APPENDIX 1 PESTICIDES USED FOR STORED PRODUCTS PROTECTION The chemicals currently (and recently) used widely are quite few in number, for a basic requirement is for a broad-spectrum insecticide and acaricide which will effectively kill all the insects and mites commonly found in stored grain and other products. For produce stores and warehouses, it is clearly advantageous to be able to have one basic treatment for the whole place, which is why fumigation with methyl bromide or phosphine was so widely practiced. But there will be occasions when commodities such as dried fish are infested with maggots, or cockroaches or rats are a nuisance, or a tobacco store is infested, and then more specific treatments are needed, and different pesticides may be recommended. The chemicals listed here are the general ones that are currently being used, or more recently widely used, in stored produce protection against insect and mite pests. Several important pesticides have recently been banned, both in the USA and the UK following reports of carcinogenic properties (methyl bromide, ethylene dibromide etc.), and the last of the organo-chlorine compounds (HCH – Lindane) were withdrawn from use because of environmental risks. But the stockpiles of these chemicals are still being sold abroad, and in some tropical countries are still very effective. In the hot tropics, Lindane does not pose the same environmental risks that it did in the UK and North America because of its more rapid degradation. It must be remembered that in some regions there is widespread resistance shown by many common pests to the major insecticides in use, so local advice should always be sought when contemplating pesticide control measures. A warning is given if any pesticide is particularly toxic to fish, for many tropical countries either have a freshwater fishing industry, or else farmers rear fish in ponds or paddy fields, and there have been many unfortunate instances in the past of farmers dumping used pesticide containers into the local freshwater bodies. Many on-farm stores are situated in the vicinity of ponds and water courses. A useful source of information on pesticides as grain protectants is the book by Snelson (1987). At present, this section does not include substances that are solely fungicidal or bactericidal. The pesticide manual (Tomlin, 1994) has to be consulted for these compounds. 415 416 APPENDIX 1 INORGANIC (INERT) COMPOUNDS/MINERALS Peasant farmers in many parts of the world have ancient traditions of adding inert materials to stored grain and seeds to reduce pest infestations. Quite how this operates is not always clear, but certain points seem obvious. Any fine material will have a filter effect, and will fill the interstices between the grains and hamper insect movement. And an abrasive material will scratch the wax from the insect epicuticle, and so increase water loss from the insect body. And some materials clearly contain chemicals that have an effect on the insect pests. Seeds and seed grain (for next season’s planting) are especially valuable to the farmer, and being small in bulk, protection methods can be used with them which would not be feasible or economic for bulk-stored grain to be used as food. For example, the production of special wood ash from selected botanical materials can often only be done on a small scale. Below are listed some of the usual additives that are being employed by farmers to protect seed and grain in storage, and also a couple of inorganic compounds which are similarly used in recent years. Ash This is usually wood ash, and is useful as a physical barrier in the grain, but it can also possess various chemical properties according to its botanical source. Foliage of Neem (and sometimes Lantana) is often burned so that the ash can be added to seeds (and grains) in storage. But rice husk ash is used in Japan and India for rice storage, and cow dung ash in India and Africa with good success rate. Coal ash is also successful. Boric acid (’Grovex’, ‘Boric Acid Powder’, ‘Cockroach Control Agent’) This chemical probably does not strictly belong to this category, but it is an inorganic compound, relatively inert, which has been used successfully to control cockroaches and crickets. Boric acid and Borax were both cleared for use on food premises in the USA (1991). Diatomaceous earths (Diatomite) These naturally occurring powders contain tiny silica crystals from the dead diatom bodies, and the abrasive powder so formed can have a wearing effect on the insect cuticle, scratching away the waxy epicuticle so that the insects suffer dehydration and can die of desiccation. But natural deposits of diatomaceous earths are limited in distribution, and only found in a few areas. Generally effective with rice, beans and wheat, and bulk maize, against the common pests. Sand Most sands are formed largely of silica crystals of very hard consistency, and fine sands are often