RESISTANCE TO BT FORMULATION IN DIAMONDBACK MOTH, PLUTELLA-XYLOSTELLA L, ON WATERCRESS

The susceptibility of diamondback moths Plutella xylostella collected from three sites in Osaka prefecture to BT formulation (Toarro CT(R)) was determined by the leaf-dipping method using 3rd instar larvae. Larvae from a greenhouse growing watercress in the Kohno area have shown a high level of resistance (LC50 > 280 ppm) to the insecticide from October 1988. The development of high BT resistance in the Kohno population has probably occurred because watercress is grown throughout the year with frequent spraying of the BT formulation (15-20 times a year) and also because there is very little immigration of the moth into the greenhouse.