Abstract
Abstract A programme to monitor Helicoverpa armigera (Hubner) for resistance to fenvalerate was initiated in 1991. Bioassays that exposed individuals to insecticide residues in glass vials were carried out on pheromone‐trapped adults and on Fl larvae reared from insects collected in the field. These assays assessed changes in susceptibility to fenvalerate during each cropping season and from year to year. H. armigera for assays were collected from processing tomato, sweet corn, and lucerne crops, mainly from the Gisborne and Hawke's Bay regions of New Zealand. Comparison of LD50S showed significant increases in resistance of 69‐fold for adults and 47‐fold for third instar larvae within a season. A diagnostic dose of 1 μg (LD95 for susceptible populations) showed a significant trend of declining mortality from 1992 to 1994 for adults, but trends were unclear for larvae. Assays with a diagnostic dose of 10 μg (LD99 for susceptible populations) showed decreasing mortality from 1992 to 1994, and at 90 μg c. 1...
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