added to stored seeds and grains. One problem is that the weight of the grains of sand will tend to result in all the sand falling to the bottom of the container. This method can be successful for stored seeds in small containers, but PESTICIDES USED FOR STORED PRODUCTS PROTECTION 417 for bulk grain is less feasible. But sands are widely distributed, and can be found in most areas, and can be reused repeatedly. Cowpeas, groundnuts, beans, peppers etc. mixed with dry sand and sealed in metal drums have been well protected in Africa, India and Indonesia. Silica gel (Silicic acid) (‘Dri-Die’,‘Silikil’,‘SprotiveDust’) Often called silica aerogel – the very fine crystals are formulated as a desiccant dust for admixture purposes. The crystals have a hygroscopic effect and absorb atmospheric moisture, and if coating the insect body, will absorb moisture from the insect, leading to death by desiccation. Most silica gels are reusable after heating to drive off the absorbed water. Approved for use on food premises in the USA (1991). Kaolin clay A very fine naturally occurring clay found in large deposits in most countries; used as a dust, sometimes with lime (calcium oxide), when it was either insecticidal or acted as an insect deterrent. The use of some of these materials, especially the naturally occurring ones, should be investigated more fully, for it would seem that they could be of use in many rural communities in the tropics, where there is of necessity emphasis on low external input sustainable agriculture. Most of these substances can be removed after storage by simply sieving, and then can be reused repeatedly. ORGANOCHLORINE COMPOUNDS These are mostly broad-spectrum and persistent poisons, and so in the past several (DDT, dieldrin etc.) were successfully used for the protection of stored products, but in more recent years, concern about environmental contamination and long-term toxicity led to their withdrawal for this purpose, and at the same time many insect pests had developed resistance to them. But in many developing countries, they are still being used, especially lindane. Gamma-HCH (= Lindane) (= Benzene hexachloride) (‘Lindane’,‘HCH’,‘BHC’ , ‘Gammalin’ etc.) Properties. HCH occurs as five isomers in the technical form, but the active ingredient is the gamma isomer. Lindane is required to contain a minimum of 99% of the gamma isomer. It exhibits a strong stomach poison action (by ingestion), persistent contact toxicity, and some fumigant action, against a wide range of insects. It is non-phytotoxic at insecticidal concentrations, but the technical product causes tainting of many crops, although there is less risk of tainting with lindane. It occurs as colourless crystals, and is practically insoluble in water; slightly soluble in petroleum oils; soluble in acetone, aromatic and chlorinated solvents. Lindane is stable to air, light, heat and carbon dioxide; unattacked by strong acids, but can be dechlorinated by alkalis. 418 APPENDIX 1 The acute oral varies considerably according to the conditions of the test – especially the carrier; for rats, it is 88-270 mg/kg; ADI for man 0.01 mg lindane/kg. Uses. Effective against a wide range of insect pests, including beetle adults and larvae, ants, fly larvae, bugs, caterpillars – in fact, most stored products pests, including mites. It was regarded as being the most potent of all grain protectants in its time. Caution. Harmful to livestock and fish. Irritating to eyes and respiratory tract. In many countries, it is no longer recommended for use on stored grain or foodstuffs, because of its toxicity and persistence, and insect resistance became widespread; in some countries, its use is banned. But it is still widely used in some tropical countries for grain protection. Formulations. E.c., w.p., dust, smoke, and many different mixtures with other insecticides and with fungicides. ORGANOPHOSPHORUS COMPOUNDS These compounds were developed in World War II as nerve gases, and are among the most poisonous compounds known to man. They are very effective insecticides, but their high level of mammalian toxicity is a serious hazard, and many of the more dangerous chemicals are now withdrawn from general use in most countries. The mode of action in both insects and mammals seems to be inhibition of acetylcholinesterase. Most of the earliest developed compounds are also quite short-lived chemicals, and so have little residual effect. But some of the more recently developed members of this group have both lower mammalian toxicity and greater persistence, and so are of great use in protection of stored products. Azamethiphos (‘Alfacron’, ‘Snip’) Properties. A broad-spectrum contact and stomach acting insecticide and acaricide, having both a quick knock-down effect and good residual activity. It forms colourless crystals, insoluble in water, but slightly soluble in some organic solvents; suffers hydrolysis under alkaline conditions.