Keeping the mice out of rice

by Arvin Donley
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Each year pests cause millions of dollars in losses through a combination of feeding, spoiling and contamination of both paddy and milled rice that is in storage. The most common pests that pose a threat to stored rice are insects and rodents.

The good news is that these losses can be prevented if facilities where rice is stored take steps to prevent infestation and contamination.

The International Rice Research Institute ( has compiled information on these pests and the ways in which rice millers and other facilities that store rice can minimize the damage.


Each insect species has its own optimum temperature and moisture range for development. The optimum temperatures for most insect species range between 25°C to 32°C, and the optimum relative humidity is 70%. Most species cannot survive extremely dry conditions or extremely cold (below 5°C) or hot (above 45°C) temperatures.

Insect activity and fertility are also affected by the change in light. Moths, for instance, are most active at dawn and dusk. Inspections to check and control flying insects are best made at these times. Artificial light can help to reduce the movement and fertility of moths.

Most storage pests are able to penetrate a stack of bags more quickly and thoroughly than bulk grain because of the gaps between the bags. The size, the surface texture and the nutrients in the grain also influence the ability of the pest to attack it.

Insects in stored rice can be classified as either primary or secondary insects. Primary insects are those whose larvae feed entirely within the kernels of grain. These include the rice weevil, Angoumois grain moth and lesser grain borer. Secondary insects, such as the saw-toothed grain beetle and the rust-red flour beetle, feed from the outside of the grain even though they may chew through the outer coat and devour the inside.

The IRRI says the management of storage insects should be done in a sequential and integrated manner. An effective pest control system involves:

  • drying and storage of clean, dry grain;
  • disinfecting the storage system; and
  • controlling or preventing pest infestation during the
  • storage period.

Grain must be dried to at least 14% moisture and seed grain should be dried to 12% moisture before storage. Grain needs to be dried so that it will not cause cracking, since cracked grain is easier for insects to infest. This requires drying the grain at a rate and temperature that will not damage it. The first stage of drying harvested grain at 25% moisture to 18% can be done at temperatures ranging from 50°C to 60°C. After this, the grain needs tempering or cooling for at least four hours. Drying from 18% to 14% moisture should be much slower, and the temperature should not exceed 42°C.

Grain storage bins must have a damp-proof floor, waterproof walls and roofs, and be well sealed so fumigation is possible. Rice stored in bags should be stacked on pallets at least 50 centimeters from the walls.

Hermetic grain storage systems have proven to be effective. By having a sealed atmosphere, the insects utilize the oxygen, exhale the carbon dioxide and eventually die through suffocation and dehydration over a 5- to 10-day period.

Disinfecting requires a systematic and thorough cleaning of all sources of infestation before storage. Old grain residues should be treated, removed or destroyed. Storage structures can be treated with insecticides such as Malathion, Fenitrothion or Deltamethrin.

The first step in controlling any pest problem is to determine the level of infestation and then select an appropriate method for control. All storage should be checked regularly, preferably every two weeks. Random samples need to be taken from all grain and tested for infestation. If there are more than four insects per kilogram, some form of treatment is required. A simple rule of thumb for the number of bags to be sampled is to use the square root of the lot size. For example, if there are 100 bags in the lot, samples should be taken from 10 bags.

Fumigants are effective against storage pests because as gases they can reach them in the most remote hiding place. The IRRI report noted that the range of safe fumigant chemicals that can be used is now restricted to phosphine and carbon dioxide.

Phosphine fumigation is undertaken using tablets and pellets which release phosphine gas when they come into contact with humid air. When insects are exposed to fumigation in a sealed environment, all stages of development — from the eggs, larvae, pupae to adults — are killed. When it is correctly applied and the grain is sufficiently aerated, phosphine does not impair the grain nor leave residues that could be hazardous to the consumer.

Care must be taken when using phosphine since the gas is very toxic to humans. Fumigation must take place in an enclosure that is tightly sealed, and once the exposure time is ended, the grain must be aerated and the bin checked for residual gas before entry.

Another type of fumigation involves using carbon dioxide to replace the oxygen in the storage bin. This suffocates and dehydrates the insects as well as producing toxic chemicals in their blood. To be effective, elevated carbon dioxide levels must be maintained until all the insects die. The required exposure time depends on the percentage of carbon dioxide and the temperature of the grain. The downside to this type of treatment is the cost of carbon dioxide fumigation is high.

Control of some insects, such as the rusty grain beetle, can be achieved by using a non-toxic dust made from prehistoric diatoms. When the insect comes in contact with this dust, the waxy covering on the exoskeleton is absorbed, leaving them prone to dehydration and death. The product, diatomaceous earth, is applied as grain is loaded into the bin and is most effective when applied to dry grain at harvest. Control can take up to five or six weeks.


Rats and mice have been estimated to damage more than 1% of the world cereal crops each year. In developing countries, estimates of 3% to 5% have commonly been reported. There are about 50 diseases that can be transferred to humans by rodents, including typhoid, paratyphoid and scabies, which is why keeping them away from stored rice is critical.

Rats consume about 25 grams of food per day and the daily intake of mice is about 4 grams. Besides eating stored grain, rodents can contaminate it with urine, feces, hair and pathogenic agents. Since it is extremely difficult to remove contamination, batches of rice that are infested must be declared unfit for human consumption. They can also cause damage to bags, pallets and the storage structure itself through chewing.

Signs that rodents are present where rice is stored include:

  • feces droppings;
  • runs and tracks (dark, greasy stains) that can be found along the base of walls, fences or across rubble;
  • footprints and tail marks in the dust;
  • urine traces, which are fluorescent in ultraviolet light;
  • chewed grain kernels and gnaw marks on sacks, doors, cables and other materials; and
  • nests in corners or roof areas or burrows just outside the building.

Rodents thrive in places where there is a sufficient food supply, protected places to build burrows and nests, hiding places and access to produce. Preventive measures that can be taken as part of an integrated control program include:

  • keeping the storage area clean by removing any spilled grain;
  • storing bags in tidy stacks set up on pallets, ensuring that there is a space of 1 meter around the stack;
  • storing empty or old bags and fumigation sheets on pallets, and if possible, in areas separate from the storage facility;
  • keeping the facility free of garbage so that rodents won’t have places to hide or nest;
  • keeping the area surrounding the building free of tall weeds, as rodents have an aversion to open spaces; and
  • keeping the area around the facility free of standing water, which rodents can use as a drinking source.

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