Variability in Response to Insecticides of Field Populations of Diamondback Moth, Plutella xylostella (Linnaeus), in the Philippines

Three populations of the diamondback moth (DBM), Plutella xylostella (Linnaeus), from crucifer--growing areas in the Philippines were tested for susceptibility to 14 formulated insecticides. The leaf dip assay was used on third instar larvae of the pest. The 72-hr LC50 values were compared to those of a reference susceptible strain (BRL Sus st). Computed resistance ratios for the three populations showed varied responses in range and levels of resistance to the insecticides. The populations from Atok, Benguet ( 7400 fast), Majayjay Laguna ( 1000 fast) and Calamba Laguna ( 100 fast) still had very low level of resistance to new insecticide molecules such as flubendiamide, chlorantraniliprole, chlorfluazuron, chlorfenapyr and diafenthiuron, and to the microbial formulation Bacillus thuringiensis var kurstaki. On the other hand, all three populations showed very high level of resistance to cypermethrin and to malathion. LC50 determination for malathion was discontinued when the populations showed no mortalities up to 3555ug/L. For the remaining test insecticides -cartap, indoxacarb, spinosad, fipronil, methamidophos and teflubenzuron -responses of the three populations in terms of LC50 values when compared to that of the reference population vary, ranging from very low to very high level. Discriminating doses (LC80-LC80 values) for the formulated insecticides are also proposed as input in development of insecticide resistance monitoring system for DBM.