How water quality influences pesticide effectiveness

The effectiveness of pesticides (i.e. insecticides and miticides) in managing insect and mite pest populations may be affected by many factors including spray coverage, application frequency, application rate and timing of application. However, one factor that is oftentimes overlooked by greenhouse producers is water quality. Water is the primary carrier associated with many pesticide applications and constitutes over 90% of most spray solutions. The three major constituents of water quality that can affect pesticide effectiveness are water hardness, water turbidity and water pH.

Water hardness

Water hardness is a measure of the total concentration of hard water cations (positively charged ions) including calcium, magnesium, sodium, iron, and aluminum. Hard water cations can react with certain pesticides, such as insecticidal soaps (potassium salts of fatty acids) and reduce pesticide efficacy. The binding of hard water cations with pesticides creates molecules that cannot enter insect or mite pests, enter at a slower rate, or precipitate out of solution.

Water turbidity

Water turbidity is a measure of the total suspended solid particles (e.g. soil or organic matter) in a water solution. Suspended solid particles, which are positively charged, are attracted to and bind with negatively charged pesticide molecules, thus inhibiting the ability of a pesticide to be absorbed by plant leaves. The inhibition of plant leaf absorption is important for pesticides with translaminar properties where, after a spray application, the active ingredient penetrates the leaf and resides within in the leaf tissues forming a reservoir that provides protection against certain foliar-feeding insect and mite pests. In addition, suspended solid particles can plug nozzles and screens resulting in uneven spray coverage.

Water pH

Water pH is associated with the acidity or alkalinity of a solution based on a scale from 1 to 14. A pH of 7 is neutral. A pH less than 7 is acidic and a pH greater than 7 is alkaline (or basic). Water pH is important to the stability and efficacy of pesticides. Stability is affiliated with the half-life of a pesticide, which is the number of days required for 50% of the original amount of pesticide active ingredient to break down in water. A reduction in stability can substantially reduce pesticide longevity and effectiveness.

When the pH of a spray solution is greater than 7, certain pesticides are susceptible to alkaline hydrolysis. Alkaline hydrolysis is a degradation process in which pesticide molecules are broken apart (fragmented) when the water pH is greater than 7. In general, pesticide effectiveness diminishes over time when the water pH is greater than 7. In addition, the rate or speed of alkaline hydrolysis increases as water pH increases. Pesticides in the chemical class, organophosphate (e.g. acephate), carbamate (e.g. methiocarb), and pyrethroid (e.g. bifenthrin and cyfluthrin) are susceptible to alkaline hydrolysis. The optimal water pH for most pesticides is between 5 and 7. Read the label of each pesticide carefully to determine the actual pH range.

Below are examples of the recommended pH ranges from several pesticide labels:

• Adjust pH of the spray solution to between 3 and 7, if necessary (Azatin O).
• A spray tank pH between 6.0 and 9.0 is suggested to achieve maximum performance (Conserve SC).
• Maintain spray mixtures containing Floramite SC within a range of pH 5.5 to 6.5 (Floramite).
• Buffer spray water to 5-7, if necessary (Mavrik Aquaflow).

So what can a greenhouse grower do?

The best way to overcome water quality issues is to incorporate adjuvants such as water conditioners or pH reducing agents into spray solutions, which will alleviate the effects of water hardness, water turbidity, and water pH. Greenhouse producers can contact their state extension office or a water-testing laboratory to obtain information on how to test for water quality.

The author is a professor and extension specialist in horticultural entomology/plant protection in Kansas State Universities’ Department of Entomology. Rcloyd@ksu.edu