RESEARCH & EVENTS
Phosphine absorption during almond fumigation
By Jo Holloway, NSW Department of Primary Industries and Regional Development, Wagga Wagga.
32
FUMIGATION is an essential part of insect pest management in postharvest almonds. Almonds may be infested with insects in the field prior to stockpiling. Insects may also move into the almonds postharvest through dispersal. Nuts can be damaged in the stockpile by the insects feeding, with stockpiles providing suitable environmental conditions for breeding, exacerbating the problem. Currently, chemical fumigation is the only option used for insect pest management in stockpiles. Although non-chemical insect pest management options have been suggested as a solution( such as low oxygen atmospheres, where oxygen levels are reduced to below 21 %), current infrastructure makes it nearly impossible to maintain these levels for the time required( 28 + days). Therefore, fumigation is our only option and needs to be conducted properly to avoid chemical resistance. Three fumigants are currently registered for use on almonds: phosphine, sulfuryl fluoride and ethyl formate. Only the solid formulation of phosphine( blankets or tablets) can be used without a licenced fumigator. This, in addition to its relatively low cost and efficacy, is the reason why phosphine is currently the most widely used. However, the use of sulfuryl fluoride and ethyl formate is increasing, possibly due to phosphine fumigation failures. Based on work with stored grain insects, phosphine rates must remain above 300 ppm for at least seven days to be considered effective. This is to manage all life stages of the pest insects( eggs, larvae, pupae and adults). Previously it was assumed phosphine fumigation failure to maintain the required concentration was due to gas leakage. Phosphine is highly mobile and easily flows through gaps or holes present and may be absorbed into the soil surface. No doubt leakage still contributes to some gas loss. However, my research has shown that almonds themselves absorb large quantities of the gas. The primary sites for the absorption
In A Nutshell- Autumn 2026 Vol 27 Issue 1
Figure 1: Relationship between rate of absorption and maximum phosphine concentration for Nonpareil, Carina and Carmel varieties of unprocessed almonds.
Figure 2: Absorption of phosphine by unprocessed Nonpareil almonds in laboratory desiccators with increasing phosphine doses.( NB: A standard dose equates to about 8 mL in these containers. Dotted line indicates parameters for an effective fumigation).
are the hull and shell( and minor absorption occurring in the kernel), with most phosphine absorbed in the first 48 hours. Unlike grains which may desorb or release phosphine post-fumigation, my laboratory studies have shown this is not the case with almonds. Since the gas must go somewhere, it appears that the absorbed phosphine may be converted to non-toxic phosphates.