The biological efficacy of pesticides gradually
decreases with time. The pesticide shelf-life is the period of time
that a pesticide can be stored before it deteriorates. Nearly all
pesticides have a limited shelf-life. As part of modern pesticide
formulation technology, packing methods and storage practice aim to
prolong shelf-life as much as possible. Manufacturers indicate the
shelf-life of the pesticide on the container, but many pesticides may
still be usable long after the indicated shelf-life has expired. Most
pesticides have an indicated shelf-life of at least two years from
the time of manufacture, but shelf-life will be shortened if
pesticides are not stored properly (e.g. if they are stored at high
temperatures). Stock turnover organization needs to take into account
the time that pesticides may have been in transit between manufacture
and reaching the store.
Pesticides in sealed containers may change over time in two main ways:
An organochlorine such as endosulfan is chemically very stable, but some formulations may break down more rapidly. Organophosphates are much less stable and therefore generally have a shorter shelf-life. Dust and wettable powder formulations tend to break down and cake together, as a result of high temperature, high humidity, strong sunlight or compaction under pressure, more than liquids in sealed containers.
The shelf-life and rate of use must be taken into account when ordering pesticides (Figure 15). Do not order more than one year's requirement. The date of manu-facture and shelf-life should be on the outside of the container. If a larger quantity is ordered than can be used during the period of shelf-life, outdated stocks will accumulate and present disposal problems, as well as financial loss.
FIGURE 15 - Storekeeper checking dates from labels on containers in a pesticide store
Stocks in a pesticide store should be inspected
regularly for signs of deterioration, such as caking of powders,
sedimentation or gelling of liquids and discoloration through
oxidation. Shelf-life declines rapidly after containers have been
opened and left partially empty. Stock turnover must be organized to
ensure that the contents of a container are used as quickly as
possible once the container has been opened. Unsealed containers of
dusts and wettable powders should not be kept for more than one
Containers are not only subject to deterioration caused by external factors (climatic, biological and mechanical), but can also be corroded internally through the action of the pesticides they contain. Emulsifiable concentrate formulations are particularly likely to affect weak spots, especially along seams (Figure 16) or where there are imperfections on the internal coating of the container. Some pesticides increase in acidity during storage and this makes them more likely to corrode containers from within. Discoloration of pesticide is a sign of corrosion of this type and should be looked for during stock inspections.
Often there is no information on shelf-life on
the pesticide container label. When this is the case, a two-year
shelf-life should be assumed, unless more precise information can be
obtained from the manufacturer or distributor at the time of
Outdated stocks may still be usable if the formulation has not broken down. The only way that this can be verified is by having a sample of the product analysed by the manufacturer or at an independent laboratory and the dose measured accordingly. The date of the test must be attached to the drums after samples have been analysed. Trial and error methods that assess the pesticide's efficacy by using more concentrated doses or higher application rates are not recommended.
The main aim of good storekeeping is to minimize the need to dispose of stocks since the disposal of pesticides presents many problems. However, on occasion, it will be necessary to dispose of old stock. Store accounting procedures should allow for old stocks to be written off, that is there should be some system by which unusable pesticides can be removed from the store. Unfortunately the storekeeper does not always have the authority to do this and stock tends to remain on record whether it is usable or not. If there is no system whereby pesticides can be written off and subsequently disposed of, old pesticides soon present hazards as their containers deteriorate and start to leak. The disposal of unwanted pesticides is considered later on.
FIGURE 16 - Pesticide container corroded and leaking from a side seam