By Mike P. Kelly
Grain storage, in any country and in any climate, is at risk from infestation attack. For many systems, rodents are the principal problem, partly because they are very visible both as pests and for their traces of droppings and footprints and damage to the structure. However, there is also the more insidious threat of attack by insects, which are often not observed until they are well established and creating excessive damage to the grain.
STORAGE SYSTEMS: PROS AND CONS
For most of the developed world, grains are stored in bulk, and many favor bin or silo structures. They generally have the advantage of being relatively easily proofed against rodent (and bird) ingress. Conversely, it is far less easy to detect the presence of insect pests within large bins and silos before they attain significant populations and cause much physical damage.
Other methods of bulk grain storage involve open-topped bins (usually of quite small individual capacity and always within buildings to protect from the weather) and flat or floor storage, where the loose grain is heaped onto a sound floor and retained by bulkheads or grain walls that can withstand the sideways pressures of grain at its natural angle of repose.
These latter storage systems are not easily proofed against rodent attack, except, where practical, by effectively proofing the building itself. Their real advantage comes with complete access to the entire grain surface for sampling or for installing insect and mite detectors (traps) across the surface and into the depth of the bulk.
Sealed external bins and silos are not accessible for such ease of sampling or trapping. Therefore, they create a mysterious world in which mites and insects can breed and change cereal grains from wholesome raw materials into germ-free rotting masses contaminated with mite allergens, insect fragments and inevitable mold toxins.
The developing countries generally remain faithful to hessian sacks for grain storage. The resulting sack stores can house huge stacks of several thousand tonnes of grain, all very neatly built into semi-pyramid stacks for great stability and ease of access for inspection — and also for rodents unless the building is effectively proofed, as mentioned for open-topped bins and flat stores. Very often these stores have excellent smooth concrete floors suitable for sheeted stack fumigation. Sack storage in temperate countries would normally be at cool temperatures, with the sacks themselves and inter-sack spaces all helping to aerate and cool the grain. However, sack storage in a tropical climate doesn’t see these advantages since the store internal ambient temperature will be warm enough to encourage rapid insect development, just as artificially-dried bulk grain does in temperate climates.
While pest problems have been recognized for decades, the extra pressures during the late 20th and early 21st century, centered on food safety, have raised their significance. At the same, however, options for pest control by chemicals are becoming more restricted each year.
RESISTANCE AND OCCURRENCE
There is virtually no evidence to show that global levels of insect and mite attacks on grain stores are reducing. In fact, there is real evidence that levels of pesticide resistance among vertebrates and invertebrates is increasing.
In the U.K., anti-coagulant rodenticides are the most commonly-used group; resistance to these compounds in wild rodent populations is quite widespread. The U.K. also uses organophosphate (O-P) insecticides as a prophylactic and to control existing insect infestations.
O-P resistance is not uncommon amongst grain mite colonies and is present in many insect populations, though seldom leading to field control failures in the insects.
Australia uses low-concentration phosphine gas extensively in well-sealed storage silo structures, and has also experienced increasing levels of phosphine resistance. Previously Australia used malathion (another O-P) very widely (and possibly not accurately enough) and was forced to change because of widespread levels of practical O-P resistance.
Against this background, what are the principal recommendations to protect the new harvests, in all types of storage, against insect and mite problems?
It has to be said that where insecticides are available, they usually represent the simplest and most effective prophylactic strategy with the greatest chance of success, particularly when following the steps below.
•Pre-harvest cleaning… At least one month before harvest, check on the risk of residual infestation. If positive, spray the store (but only after cleaning). Check again for survivors.
•Post-harvest cleaning… After loading the grain into store, monitor grain throughout storage period.
An interesting development in the pest control arsenal is desiccant dust. Following its introduction several years ago in countries such as Australia, the application of desiccant dusts has proven effective against all beetle species and grain pest mites. Desiccants work in a physical mode, removing some of the water-conserving waxy cuticle covering the insect’s body (exoskeleton), resulting in death by desiccation. Desiccants have been used successfully against cockroaches and crickets in public health pest control for many years. They don’t decay, and leave no toxic residues, but they are easily separated from grain during handling. However, they can create worker-related potential problems of air-borne silicate dust.
DETECTION METHODS IN GRAIN AND EMPTY STORES
The important pest management decisions throughout the storage period should be based on accurate information. In years past, this was not always possible. But for grain stored in open-topped bins, in flat/floor stores, and in sack stores, recent developments have made possible more accurate monitoring of different critical elements.
For temperate climates, where storage at reduced temperatures is possible just by using cool night-time air blown gently through the bulks, temperature monitoring by mobile or fixed thermocouple-probes gives vital information on the safety of that grain to sustain insect attack. In general, if the bulk is uniformly below 12°C (54°F) no significant insect development can take place. Note that these probes are NOT to detect an insect-derived "hot-spot." They are only to tell you if the grain is safe from further insect development.
For warmer climates and where low-volume ventilation is not usually effective, traps for detecting insect presence at the earliest stages are crucial. Traps do not control insects, but notify you of their existence early enough so that you have a range of options before the problem becomes critical. Ignoring ethics for a moment, one option may be to sell the grain quickly before the insects develop further. Another option may be to treat the grain with insecticide, or to have it fumigated, to remove the threat to the grain. Waiting until the insects become obvious risks everything and leaves no chance of recovering the original grain quality once the damage has occurred.
There are several designs of suitable traps. In Europe there is a "P-C" trap consisting of a clean plastic cone with a perforated lid, inserted either flush with the grain surface, or deeper in the bulk, which is effective at detecting all of the major grain pest species, without any form of lure. They are commonly called "blunder traps" relying on the almost constant activity of these insects within the grain bulks. There is also the older design from America, the probe trap. There is little to choose between these designs, all traps being at least 10-times more effective at finding evidence of insects than spear-sampling bulk grain. This means you can find insects 10-times sooner, or at 10-times lower population density. Either way, this is a big advantage and gives you far more options as discussed above.
