Pesticides are more potent at proper pH

Have you ever had pesticides that seemed less than effective? It could be due to the pH of the carrier solution.

Many greenhouse owners are very careful and deliberate when choosing which pesticides to use on pests. If they’ve chosen a product that has either been recommended by Michigan State University Extension or has been effective for many other greenhouse operations, but has not been very effective in their own operation, they may assume the pests in their greenhouse are resistant to the chosen chemistry. While this may certainly be the case, double check the pH of your water isn’t the real culprit.


Figure 1. pH scale.

pH is a measure of the hydrogen ions in solution and is measured on a scale of 0 to 14, with 7 being neutral (Figure 1). A pH of less than 7 is acidic while a pH greater than 7 is basic or alkaline. If water is alkaline (greater than 7), then a process called alkaline hydrolysis may occur to pesticides or plant growth regulators mixed into it that breaks the active ingredient down, resulting in a product that is not as effective, or possibly even phytotoxic to plants. As a general rule, insecticides and miticides are more susceptible to alkaline hydrolysis than are fungicides.

The ideal water pH for most pesticides is between 5.0 and 6.5 (Figure 2). However, always read the pesticide label for precise recommendations for that specific chemistry. To modify the carrier water pH, add an acidifying or buffering agent to the water before adding the pesticide. Also, depending on your water source, the pH can change over a period of time (years or even within a season), so test the pH of your water frequently. To ensure mixed pesticides are as effective as they can be (i.e., to limit the risk of breaking down in solution), always use the mixture as soon as possible (within six hours). 


Figure 2. Modified excerpts of a miticide label (left) and a fungicide label (right) indicating proper handling of pH.

This article was published by Michigan State University Extension. For more information, visit http://www.msue.msu.edu. To have a digest of information delivered straight to your email inbox, visit http://bit.ly/MSUENews. To contact an expert in your area, visit http://expert.msue.msu.edu, or call 888-MSUE4MI (888-678-3464).

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