Fumigation with Phosphine for storage pest management

Introduction

Safe storage of food grains is one of the crucial aspects in achieving food security worldwide. During storage several biotic and abiotic factors cause damage to the grains usually called as ‘post-harvest losses’. It includes the losses happened in food supply chain from harvesting, storage of the crop until its consumption. Moreover, pre-storage practices like harvesting, threshing, drying also have role to play in safety during storage.

The abiotic factors such as grain moisture, humidity, and temperature of surrounding and biotic factors like insects, rodents, birds, mites, fungus, and bacteria are known to cause losses upto 80% wherein losses caused by storage insects account for about 5-10% in temperate regions and 20-30% in tropical region of the world.

Insect pests infesting storage commodities can be grouped in two categories

1. Primary pests such as red flour beetle (Tribolium castaneum), lesser grain borer (Rhizopertha dominica), pulse beetle (Callosobruchus chinensis), rice weevil (Sitophilus oryzae), almond moth (Cadra cautella), Angoumis grain moth (Sitotroga cereallela), etc. 
2. Secondary pests such as saw-toothed grain beetle, Oryzaephilus surinamenisis;  long-headed flour beetle, Latheticus oryzae;  flat grain beetle, Cryptolestes minutus.

The synthetic insecticides including fumigants and contact insecticides play key role in integrated pest management in storage. Fumigants such as phosphine remain an important tool, particularly for treating stored agricultural commodities and controlling pests in structures like mills, warehouses, and shipping vessels. Apart from phosphine, contact insecticides like deltamethrin is used as prophylactic as well as curative sprays in storage facilities in India. 

Hydrogen phosphide or Phosphine

Hydrogen phosphide or Phosphine is widely used and popular fumigant all over the world because of its high efficacy against most of the storage pests, ease-of-use, cost-effectiveness and effective penetration in commodity. It is used for disinfestations of legumes, cereal grains, dried fruits, durable food commodities and also processed food. Ideally, a fumigant is a chemical that exist in gaseous state at specific temperature and pressure. Phosphine is available in solid formulation (tablets, sachets or pellets) of metallic salts of aluminium phosphide and magnesium phosphide. When placed in a storage area or among the bag stacks this formulation reacts with moisture in the air and generates phosphine gas. Once the phosphine gas is completely released these metallic formulations leaves non-toxic solid residue of aluminium or magnesium hydroxide and other inert ingredients which can be easily discarded. It has also been made possible to release phosphine gas directly into the commodity by using gas cylinders or phosphine generators. 

Pre-Fumigation Steps 

• Through inspection of fumigation site, fumigation cover for potential leak points
• Proper care of material and equipments containing copper which may get adversely affected by fumigant, if exposed
• Precise calculation of dosage based on area volume to be fumigated is necessary to acieve effective fumigation
• Pre-determine plan for even distribution of phosphine formulation inside enclosure to achieve uniform coverage by phosphine gas and also prepare plan for proper ventilation at the end of exposure period
• Appropriate warning signs at fumigation site to avoid exposure of other peoples to phosphine gas
• Ensure the availability of safety equipment such as breathing apparatus, gas detection devices and first aid at fumigation site

Fumigation using sheet/cover 

The fumigation sheet (Cross Laminated Thermoplastic Fumigation Cover CLFT) can be used for effective fumigation. Before stacking the bags, the airtight floor should be prepared on which the packed commodity, either in bags or any other packaging material, should be stacked. The stacks then be enclosed in a fumigation sheet ensuring the sheet is of adequate size so that it can extend far enough to allow sealing of its edges to the floor with the help of sand snakes to facilitate airtight enclosure. Always follow the dosage recommended by the manufacturer or specified by the Central Insecticide Board & Registration Committee (CIBRC), Government of India, as detailed below. These dosage are expressed in two ways

• based on volume of area enclosed by fumigation sheet [grams per cubic metre (g/m³)];
• based on the amount of commodity to be fumigated [usually given as grams per tone (g/ton)].

Note: Use of phosphine is restricted in India. The Pest Control Operations with Aluminium Phosphide may be undertaken only by Govt./Govt. undertakings / Govt. Organizations / pest control operators under the strict supervision of Govt. Experts or experts whose expertise is approved by the Plant Protection Advisor to Govt. of India except Aluminium Phosphide 15 % 12 g tablet and Aluminum Phosphide 6 % tablet.

Application/distribution of phosphine formulation

Before application or distribution of phosphine tablet it is important to understand that phosphine gas is 1.17 times heavier than air, and when tablets are applied at recommended dosage the generated gas gets mixed with air but the resulting air-phosphine gas mixture is not much heavier. Hence, the mixture rapidly penetrates the large stacks, tightly packed material and reaches at every corner of enclosed space. Hence, there is no need to place the tablet at top of the stacks. The Tablets or pouch formulations are usually placed among the bag stacks immediately after removing from container. The tablets can be wrapped in fine porous cloth or placed in plastic tray/ cardboard for easy handling and disposal. Moreover, it also avoids the direct contact of formulation or residue/ leftover powder with commodity. Tablets must be distributed uniformly among the bag stacks preferably on the sides of stacks. Once the application of phosphine tablet is over, the enclosure must be properly checked for any leakage for which appropriate detectors or monitoring devices can be used.

Sealing the fumigation sheet

Usually sand snakes are used for sealing the fumigation sheet to the floor to ensure air tightness. After distributing the aluminium phosphide tablets in the bag stacks the sheets can be sealed with the help of double row of sand snakes. It is important to note that in hot, humid conditions, phosphine is produced almost immediately on exposure to the air, so the dispensing process must be completed within 15 minutes.

Aeration 

After the exposure period, the storage area needs to be thoroughly ventilated to remove any remaining gas and make it safe for humans and animals. All doors and windows should be opened to provide cross-ventilation. The fumigation sheet should be opened on one end to allow the stack to aerate for at least 30–60 min before removing the sheet completely. 

Disposal of residue

At the end of the fumigation, after proper aeration/ventilation, all phosphine-generating formulations (aluminium-phosphide tablets, sachets etc.) used during the fumigation must be collected and disposed of in a safe manner. This is done because the aluminium phosphide in the formulations breaks down into a powder called ‘spent residues’. However, the spent residues still contain between 3–5% of unreacted aluminium phosphide, which is a safety hazard. The spent residues must be disposed of carefully and safely. Care must be taken when handling the residues to avoid breathing in the dust and prevent contact with eyes and skin. This step is crucial to prevent any residues of phosphine from contaminating the stored products.

Content contributors

1. Suresh M Nebapure, ICAR-Indian Agricultural Research Institute, New Delhi, India (Corresponding author: smnebapure@gmail.com)
2. Rajna S, ICAR-Indian Agricultural Research Institute, New Delhi, India 
3. Pratap Divekar, ICAR-Indian Institute of Vegetable Research, Varanasi, Uttar Pradesh, India