Pesticideshelf-life
The biological efficacy of pesticides graduallydecreases with time. The pesticide shelf-life is the period of timethat a pesticide can be stored before it deteriorates. Nearly allpesticides have a limited shelf-life. As part of modern pesticideformulation technology, packing methods and storage practice aim toprolong shelf-life as much as possible. Manufacturers indicate theshelf-life of the pesticide on the container, but many pesticides maystill be usable long after the indicated shelf-life has expired. Mostpesticides have an indicated shelf-life of at least two years fromthe time of manufacture, but shelf-life will be shortened ifpesticides are not stored properly (e.g. if they are stored at hightemperatures). Stock turnover organization needs to take into accountthe time that pesticides may have been in transit between manufactureand reaching the store.
Pesticides in sealed containers may change overtime in two main ways:
- The active ingredient may change chemically and break down into products that may no longer have pesticidal properties, thus decreasing the concentration of the original active ingredient.
- The formulation of the pesticide may break down and a precipitate of flakes, crystals or sludges may form, making it impossible to mix or use in sprayers.
An organochlorine such as endosulfan ischemically very stable, but some formulations may break down morerapidly. Organophosphates are much less stable and thereforegenerally have a shorter shelf-life. Dust and wettable powderformulations tend to break down and cake together, as a result ofhigh temperature, high humidity, strong sunlight or compaction underpressure, more than liquids in sealed containers.
Pesticide ordering andshelf-life
The shelf-life and rate of use must be takeninto account when ordering pesticides (Figure 15). Do not order morethan one year's requirement. The date of manu-facture and shelf-lifeshould be on the outside of the container. If a larger quantity isordered than can be used during the period of shelf-life, outdatedstocks will accumulate and present disposal problems, as well asfinancial loss.
FIGURE 15 - Storekeeper checking dates from labels oncontainers in a pesticide store
Stock inspection andshelf-life
Stocks in a pesticide store should be inspectedregularly for signs of deterioration, such as caking of powders,sedimentation or gelling of liquids and discoloration throughoxidation. Shelf-life declines rapidly after containers have beenopened and left partially empty. Stock turnover must be organized toensure that the contents of a container are used as quickly aspossible once the container has been opened. Unsealed containers ofdusts and wettable powders should not be kept for more than oneyear.
Containers are not only subject to deteriorationcaused by external factors (climatic, biological and mechanical), butcan also be corroded internally through the action of the pesticidesthey contain. Emulsifiable concentrate formulations are particularlylikely to affect weak spots, especially along seams (Figure 16) orwhere there are imperfections on the internal coating of thecontainer. Some pesticides increase in acidity during storage andthis makes them more likely to corrode containers from within.Discoloration of pesticide is a sign of corrosion of this type andshould be looked for during stock inspections.
Outdated pesticidestocks
Often there is no information on shelf-life onthe pesticide container label. When this is the case, a two-yearshelf-life should be assumed, unless more precise information can beobtained from the manufacturer or distributor at the time ofpurchase.
Outdated stocks may still be usable if theformulation has not broken down. The only way that this can beverified is by having a sample of the product analysed by themanufacturer or at an independent laboratory and the dose measuredaccordingly. The date of the test must be attached to the drums aftersamples have been analysed. Trial and error methods that assess thepesticide's efficacy by using more concentrated doses or higherapplication rates are not recommended.
Disposal of outdated andunusable pesticides
The main aim of good storekeeping is to minimizethe need to dispose of stocks since the disposal of pesticidespresents many problems. However, on occasion, it will be necessary todispose of old stock. Store accounting procedures should allow forold stocks to be written off, that is there should be some system bywhich unusable pesticides can be removed from the store.Unfortunately the storekeeper does not always have the authority todo this and stock tends to remain on record whether it is usable ornot. If there is no system whereby pesticides can be written off andsubsequently disposed of, old pesticides soon present hazards astheir containers deteriorate and start to leak. The disposal ofunwanted pesticides is considered later on.
FIGURE 16 - Pesticide container corroded and leakingfrom a side seam
