To understand how adjuvants work, it helps to understand how waterworks. Each water molecule is bipolar, meaning it has a negative and a positive charge, very much like a magnet. If you put several water molecules together, the positive and negative forces attract each other.
The molecules on the surface of a water droplet are held together with more force than those of the interior water molecules. This causes surface tension, which can prevent many things from going into the solution and getting wet. Surfactants overcome surface tension. Most surfactants have a water-loving polar head (hydrophilic head) and a water-hating non-polar tail (hydrophobic tail) (Figure 2).
These components of a surfactant molecule help break water surface tension, allowing the pesticide to be more evenly dispersed on a surface and to reach its target. When water molecules come into contact with unlike substances, several things may happen. If the substances have a similar charge, the two forces repel each other. If they have different charges, the two forces will attract each other. If there are no charges, there will be no reaction. Water, when placed on most hydrophobic surfaces, will bead. This beading is caused by surface tension, and this surface tension can be reduced by the addition of surfactants
The lower the surface tension in a pesticide solution, the better the pesticide coverage, allowing more pesticide to reach its target.
It’s important to know which adjuvant is best. For instance, one day we received a panicked call from a grower who indicated that his pansy crop was in trouble. Earlier that day he discovered his crop had a serious aphid infestation. He sprayed thoroughly, but he soon found that his efforts were futile, as the spray droplets ran off the foliage. Rather than investigate or consult, the grower decided to use horticultural oil with the insecticide and spray again. It was late in the afternoon, hot and sunny. By 5 p.m.. the leaves of the pansies started wilting. Panicked, he decided to apply a commonly used household dishwashing liquid solution to the pansies to rinse off the oil, knowing soaps dissolve oils.
By the time he called for assistance, 1,300 flats of pansies were already dissolving into small blobs of dark, black-green slime
Had the grower read the label, the horticultural oil he used clearly indicated it was a penetrating type oil to be used only on fruit crops, ornamental trees, and shrubs when they are dormant (not on actively growing foliage). In hot weather, oils can be very damaging to plants (Figure 5).
This unfortunate incident could have been prevented if the person applying the pesticide had taken time to read the label. When explosive thrips and viral epidemics occur or insect resistance to pesticides increases, many growers are desperate and easily tempted to use any handy surfactant wetting agent. The science of adjuvant technology has increased the effectiveness of our pesticides, but only when used properly. It is critical that a pesticide be teamed with the right adjuvant.
Adjuvants versus Surfactants
- Surfactants
Surfactants are adjuvants that reduce surface tension within the external surface layers of water. There are four different types of surfactants:
- Anionic Surfactants
Are negatively charged, and enhance foaming and other spreading properties. For example, shampoo for hair contains sodium or ammonium laureth sulfate, which is the preferred anionic surfactant for hair. Using an anionic surfactant in the greenhouse can cause problems with sprayers that have an agitator, or any system where the foam could disrupt water flow or pump suction.
- Cationic Surfactants
Are positively charged and are often very toxic to plants as they can disrupt membrane ion balance. Cationic surfactants are not widely used for pest control, but they are more commonly used in heavy-duty cleaning compounds. Don’t grab a bottle of engine wash surfactant used to clean the tractor and add it to your pesticides when spraying Easter lilies. The results can be devastating.
Amphoteric Surfactants
Are unusual in that they will form either a positive or negative charge in water, depending upon the pH of the solution. Their use in horticulture crop protection is rare. These products are used very specifically to match the properties of specific pesticide formulations to carrier components or other materials and are generally not available for use in the greenhouse as a stand-alone product.
Nonionic Surfactants
Nonionic Surfactants do not have a charge in solution and are the most commonly used surfactants for the horticulture industry. When used properly, they as a class do not harm plants, remain stable, and do a good job of breaking water surface tension. However, the application rate is critical. When applied at too high a rate plant injury may result (Figure 6).
