Scrubbing carbon dioxide prevents overestimation of insect mortality in long-duration static phosphine toxicity assays

Exposing insects to toxicants such as phosphine (PH₃) in sealed chambers is a common procedure in fumigant efficacy testing. During long exposures, carbon dioxide (CO₂) produced by metabolic processes of fed insects may accumulate in the vessel. As CO₂ enhances the toxicity of several fumigants, concomitant exposure of fumigant and CO₂ may lead to erroneous measurements of insect mortality and thus fumigant toxicity. In this study, the effect of a CO₂ scrubber such as soda lime (a solid formulation of wet hydroxides of calcium, sodium and potassium) or periodic flushing of the headspace, on insect mortality in static PH₃ toxicity assays was compared with an “unscrubbed” (no intervention) treatment. Soda lime was highly effective in removing CO₂ from the headspace of sealed chambers, without measurable loss of PH₃. Mortality of Sitophilus oryzae (L.) adults treated with phosphine in chambers scrubbed with soda lime or flushed (opened) daily was considerably lower than in unscrubbed chambers. Twenty grams of soda lime per litre of exposure chamber is recommended to obtain accurate static fumigant mortality results.