LABORATORY EVALUATION OF THE BIOLOGICAL FITNESS OF CHLORFLUAZURON RESISTANT AND SUSCEPTIBLE STRAINS FROM THE SAME ORIGIN OF THE DIAMONDBACK MOTH PLUTELLA-XYLOSTELLA

Diamondback moth (DBM), Plutella xylostella (L.), is one of the most notorious pest of cruciferous crops such as cabbage and radish throughout southeast Asian countries. The use of conventional insecticides, i.e., organochlorines, organophos-phates, carbamates and pyrethroids to control this insect pest has serious drawbacks owing to the rapid development of resistance. Insecticide-selected strains were reported to have disadvantages in biological aspects in comparison with susceptible strains (Roush And Mckenzie, 1987), and in the absence of pesticide pressure the resistance of the DBM against pyrethroid decreased after about one year (Noppun Et al., 1984; Chen And Sun, 1986). A chlorpyriphos-selected strain of the house mosquito, Culex quinquefasciatus from Tanzania, had a significantly reduced biotic fitness compared to a standard susceptible strain from Sri Lanka. However, after backcrossing the resistant gene into the genetic background of the susceptible strain, its fitness was found to be closer to that of the susceptible strain (Amin And White, 1984). In case of Australian blowfly, Lucilia cuprina, selected with the organophosphate (diazinon), similar fitness was reported among the three genotypes groups, resistant (RR), heterozygote (RS) and susceptible (SS) (Mckenzie Et al., 1982; Mckenzie And Game, 1987). The two Thai strains of DBM, TL and BK, used in this study showed 400 to 3,400 times greater resistance to the chitin synthesis inhibitor, chlorfluazuron, than the Japanese Osaka susceptible (OSS) strain, when they were collected at both Tup Luang (TL) and Bang Khae (BK). Both strains were imported from Thailand after being reared in the laboratory for two years under insecticide-free conditions. By that time, their resistance ratios became as low as 16–62 to chlorfluazuron as compared with the OSS strain. They were re-selected with chlorfluazuron in our laboratory for 14 and 15 generations for the TL and BK strains, respectively. Their resistance ratio reached 318 and 303, respectively, in comparison with the corresponding susceptible groups (Fahmy Et al., 1991). To assess the difference in biological fitness between the selected (resistant) and nonselected (susceptible) groups of TL and BK strains, the fecundity and the survival rate were compared in the four strains. © 1993, JAPANESE SOCIETY OF APPLIED ENTOMOLOGY AND ZOOLOGY. All rights reserved.