Stored Grain and Control of Insects
Major losses in storage are attributed to damage caused by stored grain insects (Labadan 1974, Casas 1980). Actual loss is through direct reduction of grain weight and commercial quality. Few farmers treat their grain with chemicals, although fumigation is the only practical method known to control these insects.
Insects consume more than their own weight. They feed on the seed embryo, which in turn greatly reduces the viability of the grain, resulting in a greater amount of seed required to plant a given area. The insects contaminate the inside and outside of the kernels by their cast skins, fecal pellets, webbings, and odors. Further, heat produced by the insects results in kernel discoloration, contamination of other grains, and development of bad odor.
Stored grain insects
The storage insects found in the Philippines are the same ones associated with stored products in most parts of the world. Of the hundreds of insects that are associated with grains and legumes, only 11 are destructive pests of stored grains and legumes.
Insects in paddy (unhusked rice)
Rice weevil(Sitophilus oryzae Linn) lesser grain weevil and Angoumois grain moth(Sitotroga cereallella Oliv.)
These insects bore into the sound kernels and, at times, almost destroy the grain completely. The rice weevil attacks only the grains with hulls which have been broken or have failed to close completely after flowering. The lesser grain borer and the angoumois grain moth can cut through the rice hull and destroy the kernel. Other insects that feed on the surface of the kernels and broken grains, and those that do damage by boring are the saw-toothed grain beetle (Oryzaephilus surinamensisLinn.), flat grain beetle (Laemophloeus pussillusSchonn.), and rice moth (CorcyracephanolicaStaint.)
The damage caused by these insects is not only seen in the actual loss in weight but more so in the resulting broken grains during milling, which in turn makes them more susceptible to the attack of other pests.
Insects in milled rice
Milled rice is attacked by more insects than paddy rice. The removal of the hull after milling and the presence of broken grains attract more pests. The following insects are encountered in milled rice.
Rust red flour beetle (Triboleum castaneum Herbst.)
Corn sap beetle (Carpophilus dimidietus Fabr.)
Coffee bean weevil (Araecerus fasiculatus Deg.)
Lesser mealworm (Alphitobius diaperinus Panz.)
Cacao moth (Ephestia elutella Hubn.)
Indian meal moth (Plodia interpunctella Hubn.)
This group of pests damages the surface of the kernels. Their presence and the injury they inflict on the grains mean high cost to the grain industry. Most of these pests become abundant under favorable conditions associated with high or low moisture or temperature, poor sanitation, and poor-quality grains.
Stored grain insects multiply at a relatively fast pace because of favorable temperature, relative humidity, moisture content of the grain, poor sanitation, and abundance of food. If they remain unchecked, their number will be in alarming proportions. Most of these pests can complete their life cycle in about a month or so.
Where to look for insects
Periodic inspection of stored grain in the warehouse and the premises is necessary to detect infestation. Insects are usually found between adjacent sacks, bags, or containers placed against the wall, in the folds and seams of sacks, on floors and walls, in cracks and crevices of old warehouses, within the grain itself, and inside the vehicles used for transporting grains and mill equipment.
The more obvious signs of infestations are the presence of live and dead insects and their feces or excreta on the surface of containers, floors, and walls of the storehouse.
Control measures
Control of stored grain insects, molds, and mites cannot be thoroughly achieved using one procedure alone. It has to be a coordinated effort, which involves proper sanitation, proper construction of storage facilities, proper drying of grains, and proper use of chemicals to prevent insects and molds. To protect the grain and maintain its good commercial quality, the following important aspects of management should be considered.
1. Sanitation
Good housekeeping is one of the simplest and cheapest preventive measures in pest control. Storage facilities should be clean and free from old infested grains, dirty containers, and foreign materials that may harbor insects. Particular attention should be given to cracks and crevices on the floors and walls of the warehouse. It is also good practice to separate the old stock from the new one to prevent cross-infestation. The principle of ‘first-in, first-out’ in grain disposal is a sound practice to follow.
2. Mechanical and physical methods
Temperature.Since most stored grain insects cannot tolerate extreme temperature, heating and cooling are logical approaches to insect control. At least once a week, the stock should be inspected and, at the first sign of insect infestation, remedial measures should be applied. Temperature within the 120–130o F range maintained for 10–12 hours is effective. However, drying systems are expensive and temperature above 115o F may damage seed viability, unless the grain is cooled soon after drying. A good substitute that small farmers may use would be sun drying for 1–2 days.
Mechanical aeration is a highly effective means of cooling stored grain to prevent moisture redistribution in the grain and render it unfit for insect development. Aeration removes the heat as fast as it is liberated in the damp grain, thus preventing a general temperature increase. This is essential to all types of storage, especially bulk storage, where turning or moving of grain is difficult.
Moisture control.Most of the stored grain insects are unable to survive and reproduce on grains with moisture content below 9%. Molds are moisture-loving organisms but they cannot thrive on grains with low moisture content. It is therefore a must to dry the grains before storing them to a moisture content of 13–14%), which is in equilibrium with 70% relative humidity or lower.
Atmosphere control.Air-tight or hermetic storage involves storing of grain protected from the exchange of gases or liquid with the outside environment. Insect control is accomplished by oxygen depletion and increased in carbon dioxide gas coming from respiring grains and insects.
Grain turning.Moving or turning grains will provide a means of the temperature below that required for insect development and will disperse pockets of moist grain to prevent deterioration.
3. Chemical control methods
Spraying, misting, fogging, and fumigation are only supplements to good warehousing practices. Chemical control will be useless without good sanitation. In the application of any chemical measure, special precaution must be observed and manufacturer’s instructions should be followed to the letter. All these chemicals should be properly handled; they are poisonous not only to insects but also to people.
Residual spraying.Residual spraying of either 2% malathion or lindane may be used on the floor, walls, and ceilings of storage facilities 2–3 weeks before grains are stored. Other insecticides recommended at 2–4% are tetrachlorvinphos, primiphos methyl, and chlorpyrifos methyl. Pyre-thrumpiperonyl butonxide is sprayed at 0.5% concentration.
Fumigation.Most fumigants are highly toxic to man and animals. The products are available in solid, liquid, or gas formulation. At the required temperature and pressure, the fumigants can exist in the gaseous state, which can diffuse and penetrate tightly packed materials to be lethal to insects. The present practice is to use either methyl bromide or phostoxin under gas-proof plastic sheets that cover the grains to be treated.
Surface and space treatment.This spraying prevents infestation but generally does not kill the insects which are already established in the grains. The practice is good for killing moths and adult beetles flying around the warehouse and to protect bins or grains against invasion from outside. The commonly used chemicals are lindane and malathion, sprayed at 2% per 1,000 ft2of surface area.
Sack treatment.Grains are stored in sacks or jute bags treated (air-dried after treatment) with 24% of any of the following insecticides: malathion, primiphos methyl, and tetrachlorvinphos. Grains stored in 4% lindane or malathion treated sacks can be stored for 35 months; those in 2% can last for 6–12 months, depending on the type of grain and insect infestation (Author’s experience).