Adjuvants in the Airblast Tank

Note how little of the droplet contacts a waxy leaf (above). This hydrophobic reaction between water and wax can be overcome using a non-ionic spreader. Similarly, note how the droplet gets hung up on the trichomes (hairs) on a leaf before it reaches the leaf surface (below). Again, a non-ionic spreader would reduce droplet surface tension allowing it to splash onto the leaf. Photo Credit – Dr. H. Zhu, Ohio.

Note how little of the droplet contacts a waxy leaf (above). This hydrophobic reaction between water and wax can be overcome using a non-ionic spreader. Similarly, note how the droplet gets hung up on the trichomes (hairs) on a leaf before it reaches the leaf surface (below). Again, a non-ionic spreader would reduce droplet surface tension allowing it to splash onto the leaf. Photo Credit – Dr. H. Zhu, Ohio.

Spray adjuvants are tank mix additives that either physically or chemically influence the efficacy, consistency or safety of pesticides. For example, adjuvants can improve the handling characteristics of a spray solution (e.g. water conditioners, de-foamers, emulsifiers). They can improve uptake into a target plant and/or improve the amount of contact between spray droplet and target surface (e.g. non-ionic spreaders). They can also modify droplets to reduce the potential for wastage from drift or run-off (e.g. anti-drift additives, stickers).

Some pesticide labels require the use of adjuvants in the tank mix for the pesticide to work correctly. They are not formulated with the product because of expense, bulk, or product stability, and must be added during loading. In order for a pesticide to work as advertised, it is important to include any adjuvants required by the label. In some cases, we are encouraged to use adjuvants to improve an application, even though they are not on the label.

There are potential benefits to introducing some unlabelled adjuvants, but there are also potential problems. The difficulty is that unless someone tests a specific tank mix combination for a specific crop, the results cannot easily be predicted. For example, when a tank mix is incompatible, an adjuvant could cause phytotoxicity, create more drift when used with the wrong nozzle, deactivate or enhance a tank partner, and/or potentially reduce spray coverage.

We once conducted a trial to test an adjuvant intended to reduce run-off and drift. Water-sensitive papers were placed in the canopies of a 40 year old McIntosh orchard, which was then sprayed from one side in late May. The papers in the left panel (dilute control) were sprayed with 600L/ha (~60 g/ac.) of water. Those in the right panel (adjuvant) were also sprayed with 600L/ha but included the label rate of 500 ml of adjuvant. The water-plus-adjuvant reduced drift and runoff, as advertised, but did not penetrate as deeply into the canopy or spread on the papers, which is a concern if the operator was performing alternate-row middle spraying or needed better coverage (e.g. for mites). It was an unexpected side effect.

For better or worse, even small amounts of adjuvants can have a significant effect on spray coverage. Always test spray coverage when using a new adjuvant in a tank mix.

For better or worse, even small amounts of adjuvants can have a significant effect on spray coverage. Always test spray coverage when using a new adjuvant in a tank mix.

There is no simple answer regarding unlabelled adjuvants; there are too many possible product/adjuvant/plant combinations. If you intend to experiment with an adjuvant, perform a jar test to test for physical incompatibility. Then spray a small volume of the tank mix on a few trial plants to ensure there are no unexpected chemical issues (e.g. phytotoxicity or inactivating tank mix partners) or coverage issues.

DOWNLOADIt is highly recommended that every sprayer operator have a copy of Purdue Extensions’ 2015 “Adjuvants and the Power of the Spray Droplet – PPP-107”. It is a comprehensive handbook describing of how water quality and adjuvants affect the performance of pesticide applications. I consult it regularly.

Here are two videos from Dr. H. Zhu, Ohio showing how adjuvants that affect surface tension can help improve the level of contact between spray droplet and target surface.

Dr. Zhu makes a controlled droplet application to a hairy (i.e. trichomes) leaf. The surface tension causes it to get caught up in the hairs, and it never reaches the leaf surface. (2014).

Dr. Zhu makes a controlled droplet application to a hairy (i.e. trichomes) leaf. The surface tension is reduced with teh addition of a non-ionic spreader, and it reaches the leaf surface. (2014).