Changes in susceptibility to various pesticides of diamondback moth (Plutella xylostella L.) in Gobo, Wakayama Prefecture.

The susceptibility to insecticides of the diamondback moth, Plutella xylost ella L. collected during 1985-1997 from commercial fields of cabbage and br occoli in Gobo, Wakayama Prefecture was determined at the 3rd-instar larvae by the leaf-dipping method. The resistance to Bacillus thuringiensis toxin (Toarow CT(R)) varied with the frequency of application. Higher LC50 value s were observed during 1986-1987 when BT products were applied often and lo wer LC50 values were observed during 1988-1990 when alternative pesticides were used due to the lower effectiveness of BT formulations. Chlorfluazuron was remarkably effective when it was introduced. However, extended treatme nt promoted development of high resistance to chitin synthesis inhibitors i n early 1991. With organophosphates, LC50 values of prothiofos and phenthoa te fell because applications were less frequent. The moth population mainta ined high resistance to the pyrethroid group from 1986 when they first deve loped resistance to fenvalerate.