EVIDENCE FOR RESISTANCE TO BACILLUS-THURINGIENSIS (BT) SUBSP KURSTAKIHD-1, BT SUBSP AIZAWAI AND ABAMECTIN IN-FIELD POPULATIONS OF PLUTELLA-XYLOSTELLA FROM MALAYSIA

The efficacy of Bacillus thuringiensis (Bt) subsp. kurstaki HD-1 ('Dip el'(R); Btk; CryIA & CryII) and Bt. subsp. aizawai ('Florbac'(R); Bta; CryIA & CryIC) was assessed against larvae from various field populat ions of Plutella xylostella (F2 generation) collected in the Cameron H ighlands, Malaysia in April 1994 and a lowland population (SERD 2; F10 generation) collected in December 1993. Evidence of resistance to Btk and to a lesser extent Bta is reported in these populations (LC(50) T oxicity Ratios [TR] = 3-14 and 2-8 respectively), most notably in SERD 2. The first recorded evidence of resistance to abamectin (TR = 17-19 5-fold) in field populations of P. xylostella is also reported. In an unselected sub-population of SERD 2, the TR values for Btk, Bta and ab amectin declined 2- to 3-fold (P < 0.01) over six generations in the l aboratory (F10-F16) while in sub-populations of SERD 2 selected with t hese products (F11-F15) there was a significant (P < 0.01) increase in the TR (15-, 3- and 2.5-fold respectively) when compared with the F10 generation This suggests the presence of marked resistance to Btk and some resistance to Bta and abamectin. There is also evidence of sligh t cross-resistance to Btk in the Bta-selected sub-population but no ev idence for the reverse selection of resistance or for cross-resistance between Btk and abamectin. Concurrent selection studies (F11-F15) wit h another sub-population of SERD 2 demonstrated resistance to the acyl urea insect growth regulator, teflubenzuron ('Nomolt'(R)) (29-fold inc rease in TR). Based on the selection experiments with SERD 2, estimate s of realised heritability (h(2)) of resistance gave very high values for teflubenzuron and Btk (c.07) and moderate values for abamectin and Bta (c.0.3). The results are discussed in relation to integrated pest management (IPM) and insecticide resistance management (IRM) strategi es for P. xylostella.