A wide range of insect pests attacks all stored sorghum grains and grain products in the arid and semi arid tropics. Typical conditions for infestation and damage for stored sorghum grains are summarised in Table 5.
Table 5. Conditions for infestation and damage for stored sorghum grains
Insect |
Optimum conditions for infestation |
Damage symptoms and losses |
Rhizopertha dominica |
Heating follows infestation |
Adults and grubs make ragged holes in grain; losses in improved cultivars range from 17-66% (Gupta et al., 1977). |
Sitophilus oryzae |
Preferred moisture content 10-16%. Red varieties with soft endosperm are more susceptible. |
Adults and larvae feed on grain; losses 2-21% for improved cultivars in India (Gupta et al 1977). |
Tribolium castaneum |
Does not attack whole grain; feeds on damaged grains; fairly resistant to high temperatures. |
Tunnelling damage; when attack is severe, grains turn greyish yellow and mouldy with pungent smell. |
Sitotroga cerealella |
Infestation starts in the field; develops rapidly on low moisture content sorghum stored on the panicle. |
Field infestation on the upper layer of the grain; larvae bore and consume grain contents adding excreta and webbing. |
Ephestia cautella, Plodia interpunctella, Corcyra cephalonica |
Adult moths are active at dawn and dusk, particularly during periods of high humidity; inactive in complete darkness |
Attack the embryo of whole grains; thick web left on grain surface; can cause losses in excess of 10% |
Sorghum is a vital subsistence crop for rural populations in semi-arid regions and most farmers are aware that newer high yielding varieties are more susceptible to storage insects. Good management practices, including: examining stores regularly for signs of insects, moulds and rodents; cleanliness in and around the grain store; and cleaning, and keeping grain dry, all contribute to quality and reduce the extent of losses. Varietal resistance to insect attack, physical, mechanical and botanical pest control measures integrated with minimum use of insecticides reduce considerably the damage done by pests.
Fumigation of grain is a widely used method of controlling storage insects. The practice is usually restricted to commercial stores where adequate safety precautions can be maintained. The two most commonly used fumigants are methyl bromide and phosphine. Methyl bromide is in the process of being withdrawn from use because of its ozone depleting properties, and is no longer recommended.
It may well not be cost-effective to treat all grain immediately after harvest but rather only the proportion of the produce which is to be stored for three months or more. Such advice helps the farmers to decide on actions to be taken, encouraging assessment of the need to store or sell, given market prices, the quantity harvested and so on.
For the last 20 to 30 years, it has been a standard recommendation that farmers should treat the crop to be stored with an organophosphate (OP) dust. Several OPs, including malathion, fenitrothion, iodophenphos and pirimiphos-methyl have been approved by the UN Codex Alimentarius Commission for application to raw cereal grains, flour, pulses and some oilseeds; acceptable daily intakes and maximum residue limits have been prescribed.
Many of these compounds are commonly applied as sprays for other agricultural purposes. However, it requires the use of a sprayer which most farmers do not possess or cannot afford. Furthermore, spraying requires dilution of an insecticide concentrate, a hazardous process especially for untrained small-holder farmers. Dilute dusts are composed of at least 95 percentage inert material. They are therefore very bulky, difficult to package in quantities suitable for an individual producer's needs, and difficult to distribute to rural communities at a time when they are most needed and to where they can be conveniently obtained. Families frequently complain they have no access to insecticides when they want to apply them, and whatever little is available is expensive. Farmers who cultivate cash crops or have other sources of income may be able to afford these high prices, but most farmers in the semi-arid tropics are poor and do not have cash or credit to be able purchase insecticides. During the last decade, efforts have been made to identify alternative methods of protecting grain against insect damage which are of low or no cost to the farmer and are therefore likely to be acceptable and utilised. Furthermore, these methods are more environmentally sustainable and less of a health risk than the synthetic insecticides.
Botanical pesticides:
Throughout the developing world, farmers have traditionally used the plants and trees around them as sources of insecticides. The number and type of species used for this purpose is considerable. The literature contains many references to the effect of plants on storage insects (Dales, 1996), but this work is mostly confined to laboratory investigations. There is very little direct evidence which demonstrates that plants used by farmers are effective grain protectants. Few studies have been conducted to replicate farm methodologies, one reason being that farmers themselves are uncertain of the details of the methods and can only provide subjective assessments of effectiveness.
Plants with known insecticidal or repellent properties include: Securidaca longipedunculata, Chrysanthemum cinnerariafolium (pyrethrum), and Azadirachta indica (neem). Both pyrethrum and neem are relatively unstable and may not be suitable for long term grain storage. Although more than 130 plants have been reported as being used as storage protectants, only pyrethrum and neem have been used commercially.
In some countries, palm and coconut oils are used to protect grain, particularly those for seed. However, they are usually considered too costly for storage insect control.
Mechanical control:
Sieving, winnowing combined with sun drying, sticky bands, water traps and baits can be used for insect control in storage. Turning grain to interrupt population development is quite feasible for bulk grain in silos.
Physical control:
Low and high temperatures (> 60_C) inhibit development, and may kill several insects. Dusts such as ash, sand and other mineral powders can be used to fill the interstitial spaces in grain bulk. These provide a barrier to insect movement and damage the insect cuticle causing death by dehydration. Diatomaceous earths (naturally occurring aluminium silicates) absorb waxes from the insect cuticle and cause death by dehydration. These are `Generally Regarded As Safe' (GRAS) by the US Environmental Protection Agency for use with grain.