Insect mortality at doses of phosphine which produce equal uptake in susceptible and resistant strains of Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae)

Abstract

The rate of uptake of [ 32P]phosphine was studied in a susceptible and two phosphine-resistant strains of Rhyzopertha dominica. Under similar test conditions, live susceptible insects absorbed much more [ 32P]PH 3 than the live resistant or dead insects of any strain. Although treatment of resistant strains at higher doses of [ 32P]PH 3 produced equal or even higher uptake than the susceptible strain treated at a comparatively lower dose, the resulting mortality was still very low in the resistant strains against a high level of kill of the susceptible strain. There was an increasing order of mortality in the resistant strains when equal uptakes were achieved after 20, 48 and 72 hr exposure to different doses of [ 32P]PH 3. Resistant insects appeared to have some metabolic detoxicating process, in addition to a reduced uptake of phosphine. The process of [ 32P]PH 3 uptake in the susceptible insects continued even after insects were knocked-down or killed during a 24 hr exposure to 0.02 mg/l of [ 32P]PH 3. Mechanisms of insect resistance to phosphine are discussed in the light of these results.