Variability in response to insecticides of field populations of diamondback moth, Plutella xylostella (Linnaeus)(Lepidoptera:Plutellidae) in the Philippines

Three local populations of the diamondback moth, Plutella xylostella (Linnaeus), were collected from major crucifer growing areas in the Philippines and tested for susceptibility to 14 formulated insecticides. The leaf dip technique was employed on third larval instar of the pest. Larval mortality was compared with 2 reference strains (BRL Sus st and BRL Bt Res st). Data showed varied responses, in range and levels of resistance to the insecticides based on computed resistance ratios. The population from the highlands of Atok, Benguet [Philippines] showed very low levels of resistance to flubendiamide (1.1-fold), chlorantraniliprole (1.7-fold), Bt var kurstaki (2.5-fold), chlorfluazuron (2.5-fold), chlorfenapyr (3.2-fold) and diafenthiuron (4.0-fold), low level of resistance was observed with indoxacarb (8.5-fold), moderate resistance to cartap(17.0-fold), spinosad (17.7-fold) and methamidophos (45.5-fold), while fipronil (62.5-fold) had high level of resistance and extremely high level of resistance was recorded for cypermethrin (565.9-fold). DBM population from City of Calamba recorded very low levels of resistance to eight insecticides; chlorantraniliprole (2.7-fold), diafenthiuron (2.7-fold), Bt var kurstaki (2.8-fold), flubendiamide (2.9-fold), chlorfluazuron (3.1-fold), chlorfenapyr (3.2-fold), teflubenzuron ( 3.3-fold) and indoxacarb (4.4-fold) and low resistance level to cartap (5.2-fold) and spimosad (12.9-fold). Meanwhile, fiproil (33.6-fold), methamidophos (53.5-fold) and cypermethrin (380.9-fold) were moderate, highly resistant, and very highly resistant, respectively. The Majayjay (Laguna) population generated very low levels of resistance to flubendiamide (1.3-fold), chlorantraniliprole (1.7-fold),Bt var kurstaki (1.7-fold), indoxacarb (1.8-fold),chlorfenapyr (2.7-fold), diafenthiuron (2.8-fold), cartap (3.1-fold) and chlorfluazuron (4.1-fold). Fipronil provided low level of resistance, while methamidophos (19-fold) and spinosad (22.3-fold) and teflubenzuron (31.2-fold) were moderately resistant while cypermethrin (264-fold) was highly resistant. Malathion was extremely highly resistant to all populations since no LC sub 50 and LC sub 90 values were generated up to 3555 ug/L. The results of the various bioassays conducted validated the reproducibility of results of leaf-dip technique. Diagnostic doses (LC sub 80-LC sub 90 values) for all formulated insecticides were also established to assist in immediate monitoring of insecticide resistance.