Prospects for predicting insect mortality in relation to changing phosphine concentrations.

Abstract

Typically, in bag-stack or silo fumigations the concentration of phosphine is not constant, and yet most of what is known about phosphine efficacy against grain insects comes frorn studies with fixed concentrations. Indeed, where changing concentration experiments have been performed, researchers have been unable to explain observed efficacy on the basis of data frorn fixed concentrations. The ability to predict insect mortality in relation to changing phosphine concentrations would facilitate the development of effective fumigation protocols. In this paper, we explore the prospects for making such predictions. After reviewing published and new results, we conclude that the commonly used concentration x time (Ct) product is y e l iable for this purpose. New results, for a strongly resistant strain of Rhyzopertha dominica frorn Australia, suggest that the relationship Cnt = kmay be useful for predicting mortality of this type of insect in changing concentrations. However, in the case of a strain of Sitophilus oryzae with a type of resistance common in Australian S. oryzae, the relationship C t = k proved to be less reliable.

Diagnostic Molecular Markers for Phosphine Resistance in U.S. Populations of Tribolium castaneum and Rhyzopertha dominica

“…The technique is relatively easy to perform, inexpensive and can be readily applied by research laboratories with a basic infrastructure for molecular techniques. The ability to use a PCR marker in the study of phosphine resistance dynamics opens many opportunities for future work not afforded by the standard FAO-type bioassays or the so-called “quick tests” for weak and strong resistance [ 19 , 21 ]. Phosphine bioassays require living adults recently collected from the population of interest, access to quantitative analytical chemistry and handling of dangerous phosphine gas by researchers, which are severe limitations.…”

“…In order to score both weak and strong phosphine resistance in these populations we carried out a modified FAO (1975) bioassay with 50 individuals/vial and 3 replicates per population. The discriminating dose for weak resistance was 30ppm for 20 hrs, as described above, and the discriminating dose for the strong resistance phenotype was set at 180ppm for 20 hrs [ 19 ] in separate experiments. Molecular marker analysis for the strong resistance SNP marker was performed on all seven populations as described above and its frequency was calculated for each population from the individual genotypes inferred by the CAPS markers.…”