Pesticide shelf-life

   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.

Pesticide ordering and shelf-life

   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 If a larger quantity is ordered than can be used during the period of shelf-life, outdated stocks will accumulate and present disposal problems, as well as financial loss.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

Stock inspection and shelf-life

   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 year.
   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.

Outdated pesticide stocks

   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 purchase.
   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.

Disposal of outdated and unusable pesticides

   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