Those who think that pesticide applications work the same on all target surfaces, think again.

Ohio State University researchers have found that the surface makeup of a plant’s leaves — hairy or waxy or varying degrees of both — plays a huge role in the effectiveness of liquid applications.

“The ultimate goal is to get the most out of pesticides, to get the biggest bang for your buck — achieving maximum efficiency while promoting environmental sustainability,” says Erdal Ozkan, an Ohio State University Extension agricultural engineer. “One way of doing that is to make sure that the droplets hit the target, remain on the target and the pesticide active ingredients in droplets are taken up by the target.”

How that product is deposited is just as important as how much is being deposited, and knowing what kind of plant surface a grower is dealing with can help tremendously.

Ozkan and his colleagues at the U.S. Department of Agriculture-Agricultural Research Service Application Technology Research Unit studied the impact of liquid pesticide applications on waxy and hairy geranium leaves. They found that hairy leaves captured and retained the droplets more effectively than waxy leaves.

In addition, droplets deposited on hairy leaves covered more surface area as they evaporated and spread over the leaf more quickly, increasing plant uptake of the pesticide by 50%.

“Foliar uptake efficiency is affected by chemistry of the product applied, target surface characteristics, droplet size, evaporation time and how much the surface area is wetted,” Ozkan says. “Our objective was to investigate those parameters.”

Ozkan says that the “hairy” characteristic of the plant leaf is more effective than a smooth or waxy leaf because the hairs protect the droplets from rebounding and drifting and allows the droplets to spread out over a larger contact area.

“Hairy leaves are hydrophilic (water-loving) and so evaporation times are short,” Ozkan says. “Waxy leaves are hydrophobic (water-hating) and so evaporation times are much longer for those droplets that do manage to remain on the leaf.”

In addition, researchers found that by adding a surfactant — a wetting agent that lowers the surface tension of a liquid — to the pesticide application mix, droplets cling better and are taken up by the plant much more readily.

“Adding a surfactant reduces the contact angle the droplet has with the leaf, making the droplet more stable and less likely to roll off,” Ozkan says. “We found that in medium-sized droplets, the surfactant helped increase the contact area with the leaf by a faction of 2 for waxy leaves and a faction of 4 for hairy leaves.”

Ozkan says that the findings are useful for chemical companies who can recommend specific pesticide dosage and application methods when target surfaces are known to achieve maximum product benefit.

“Companies can invest into making more detailed descriptions of how a product will work for different types of surfaces,” Ozkan says. “By doing so, you could be saving the grower money on pesticide application costs.”

For example, Ozkan says, a farmer may only need to apply half of the application on surfaces with hydrophilic characteristics.

The research, funded by the USDA-ARS, helps to deepen the understanding of how droplets behave when they impact plant surfaces. But not all mysteries have been solved.

Ozkan says that the droplet sitting on the middle of a leaf surface behaves differently than those resting along leaf veins or leaf stems, and researchers are still trying to figure out why.