In sack stores, sampling and visual inspection works much better than in bulk grain. Insect behavior is quite different in a sack store, and lends itself to different techniques. The best form of trap is the bait bag — a plastic mesh bag about 10cm x 20cm and containing kibbled carob pod (locust bean), wheat and groundnuts. Other local ingredients suitable to the grain in storage could also be used, for example brown rice in a rice store, and a nut-free version for certain sensitive food sectors. The bait bags are spread around the less-accessible corners and machinery in the store, left for a few days before removal and shaking to self-sieve insects that have wandered in.
There is a newly-developed international standard relating to detecting invertebrates in grain, which deals extensively with trapping types and methods: ISO 16002 Stored grains — Guidance on the detection of live invertebrate infestation by trapping.
This guide also covers flying insect pests of stored grain in the tropics.
Results from all trapping exercises need to be compared with temperature of the grain (temperate climates particularly), the expected market place and its quality standards, the expected length of storage period, and the realistic options for control. However, identifying the trapped insects is the number one priority, since no decisions can be made unless you know what is in the grain.
All of the other decisions as above depend on that information.
THE LAST RESORT
Fumigation was mentioned earlier, and essentially it is the use of a poison gas to control grain pests. However, all fumigants are at least as poisonous to humans as they are to insects (and usually more so). The fumigant most commonly used in grain storage, phosphine, takes several days to kill all stages of insects, and can only be used where grain is stored in gas-tight conditions. This is where silo and closed bin storage might score over other methods, since these can usually be sealed enough to maintain the required gas concentration over a week or so. Nests of bins in a building and floor-storage are very difficult to seal adequately, and it may not be possible to use fumigation to eliminate an insect or mite infestation in very leaky situations. In most countries, fumigation of stored foods can only be done by trained and government-certificated specialist technicians, due to the great risk to human life if the fumigation is not done correctly.
Many cleaning details vary according to local situations. Here are some frequently asked questions:
Can grain sweepings be re-used?
Possibly, but disinfest by heat or fumigation if risk of infestation is high. Don’t leave bags of sweepings inside the store.
Is vacuum or compressed air better for cleaning?
Vacuum: it removes eggs and insects and mites, rather than blowing them into another less-accessible area.
Will a brush be acceptable?
Yes, but it takes longer and results depend heavily on worker conscientiousness.
How much needs to be sprayed, and what with?
All floor areas, sub-floor ventilation ductings, intake pit, wall/floor corners, wall tops (if stub walls), and all areas which are difficult to clean.
Use an insecticide high-volume spray to produce a complete coverage of all selected areas. Choose an insecticide approved for safe use in grain/food stores. Where appropriate, choose a product that also has approval for use directly on the grains, or the sacks.
Can I spray inside machinery used for processing grains?
I don’t know of any country that permits insecticides to be used directly on surfaces on which grain used for human consumption will be handled. Machinery must be effectively cleaned then bait-bagged to check for residual infestation.
**Note that any insecticide used in grain stores MUST comply with local and National regulations, and with the terms of any trade agreement/customer specification.
New Storage Resource:
The new book, Advances in Stored Product Protection, presents the proceedings of the 8th International Working Conference on Stored Product Protection held on 22-26 July 2002, York, U.K.
The IWCSPP, held every four years, is the premier world forum for the presentation of research results and reviews on the safe storage of durable foodstuffs, of which cereal grains, pulses and oilseeds make up the largest components.
The book highlights work on the pests and diseases that may cause spoilage, adverse health effects and grain loss, and discusses new techniques for the safe, effective and environmentally friendly management of stored commodities. With nearly 200 papers and contributions from leading experts around the world, the contents cover the future of stored product protection and the impacts of global issues, food safety, chemical and physical control, and processing and applications.
Published by CABI Publishing, the book was edited by grain storage industry leaders, P.F. Credland, Royal Holloway, University of London, UK; D. M. Armitage, C. H. Bell and P. M. Cogan of Central Science Laboratory, York, U.K.; and E. Highley, Clarus Design, Canberra, Australia.
Purchase the book online at: www.cabi-publishing.org/Bookshop/ and search by book title.
Common grain pests
1 Hairy fungus beetle, Typhaea stercorea. A fungus feeder, usually dusty, and an indicator of damp conditions.
2 Grain weevil, Sitophilus granarius. A primary grain pest in temperate climates. The rice and maize weevils (S. oryzae and S. zeamais) take over in hot climates. Larvae feed internally in grains.
3 Saw-toothed grain beetle, Oryzaephilus surinamensis. The U.K.’s most common grain pest, needing about 18°C (64°F) to breed.
4 Australian spider beetle, Ptinus tectus. A nocturnal scavenger from birds’ nests, often on the grain surface.
5 Flat (rust-red) grain beetle, Cryptolestes ferrugineus. Another primary grain pest, tropical in origin and needing 22°C (72°F) to breed.
6 Rust-red flour beetle, Tribolium castaneum. A mill pest, and often found in grain stores in the tropics.
7 A White-marked spider beetle, Ptinus fur. A nocturnal scavenger from birds’ nests. Male has white patches.
8 Foreign grain beetle, Ahasverus advena. Another mold feeder sometimes in vast numbers if the grain surface is damp.
9 Plaster beetle, Enicmus minutus. Very small and common fungus beetle, often dark gray or nearly black.
10 Flour mite, Acarus siro. Very common grain pest mite species, with characteristic smell when squashed.
11 Grain mite, Lepidoglyphus destructor. Another common grain pest mite species. Rapid jerky gait, and very hairy.