BIOLOGICAL, MICROCLIMATE, AND TRANSPORT PROCESSES AFFECTING PEST CONTROL APPLICATION TECHNOLOGY
Location: Application Technology Research Unit
Title: Evaporation and Coverage Area of Pesticide Droplets on Hairy and Waxy Leaves
Submitted to: Biosystems Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 16, 2009
Publication Date: September 27, 2009
Citation: Yu, Y., Zhu, H., Frantz, J., Reding, M.E., Chan, K.C. 2009. Evaporation and Coverage Area of Pesticide Droplets on Hairy and Waxy Leaves. Biosystems Engineering. 104:324-334.
Interpretive Summary: Although numerous studies have been reported on methods to maximize deposition and coverage of droplets on target surfaces, current application technologies are still highly inefficient. This can lead to excessive amounts of pesticides being applied with greater cost and increased contamination of the environment. Little research has been carried out on how droplets spread and how long the droplets last on target surfaces; i.e., the evaporation time of droplets on different types of leaves, which directly influences the morphology and absorption of active ingredients. This research determined effects of individual variables including surfactant, drift retardant, droplet size, and RH on the evaporation and maximum coverage area of single droplets deposited on leaves with smooth wax or hairy surfaces. The information from this research on the evaporation time and spreading area of pesticide droplets on plant leaf surfaces will assist pesticide formulators to develop better products that can maximise uptake by leaves. It will also help spray applicators to maximize efficacy and minimize chemical use by selecting optimal droplet sizes and chemical formulations for the specific crops under specific environmental conditions.
The fate of pesticide droplets on leaves is significantly influenced by the fine structures found on leaf surfaces. Evaporation times and the maximum coverage areas of single droplets (246, 343, 575, 762, and 886 µm) on hairy and smooth waxy geranium leaf surfaces were determined under controlled conditions. Stereoscopic sequential images of the droplet evaporation processes were taken for five droplet sizes, three relative humidity (RH) conditions and 13 different sprays. The sprays were combinations of water, a non-ionic colloidal polymer drift retardant, an alkyl polyoxyethylene surfactant, a fungicide and three insecticides. The evaporation time and maximum coverage area of droplets were significantly changed by adding the surfactant or drift retardant to the sprays, but not by adding the fungicide or insecticide. Droplet evaporation times on waxy leaves were longer than those on hairy leaves. Evaporation times increased exponentially as droplet diameter and RH increased with limited variability of regression coefficients independent of spray type and leaf surface. The maximum coverage area of droplets also increased exponentially as droplet diameter increased but it was not significantly affected by RH. On the waxy geranium leaf surfaces, the coverage area of pesticide droplets decreased throughout the evaporating process and at all RH conditions, while, on hairy leaf surfaces for the same size droplets and at the same RH conditions, the coverage area continued to spread until evaporation was nearly completed. Given that the duration of evaporation time and the extent of the coverage area affect pesticide distribution on waxy or hairy leaves, recommendations for pesticide dosage and spray methods should be take into account different leaf surfaces to obtain the optimum biological effect and reduced pesticide use.