Phosphine fumigation of silo bags

Fumigation with phosphine has the potential to disinfest grain stored in silobags but only limited research has been conducted on whether phosphinefumigation can be undertaken effectively and safely in this form of storage. Fumigation with phosphine was tested on two (70 m) replicate silobags each containing 240 t of wheat (9.9 and 9.2% m.c.). The target application rate of phosphine was 1.5 g m−3 with a fumigation period of 17 days. Aluminium phosphide tablets were inserted into each bag at ten release points spaced at 7 m intervals starting 3.5 m from either end of the bag. A total of 14 bioassay cages containing mixed age populations of strongly phosphine resistant Rhyzopertha dominica (F.) were inserted into each fumigated silobag. Complete control of all life stages of R. dominica was achieved at all locations in the fumigated silobags. Phosphine concentrations at release points increased rapidly and remained high for the duration of the fumigation. Concentrations at midway points were always lower than at the release points but exceeded 215 ppm for ten days. The diffusion coefficient of available phosphine averaged over the first three full days of the fumigation for both fumigated silobags was 2.8 × 10−7. Venting the silobag with an aeration fan reduced the phosphine concentration by 99% after 12 h. Relatively small amounts of phosphine continued to desorb after the venting period. Although grain temperature at the core of the silobags remained stable at 29 °C for 17 days, grain at the surface of the silobags fluctuated daily with a mean of 29 °C. The results demonstrate that silobags can be fumigated with phosphine for complete control of infestations of strongly phosphine resistant R. dominica and potentially other species.