On Criteria for Success of Phosphine Fumigations Based on Observation of Gas Distribution Patterns

Abstract

Abstract: Recent changes in the basis of practical use of phosphine have created a need to re-evaluate the distribution of gas produced during fumigation. This evaluation must be made against an awareness that survival of insects may increase the risk of selection of strains resistant to phosphine. Using examples from recent Australian full-scale field trials, illustrations are given of various defects in fumigant retention distribution and application that may lead to inefficient use of material and survival of insects. The ratio of minimum to maximum concentration is used as an indicator of distribution in unsealed systems with uniform admixture of phosphine releasing agents and sealed systems with surface application. Based on the gas distribution patterns in the examples presented, a set of criteria are proposed by which the success of a treatment can be judged, and a commercially successful result and a completely successful one, i.e. one in which complete insect kill may be expected, can be differentiated. In increasing order of stringency the criteria are (a) the grain be found free of insects by inspection after treatment, (b) the average maximum concentration of phosphine be >50% of that expected theoretically, (c) the concentration at the end of the exposure period be greater than the minimum effective against insects and (d) that the ratio of minimum to maximum concentration exceed 0.25 after not more than 25% of the exposure period.