What's the shelf life of a pesticide? Pesticides used in the landscape are manufactured, formulated, and packaged to specific standards. However, when stored improperly, they can break down, especially under conditions of high temperature and humidity. Some pesticides can lose their active ingredients through chemical decomposition or volatilization. Dry formulations (wettable powder) can become caked and compacted; emulsifiable concentrates (ECs) can lose their ability to form emulsions. Some pesticides become more toxic, flammable, or explosive as they break down.

Pesticide formulations with low concentrations of active ingredients generally lose effectiveness faster than more concentrated forms. Sometimes a liquid pesticide develops gas as it deteriorates, making opening and handling containers quite hazardous. In time, the gas pressure may cause the container to rupture or explode.

Certain pesticides have a characteristic odor. A strong odor in the storage area may indicate a leak, spill, or improperly sealed container. It may also be a clue that the pesticide is deteriorating, because the smell of some chemicals intensifies as they break down. If none of these problems are found, chemical odors can be reduced by installing an exhaust fan or lowering the temperature of the storage area.

The following pesticide product characteristics affect shelf life:

• the formulation (liquid concentrate, wettable powder, granules)
• the types of stabilizers and emulsifiers used in the product
• the chemical nature and stability of the product
• the type of container and its closure

Pesticide containers (including fiber and metal drums, pails, cans, bottles, bags, boxes, and overpacks with liners) have an important effect on storage and shelf life. If stored for long periods, these containers may eventually corrode, crack, break, tear, or fail to seal properly. Also, the label may become illegible. If a damaged container is found, transfer its contents to a similar sturdy container that can be sealed. Be sure to transfer the label to the new container. Never put a pesticide in a food or drink container!

Even with careful planning, it is sometimes necessary to carry pesticide stocks over from one year to the next. Check dates of purchase at the beginning of each season and use the oldest material first. To keep the label on a container intact and legible, cover it with transparent tape or lacquer.

Given proper storage, some pesticides may remain active for a number of years. By following the above guidelines, you can maximize their shelf life and your investment.