Laboratory Selection for Phoxim Resistance, Cross Resistanceand the Synergism of Several Synergists in Helicoverpa amigera Hübner

A field collected strain (HNYS) of Helicoverpa armigera in cotton in 1991 had 2.7 fold resistance to phoxim compared to a laboratory susceptible strain (SUS). After HNYS strain was selected with a mixture of phoxim+cyhalothrin (25∶1 A.I.) for 27 generations, the resistance to phoxim increased from 2.7 fold to 9.4 fold. Then this strain was further selected by phoxim alone for 21 generations and the phoxim resistance increased from 9.4 fold to 63.0 fold. The phoxim resistant strain had high level cross resistance to methomyl (45.5 fold), middle level to monocrotophos (17.9 fold), and low level to methyl parathion (8.7 fold) and profenofos (4.8 fold). Synergism ratios of TPP, DEF and PBO to phoxim in the selected HNYS strain were 1.4, 24.8 and 1.0 fold respectively. Significant synergism of DEF to phoxim (resistance decreased from 63.0 to 2.5 fold) indicated esterase appeared to be an important resistance factor in this laboratory selected resistant strain. These results are indicative in designing phoxim resistance management strategies such as alternating, rotating and mixing insecticides.