Helicoverpa armigera (Hübner) is a notorious pest of various field crops. A contributing factor in its pest status is the development of resistance to insecticides, making the insect difficult to control. The objective of this study was to assess the toxicity of selected insecticides against natural populations of H. armigera in Pakistan and thus identify possible levels of insecticide resistance. Insects were collected from three locations in the province of Punjab, Pakistan, in three consecutive years. The median lethal concentration (LC50) of selected insecticides was determined for each field population as well as a susceptible lab strain designated as Lab-PK. Resistance ratios (RRs) for each insecticide were calculated as the ratios of the LC50 for each field population relative to that of the Lab-PK strain. Based on the calculated RRs, the field populations tested were highly resistant to bifenthrin (RR = 34.1 to 48.0), moderately to highly resistant to lambda-cyhalothrin (RR = 19.6 to 68.2) and deltamethrin (RR = 19.3 to 37.2), and minimally to moderately resistant to profenofos (RR = 9.80 to 12.11), methoxyfenozide (RR = 6.0 to 11.8) and thiodicarb (RR = 5.6 to 11.5). Resistance was low for emamectin benzoate (RR = 1.7 to 5.2), chlorpyrifos (RR = 3.5 to 9.6), and lufenuron (RR = 1.0 to 2.2). Pairwise comparison of the log LC50 of the insecticides against all populations showed a correlation among the various insecticides, suggesting possible development of cross-resistance.
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Vol. 50 • No. 2