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Effect of Water pH on Pest-Control Materials

September 20, 2000

Resistance is often blamed for the failure of an insecticide or miticide to manage a given pest or pests. However, another possible reason for lack of control may have to do with the pH of the spray solution. pH is a measurement of the concentration of hydrogen ions [H+] in a solution. It is a scale indicating the degree of acidic and basic properties of water. The pH scale ranges from 0 to 14. A pH value below 7 is acidic, whereas a pH value above 7 is basic or alkaline. A pH of 7 is considered neutral.

Many common insecticides and miticides are susceptible to breakdown if the pH of the water is not within an acceptable range. When the pH is greater than 7, a process known as alkaline hydrolysis occurs. Alkaline hydrolysis is a degradation process in which the alkaline water breaks apart insecticide or miticide molecules. This process releases individual ions (electrically charged atoms), which may then reassemble with other ions. These new combinations may not have any insecticidal or miticidal properties.

Insecticides and miticides are more susceptible to alkaline hydrolysis than fungicides and herbicides. Many insecticides and miticides degrade under alkaline conditions. For example, Malathion, Kelthane, Dylox, and Turcam are very sensitive, degenerating within a few hours after being diluted in alkaline water. In general, the carbamate (for example, Sevin) and organophosphate (for example, Dursban) chemical classes are more susceptible than chlorinated hydrocarbons (for example, Lindane) or pyrethroids (for example, Talstar). However, other pest-control materials can be affected by high pHs. For example, a pH above 8 can reduce the efficacy of the Bacillus thuringiensis (Dipel, Thuricide, and Javelin) toxin and the insect-growth regulator azadirachtin (Azatin).

Higher temperatures can increase the rate of insecticide degradation. Alkaline hydrolysis occurs more rapidly when temperatures are high. For example, at a pH of 9 and a water temperature of 77įF, acephate (Orthene) loses 50% of its activity in about 5 days, and fenvalerate (Mavrik) loses 50% of its activity in 1 to 2 days.

The following are ways to avoid water pH problems:

  1. 1. Follow manufacturer directions regarding the desired water pH. The ideal pH range for most insecticides and miticides is between 5.5 to 6.0.
  2. 2. Regularly test water pH because the pH of water can change during the season.
  3. 3. Apply insecticides and miticides as soon as possible after mixing. It is advisable to use a spray mixture within 6 hours or less to avoid potential pH problems.
  4. 4. Donít leave insecticides or miticides sitting in a spray tank for an extended time.
  5. 5. Adjust water pH with buffers or water-conditioning agents. Buffers or water-conditioning agents are compounds that reduce the damage caused by alkaline hydrolysis and adjust the pH of the spray solution to maintain it within a pH range of 4 to 6. In addition, other materials, such as vinegar (acetic acid), are often used to acidify water.

Author: Raymond Cloyd


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