EXPERIMENTS ON DIFFUSION OF PHOSPHINE IN CUT FLOWER BOXES

Abstract

One of the most effective fumigants for disinfestation of cut flowers is currently methyl bromide, soon to be restricted. Toxicity of a potential alternative fumigant, phosphine (2% PH 3 + 98% N 2 ), was tested earlier on insect pests of cut flowers. Effective concentrations and durations of fumigation by phosphine have been ascertained in vitro, but the efficacy of phosphine under commercial situations needs to be examined. Numerical modelling indicates that in still conditions it may take over 700 min for phosphine to equilibrate at the center of an empty fibreboard box 1.0 m X 0.3 m X 0.21 m. For flowers such as proteas with large, tight bracts covering the flower, the time for these fumigants to penetrate will be longer. In laboratory experiments we measured the change in concentration of phosphine with time in different locations of a half-sized fibreboard flowerbox. Samples taken from various locations in the box were analyzed for phosphine using gas chromatography, and the time - concentration relationship of phosphine diffusion in a box with or without flowers in still or moving air was determined. Diffusion of phosphine into flower heads of Protea was also measured. In an empty flower box, phosphine took 240 min to attain equilibrium in still air compared with 800 min predicted if mass transfer were governed by pure diffusion. Equilibrium was reached in an empty box in 35 min; in a box packed with flowers it took 60 min, whilst diffusion into a protea flower head took 75 min with some bulk movement. The results highlighted that a box full of proteas cause resistivity to the free movement of phosphine and that the time required to attain equilibrium (130 min) inside a flower head can be as much as 50% of a standard fumigation time of 4 h.