H. armigera and H. punctigera are important pests of cotton in Australia. Transgenic cotton (containing Cry1Ac) has been commercially grown in Australia for 5 years, The susceptibility of both species to Bacillus thuringiensis (Bt) toxins in Australia field populations has been monitored since 1993.

This paper reports results of Bt resistance monitoring for the 2000/2001 cotton season as compared with results from previous seasons. Our studies indicate the development of resistance in H. armigera and possible esterase mediated mechanism of resistance in H. armigera.