Status of fumigation in stored grains in India

India, a leading producer of food grains with an estimated output (inclusive of pulses) 257.07 Mt in 2014–15 has been storing grains as bag- stacks in warehouses (covered godowns). Due to shortage of space in conventional warehouses, about 20% of the stock (wheat and paddy rice) has been held outdoors in open storage called cover and plinth (CAP) storage. Lately, under new schemes such as Private Entrepreneurs Guaranteed Scheme, 2008 (PEGS) and Public Private Partnerships (PPP), steps have been undertaken to augment the overall storage capacity. Accordingly new warehouses have been built, metal silos erected and silo bags for wheat introduced in place of CAP storage. Grains stored in all types of storages are infested by beetle and moth pests. The pest activity is exacerbated by bag-stack storage system and favorable climatic conditions (25–35°C and 50–80% RH). Phosphine (from aluminium phosphide tablet and powder formulations) is the only fumigant used for grain protection in India since 1970s. In recent years, indigenous onsite phosphine generators have been deployed for fumigation of grains stored in bulk in metal silos. Multiple phosphine applications (every 3 months during a storage period of 1 to 3 years) of grain stacks are common. Inadequacies in current fumigations such as use of substandard gas proof sheets and sand snakes, shorter exposure periods (3 to 5 days), failure to measure gas levels, and poor maintenance of gas concentrations during exposure period have been discussed.

Phosphine resistance in grain beetle pests particularly in Rhyzopertha dominica, Tribolium castaneum, Cryptolestes spp., has been elaborated. In order to overcome the deficiencies, it is recommended to revise phosphine dosages taking into consideration of commodity, pest type and resistance factors. In addition it is necessary to achieve a minimum of 500 ppm for the control of normal insects and at least 1000 ppm when phosphine-resistant insects are present as target end concentrations. Alternatives such as ethyl formate, sulfuryl fluoride and CO2-rich atmosphere have been studied both at laboratory and field levels and efficacy proven. However, these have not been used yet for grain preservation in India. Several plant compounds have been studied but at laboratory level only. Overall, while there is an appreciable change in Indian grain storage system by the use of silo bags substituting CAP storage and by expanding storage capacity by erecting more metal silos across the county, rigorous changes in fumigation of food grains are yet to take place. Essential changes include revised phosphine dose and exposure period, use of standard fumigation accessories and compulsory gas monitoring during fumigation to ascertain achievement of target terminal gas levels. Involvement of fumigant manufacturers, grain storage agencies and regulatory bodies is necessary to implement good fumigation practices for sustained effective use of phosphine for grain preservation.