Pesticides Leftover

Often times gardeners will  mix up an insecticide spray and store any leftovers for the next application.  But is it still good?  Maybe.  Mixed pesticides lose effectiveness depending on their type and the time stored. When powdered pesticides are kept dry and sealed, they can last for quite some time; the same is true for liquid formulations. But once they are mixed with water the solution may quickly lose it's potency.  

 It is the pH level of the water used in the spray mix that influences the chemical breakdown of many pesticides. The pH of a liquid is the measurement (ranging from 0 – 14) of its acidity or alkalinity. Seven is neutral, above 7 is alkaline and below 7 is acidic. Some insecticides, particularly the groups known as carbamates and organophosphates, undergo a chemical reaction in the presence of alkaline water. The reaction is known as alkaline hydrolysis, and reduces the effectiveness of the pesticide’s active ingredient. 

The chemical breakdown of a pesticide is commonly referred to in terms of its half-life. A half-life is the period of time it takes for one-half of the amount of pesticide in the water to degrade. A common insecticide, sevin, has a half life of 100-150 day in solutions whose pH is at 6. That number rapidly decreases to 24-30 days at a pH of 7. At a highly alkaline solution of 9, its half life is only 1-3 days.  Malathion’s half life goes from 8 days to 19 hours as the pH shifts from 6 to 8. One of our old fungicides, captan, last about 8 hours at neutral (7), and only 2 minutes when the spray solution is at a pH of 9.  When mixed pesticide solutions are stored for days, weeks or months, their effectiveness can greatly diminish.