BIOLOGICAL, MICROCLIMATE, AND TRANSPORT PROCESSES AFFECTING PEST CONTROL APPLICATION TECHNOLOGY
Location: Application Technology Research Unit
Title: Adjuvant Effects on Evaporation Rates and Wetted Area of Droplets on Waxy Leaves
Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 1, 2010
Publication Date: May 29, 2010
Citation: Xu, L., Zhu, H., Ozkan, E., Bagley, B., Derksen, R.C., Krause, C.R. 2010. Adjuvant Effects on Evaporation Rates and Wetted Area of Droplets on Waxy Leaves. Transactions of the ASABE. 53(1): 13-20.
Interpretive Summary: Pesticide efficacy is often correlated with its spread area (wetted area) and evaporation time on a leaf surface, and it may be reduced if the active ingredients in droplets do not uniformly spread out and remain on a leaf surface especially for the surface of difficult-to-wet plants. Also, chemical residues may form large crystals that cannot remain on foliages after droplets dry. This research investigated the effect of different classes of spray additives on droplet evaporation, spread and residual pattern after droplets deposited on various types of waxy leaf. The investigation quatitatively demonstrated that droplet evaporation, spread area and residual pattern formation varied greatly with spray additive type and leaf waxy level. In general, selection of the appropriate class of adjuvants significantly improved deposit formation on waxy leaves, leading to more effectiveness of pesticides. Therefore, to maximize efficacy and minimize chemical use, pesticide dosage, chemical formulations and application methods should be recommended differently for different crops under specific environmental conditions.
The use of an appropriate adjuvant for pesticide applications is a critical process to improve spray deposit characteristics on waxy leaves and to reduce off-target losses. After deposition and evaporation, residue patterns of 500 µm sessile droplets that incorporated four classes of adjuvants on five different waxy plants were investigated. Droplets were generated with a single-droplet generator and deposited on target leaves placed in an environmentally-controlled chamber at 60% relative humidity and 25ºC. Adjuvants tested were two types of oil-based Crop Oil Concentrate (COC) and Modified Vegetable Oil (MSO), a Nonionic Surfactant (NIS), and a type of mixture Oil Surfactant Blend (OSB). Water-only droplets were also tested for comparative purposes. The five waxy plants were difficult-to-wet and had a water contact angle greater than 90º. The water-only droplets did not spread at all and formed extremely small wetted areas on the leaf surface. The addition of an adjuvant to the spray solution significantly reduced the contact angle and increased the wetted area, but the change/improvements varied with the plant specie and the adjuvant class. In general, the MSO and NIS enhanced the droplet spread and maintained the droplet evaporation rate on the waxy leaf surfaces. After evaporation, the residues formed patterns of “coffee rings”. Droplets with oil-based adjuvants had more uniform residual distribution in the deposition patterns than droplets with the surfactant adjuvant. Results of this study demonstrated that selection of the appropriate class of adjuvants significantly improved deposit formation on waxy leaves, leading to more effectiveness of pesticides.