Despite increased food production from agricultural advancements, significant quantities of harvested food are lost due to inadequate protection of stored products. An FAO estimate indicates that insect infestation causes about 10% or more loss in developing countries.
Losses may be greater now, with growing emphasis on establishing national and international buffer stocks to mitigate production irregularities caused by unpredictable climatic conditions.
Public concern over synthetic pesticides in agriculture cannot overshadow their necessity, nor is a rapid decline in their use anticipated, making consumer protection and user education imperative.
Chemical pest control methods, when applied intelligently and knowledgeably, can be both effective and safe.
Understanding the classification, mode of action, properties, metabolism, and residues of pesticides is essential for users to assess benefits and potential hazards accurately, enabling judicious insecticide selection and formulation of efficient control measures for specific circumstances.
Pesticide application refers to the practical delivery of pesticides to biological targets (e.g., pest organisms, crops, or plants).
Public concern about pesticide use underscores the need to optimize this process to minimize environmental release and human exposure, including to operators, bystanders, and consumers of produce.
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Pest Control Strategies for Stored Agricultural Produce
Various techniques control insect pests in stored produce, ranging from traditional sunning and smoking on farms to irradiation in large-scale bulk handling.
Specific recommendations are challenging, as techniques must be tested for particular situations and may become unsuitable due to changes in economics (product value relative to material and labor costs), pest problems (occurrence and resistance), and farming system techniques or new product availability.
Consideration of economics and technical specifications for effectiveness against target pests and hazards to farmers and consumers is crucial.
Field trials supported by effective loss assessment are necessary to determine if a control technique’s benefits justify its costs.
Insect Control Methods in Warehoused Sacked Grain
Three common chemical methods control insects in sacked grain stored in warehouses: admixture of insecticidal dusts with produce before sacking, spraying or dusting successive sack layers with insecticides during stacking, and enclosing a fumigant with sacks under a gas-proof sheet.
Admixture of insecticidal dusts is highly effective with suitable insecticides. Synthetic pyrethroids and pirimiphos methyl dust, applied at 2.5–15 ppm active ingredient, can eliminate insects in stored bags for at least eight months.
Dust mixing methods include shovel mixing on a tarpaulin or, for large-scale operations, using a drum with an eccentric axle.
However, dust admixture poses health hazards and is not recommended unless a safe insecticide is used and grain is consumed after prolonged storage.
Spraying or dusting sack layers is less hazardous but not always effective.
Pirimiphos-methyl (Actellic 50 EC), applied undiluted at two to three strokes per bag with a domestic applicator, significantly reduces weevils in heavily infested maize sacks for eight months, though applying undiluted insecticides carries risks.
Fumigation is the most effective method for insect elimination in bagged grain. A gas is released among bags covered by a gas-proof sheet, secured with “sand snakes” or a heavy chain wrapped in hessian, and left for at least three days.
For small-scale storage (100–300 tonnes), aluminum phosphide, releasing phosphine gas upon moisture absorption, is convenient, using one tablet per two bags in a stack hermetically enclosed within two hours. Phosphine and other fumigants are effective for larger grain quantities.
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Comprehensive Pest Control Techniques for Stored Produce
1.Sanitation Practices for Storage Facilities: Reducing initial pest populations and preventing insect development in stored crops is critical. Before storing new crops, infested material must be removed, new grain must not be mixed with old, and old material retained must be fumigated.
Storage structures must be cleaned by brushing away spilled grain and dust, disinfecting sacks and baskets through sunning or chemical treatment, and using smoke, sun, or rain for small rural structures to encourage insect departure. Early control measures prevent field infestations.
2. Natural Resistance in Crop Varieties: Crop varieties vary in susceptibility to storage pests, with traditional varieties often more resistant than new ones.
New maize and cowpea varieties with improved storage resistance are becoming available. Resistant characteristics include good husk cover in maize, glume-covered sorghum grains, paddy rice, intact cowpea pods, and grain hardness in cereals.
3. Hermetic Storage for Pest Suppression: Airtight conditions reduce oxygen and increase carbon dioxide, halting insect and mold development. Dry grain is essential for human consumption or seed, as damp grain undergoes bacterial and enzymatic action, causing tainting and viability loss.
Bagged material must remain sealed to prevent severe losses from insects, rodents, or careless handling. In North Nigeria, storing threshed cowpeas in sealed plastic bags with cotton liners prevents insect perforation.
4. Chemical Control Methods for Stored Products: Insecticides are applied via dusting, spraying, or fumigation. Dusts (2.5–15 ppm active ingredient) are sprinkled layer by layer or mixed using tins, groundsheets, or revolving drums.
Sprays (10–15 ppm active ingredient in 0.3–2 l/tonne water) are applied with domestic or knapsack sprayers, targeting bagged produce, warehouse spaces, or cribs.
Fumigation uses phosphine (three to four days) or ethylene dibromide (less than one day) in airtight conditions, essential for export crops like groundnuts and cocoa but hazardous if misused.
Classification and Characteristics of Pesticides for Stored Produce
Insecticides are classified by mammalian toxicity, chemical origin, composition, mode of entry, and formulation.
1. Mammalian Toxicity Assessment: Toxicological studies determine safe chemical thresholds using LD50 (dosage causing death in 50% of test animals), evaluated through acute oral, dermal, inhalation, and chronic tests.
Insecticides for stored products are low in mammalian toxicity, effective against target species, persistent under storage conditions, and do not alter commodity flavor, color, or odor.
2.Chemical Origin and Composition:
i. Botanicals: Extracted from plants (e.g., pyrethrum from Chrysanthemum), with low mammalian toxicity but high insect toxicity.
ii. Synthetic Insecticides: Include organochlorines (toxic, not used on stored food), organophosphates (e.g., malathion, pirimiphos methyl, safer due to rapid decomposition), and carbamates (e.g., carbaryl, limited use).
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Frequently Asked Questions
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