Is That Pesticide Too Old?

#1 Time

Everything ages and very few things (maybe wine and cheese) get better with time. As a general rule, any pesticide that has been opened and stored for more than one year should be checked for effectiveness. Open containers of dry pesticides should be disposed of after one year. Most products, though, will remain effective for at least two years, some longer when stored between 40-85°F (4.4-29.4°C), unopened, in original containers.

#2 Storage Conditions

Even pesticides that have a long shelf life under ideal conditions can deteriorate rapidly when exposed to environmental extremes. Overexposure to humidity, air and light, and especially temperature, can cause chemicals to lose their effectiveness much sooner than expected. Pesticides last longer when stored in a cool, dry place. Storage areas should be ventilated with temperatures between 40-85°F (4.4-19.4°C). Pesticides should never be placed in direct sunlight either in storage or inside your vehicle. Exposure to very cold temperatures can cause pesticides to separate or gel. This can be a permanent, irreversible change, but sometimes the pesticide can be restored to normal with warming and shaking.

#3 Stability of the Container

While a pesticide still sealed in its original container should last for years, once the container is opened, deterioration begins. To slow the breakdown, reseal opened containers as tightly as possible. Over time, pesticides can increase in acidity and containers can corrode, crack, seams tear or seams fail. Containers vary greatly in their ability to protect the pesticide in case of flooding or other moisture infiltration. Pesticides in glass, metal or plastic containers have the greatest protection; pressurized spray cans can corrode; and paper or cardboard packaging offers little protection at all in wet conditions.

#4 Stability of the Formulation

Whether the product is dry, liquid, concentrate or ready-to-use makes a difference in its shelf life. Dry formulations such as dusts, wettable powders or granules usually store better at low temperatures than liquids, but they break down more easily than liquids when exposed to high temperatures, humidity or sunlight. Formulations that contain low concentrations of active ingredient generally lose effectiveness faster than more concentrated formulations. Certain inert ingredients in the product, like stabilizers and emulsifiers, also will affect its shelf life.

TOO OLD? It’s normal for many pesticide formulations to separate or clump to some extent as they sit, but excessive separation or clumping that cannot be remixed is a clue that the product has deteriorated. If you suspect a pesticide may have deteriorated, mix a small amount in a jar to see how it mixes:

• If an emulsifiable concentrate forms a sludge or the mixture separates when water is added, instead of forming the normal milky coloration, it means the product has lost its ability to form an emulsion.
• If a wettable powder is cakey and will not mix with water, the product has deteriorated.
• If a dust or granular is clumping and cannot be separated by shaking, the product may be too damp to be effective.
• If a normally clear liquid has developed a milky appearance, water has probably gotten into the container. If moisture gets into a container of oil-based pesticides, you will be able see it as a separate layer.

Pesticides last longer when stored in a cool, dry place. Storage areas should be ventilated with temperatures between 40-85°F (4.4-19.4°C).

Many pesticides change properties as they break down. Some become more toxic, flammable or explosive. Some liquid pesticides build up gases, which can rupture a container, or put you at risk when you open the container. Contrary to what you might think, the characteristic smell of certain pesticides becomes even stronger as they deteriorate. An unusually strong odor in the storage area may mean there is a pesticide leak or spill, but it also can be an indication of deteriorating pesticides.

Old pesticides not only don’t work as well, they also can clog and damage your application equipment. Mix some in a jar to check the consistency before you add it to your equipment. Unfortunately, there is no good way (outside of a laboratory) to check whether an old product will still kill pests other than to treat an infestation and monitor the results.

Manufacturers often list the shelf life of the pesticide on the container. If you know when you purchased it, you can determine if it should still be viable. If you don’t have that information, check with the manufacturer. A check of the lot number, stamped on the container, can tell you when the product was manufactured. Most pesticides are not backed by the manufacturer if stored longer than two years.

The authors are well-known industry consultants and owners of Pinto & Associates.