Only vigorous males are used in bioassays. Male moths are placed individually in insecticide-treated vials. As treatment controls, males are placed in vials treated with acetone only. Infested cotton squares are brought to the laboratory and placed in a cage for weevil emergence. Weevils are tested in groups of 2-5 per vial.
All weevils tested are less than four days old.
After 24 hours exposure to an insecticide, adults are considered susceptible if there is no leg movement when the snout is pinched with forceps or if they are unable to walk for 0.3m without rolling onto their back. Determination of appropriate doses is an important consideration. If resistance is due to a single gene, three geneotypes of insects, homozygous susceptibles, heterozygotes and homozygous resistants, are possible. Rather than try to separate the three genotypes, we concentrate on identifying susceptible individuals only. We determine the decline in mortality in a test population and compare observed mortality with a susceptible population.