Is Water Reducing your Pesticide Performance?

Since water is normally used to deliver the chemical to the target pest, it should be considered the foundation for the application process. Whether it is from a lake, stream, or pond, it may very well be the deciding factor between ineffective and optimum product performance.

Suspended, positively charged organic pesticides are attracted to and bind with negatively charged particles found in the water.

Some products (e.g., glyphosate) bind to suspended sediments, rendering them unavailable for plant uptake.

One label states,

“Product performance may be significantly reduced if water containing soil sediment is used as a carrier. Do not mix this product with water from ponds or ditches that are visibly muddy or murky.”

Water hardness can affect some pesticides negatively. As in magnets, opposite charges attract: negatively charged pesticide molecules attach to the positively charged iron, calcium, and magnesium molecules (cations) in hard water. The binding of pesticides with these minerals creates molecules that cannot enter the target pest, which enter at a much slower rate, or which precipitate out of the solution.

The following cations, if present in water, can cause problems and may contribute to water hardness. They are listed in the order of greatest potential to bind to pesticides:

• Aluminum (Al+++)
• Iron (Fe+++, Fe++)
• Magnesium (Mg++)
• Calcium (Ca++)
• Sodium (Na+)

The chemical characteristics of the pesticide change once the pesticide recombines with the positively-charged ions such as calcium or magnesium.

The label for one herbicide indicates that a water conditioner

“…may increase the performance of this product on annual and perennial weeds, particularly under hard water conditions.”

In one sense, the more the pesticide is bound to minerals, the more “diluted” the product becomes in the tank. In some cases, the chemically-altered molecule may be unable to dissolve in water, penetrate the leaf tissue, attach to the site of activity in the pest to disrupt biological functions, or perform as a pesticide.

These effects are not limited to the spray tank environment but extend to the spray solution on the leaf surface, which can affect product uptake.

Pesticides normally are formulated as weak acids or neutral to weakly-alkaline products. As a general rule, herbicides, insecticides, and fungicides perform best in slightly acidic water, pH 4–6.5.

Pesticides such as the sulfonylurea herbicides perform better in water that is slightly alkaline (above pH 7). When water pH falls outside of the preferred upper or lower boundaries, product performance can be compromised. In some cases, the pesticide can fall out of the solution.

The bottle on the left (in each photo) contains distilled water with zero hardness; the bottle on the right contains hard water. A material that mimics glyphosate is added to both bottles of water. Notice that the water on the left remains clear, indicating the added product is in solution. The water on the right is cloudy, indicating that the calcium has bound to the glyphosate mimic.

The pH of the solution also can influence how long a pesticide molecule remains intact. A higher or lower than optimal pH causes some pesticides to begin degrading or “hydrolyzing.”

When a weakly acidic pesticide is placed in water that is slightly acidic, it stays largely intact. Certain insecticides and fungicides have been shown to break down in alkaline water, and the effect of pH usually proceeds faster as the temperature of the water increases.

Many products have a weak electrical charge. The pH also can change the chemical charge of a pesticide molecule, limiting its ability to penetrate the leaf cuticle and reach the site of action, thus reducing its efficacy.