Temporal dynamics of phosphine fumigation against insect pests in wheat storage

Methyl bromide (MBr) fumigant has been phased out globally honouring ‘Montreal Protocol’. However, MBr is still used in India in various commodities for controlling pests as well as international trade requirements. Reports on phosphine fumigant efficacy under different ecological zones in India are scanty. In the present study fumigation trials were conducted in warehouses on 5 Metric-Tons of wheat stacks per replication of a treatment for testing phosphine efficacy against laboratory cultured and warehouse resident population of Tribolium castaneum (Herbst), Sitophilus oryzae (Lin), and Rhyzopertha dominica (Fab). The trials were organized in triplicate including untreated control at FCI warehouses at two locations, Pithoragarh in Uttrakhand and New Delhi, with varying climates. The conventional 56% Aluminium phosphide (AlP) tablet at 2 and 3 tablet/MT, and gas derived from on-site phosphine generator using 77.5% granular formulation at 1.0 and 1.5 g phosphine/m3 was tested at these warehouses against test insects for 7 and 10-days exposure periods for each dosage. Phosphine concentration was monitored every 24 h till end of the exposure periods using gas monitors. No emergence of insects was observed until 60 days after treatment in both the exposure periods using all dosages of phosphine indicating mortality of all life stages of laboratory cultured insects and resident population at both the locations. Phosphine at 1.5 g/m3 was observed as an effective alternative to methyl bromide fumigant for controlling S. oryzae, R. dominica and T castaneum at 250–27 °C and humidity 38–45% with an exposure period of 7-days, with a terminal phosphine concentration of 500–600 ppm. Locational effect was observed for gas loss and phosphine gas concentration. During fumigation, phosphine gas loss was observed to be inversely related with grain moisture and directly related with Ct values.