Q. Do insect sprays lose their effectiveness by being carried over from the previous year? Bill McMonagle – Tacoma, WA
A. Yes. Although manufacturers are not required to put an expiration date chemicals can slowly loose their effectiveness over time. In most cases you can expect to get 1-3 year shelf life from the average spray. In other cases they may be effective for over 5 years. But there is no sure way to gauge this gradual loss of effectiveness. There are many variables that come into play. For example insect sprays that depend on living organisms like Bt (Bacillus thuringiensis) can loose their effectiveness faster than other types of sprays depending. But this also depends on the formulation of the Bt. The liquid formulation has a shorter shelf life than the powder formulation when stored under the same conditions. Also, once the seal has been broken and the contents have been exposed to air the spray will loose effectiveness more rapidly than if the product has not been opened. The temperature and storage conditions will are also critical factors. Extreme high or low temperature storage, exposure to sunlight, or high humidity will shorten the shelf life of sprays. The process is gradual. Although the product may not be fully effective it still may provide control of the insect. Since there is no way of knowing the loss of effectiveness I would not try to compensate by using more than the recommended amount or you run the risk of an over application which can be detrimental. Store sprays in a cool dry place that is out of the reach of children and pets. Areas that stay between 40 and 80 degrees are best. I suggest that you only purchase as much as you will use in a season or two. If you can’t use the smallest bottle of concentrate then consider using a ready-to-use (premixed in a hand sprayer) or ready-to-spray (premixed in a hose-end-sprayer) formulation.