagous leaf rollers which also attack white clover, a crop which is a further candidate for transformation with Bt genes. This c rop and its pest complex require full assessment of resistance risk be fore this step is taken.