A crucial part of applying all plant protection products like herbicides, insecticides, and fungicides is making the product cover and stick to the target.
Spray droplets can bead-up, run off, or bounce away from the weed or an insect or sometimes sit on top of the leaf hairs where it dries up without ever contacting the rest of the plant. Plant leaves and insects can be hard-to-wet, or hydrophobic, which limits the effectiveness of the pesticide.
These problems are easily fixed by adding a surfactant adjuvant to the spray application.
The word “surfactant” was originally derived from “surface active agent” and sometimes that phrase is still used on product labels.
Surfactants have two unique chemical abilities: to break the surface tension of water—that’s what makes the water bead-up and run off instead of spreading across a surface— and to combine liquids together that normally resist mixing, like oil and water.
Surfactants used to keep oil and water mixed are called “emulsifiers.” Surfactants and emulsifiers are the same chemistries but have different purposes. When using a wettable powder or an emulsifiable concentrate pesticide, a surfactant/emulsifier may need to be added to help keep the spray mix from phasing or settling into visible layers in the tank, and to form a stable emulsion.
Together oil and water will form separate layers rather than mix. Add an emulsifier, and oil and water will mix, forming an emulsion or stable mix. Different combinations and types of oil, emulsifiers, and water can result in more or less stable emulsions.
When a surfactant is added to break the surface tension of the mixing water, the pesticide application droplets will hit the leaf and spread into other droplets eventually covering the entire surface with the pesticide.
If the pest is a weed resistant to wetting like velvet leaf or lambsquarters, the surfactant instantly overcomes the waxy or hairy cuticle and contacts the leaf surface, carrying the pesticide along as it spreads. This is why surfactant adjuvants are often labeled as “spreader-wetters.” They spread across the canopy and wet the foliage with the pesticide.
With adequate spreading, the pesticide will be absorbed through the surface, into the leaf cells where it can be transported to the mode of action site and work to kill the weed.
With adequate spreading, the pesticide will be absorbed through the surface, into the leaf cells where it can be transported to the mode of action site and work to kill the weed.
The active ingredient label will say which surfactant to use and what use rate to add. If the label requires a surfactant adjuvant, a spreader-wetter, to be added to an application spray mix, it will always say to add a nonionic surfactant (NIS).
Surfactant chemistry can get complex because surfactants are also used in mining applications, kitchens, bathrooms, laundry rooms, and medical treatments like vaccines and infusions. There are surfactants with different chemical characteristics for each of those purposes.
Surfactants used in the kitchen and laundry will often be anionic or cationic. These surfactants can become very foamy and are harsh on surfaces. That’s one of the reasons that common dish soap is not effective as a spreader-wetter adjuvant in a pesticide application – it is harsh on plants. This might be okay for a weed that needs to be killed with an herbicide but could be problematic for the crop or a fungicide application on a fruit tree or turf area.
Household surfactants also produce high amounts of foam. Foam is a good thing when washing clothing, but not for mixing the spray application and applying it with a sputtering nozzle. NIS do not carry a positive or negative charge and are generally highly effective with no risk of phytotoxicity.
Organosilicone surfactants (OSS) are a specialty class of surfactants that spread and wet so rapidly that droplets can spread right off the target. OSS are super effective at penetrating through the hardest-to-wet surfaces at very low use rates. However, some active ingredients may not work well with this characteristic, so following label instructions is key to success.
The pesticide product label will always say if a surfactant is required and usually, how much to add to the spray mix. Some branded pesticides may have surfactants and emulsifiers included in the product and others, like generic products, may have none. Sometimes, the surfactant is included in the pesticide/active ingredient to act an emulsifier, keeping the chemistry stable and ready to mix with other active ingredients. More surfactant may be needed to spread and wet the target after spraying.
In all cases, the pesticide label requirements must be followed for an application to be effective and legal.
Pre-emergence herbicide applications offer a valuable tool to prevent or reduce weed infestations early in the season. While these applications are helpful, it can be a challenge to achieve optimum results. Adjuvant use is increasing and studies continue to prove their effectiveness.
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