Waxy leaves or hairy leaves? This might be a good question to consider before you attack your next batch of weeds. Leaf-surface types affect how readily pesticides are absorbed, along with humidity levels, size of contact area and droplet size.

New research into pesticide uptake may open the way for farmers to get more effective results from the same application dose or reduce pesticide use.

"You can improve pesticides’ efficiency considerably if you can retain them on the target and find a way to spread them out," explains Erdal Ozkan of the Department of Food, Agriculture and Biological Engineering at Ohio State University

Efficient herbicide uptake by a plant’s leaves depends on more than the applied pesticide dose, says Ozkan. The chemistry of the sprayed solution, spray droplet size, target surface and evaporation time all play a role.

“If a droplet evaporates immediately, it may not have a chance to spread,” he explains. “The size of the wetted surface area is a key factor in uptake. If you get only two or three droplets on each leaf, and they don’t spread, you may reach only 10% of the available area, and that may not be enough chemical to kill the weed.”

Ozkan cooperated on research led by Heping Zhu of the USDA-ARS application technology research unit to study how plant surface characteristics, spray formulations, droplet size and relative humidity affected insecticide and herbicide coverage and evaporation.

Among their findings:

  • As droplet size and relative humidity increased, droplet evaporation times increased exponentially.
  • Larger droplets also increased the maximum wetted area exponentially, especially when a surfactant was added to the spray.
  • Drift retardants did not improve evaporation times.
  • Droplet evaporation times and spread were different on waxy-leaved plants (characterized as hydrophobic) than on hairy-leaved plants. “When a droplet hits a hydrophobic (waxy-leaved) leaf, it just sits there and the contact point is very small,” explains Ozkan. “On hairy leaves you can see the droplet spreading.”

The microscopic photographs revealed, however, that on some hairy leaves, droplets sit on top of the hairs until they disappear without ever contacting the leaf surface.

Adding surfactants greatly improves droplet spread and maximum contact on waxy-leaved plants more than for hairy-leaved plants.

The results have implications for pesticide dosage and application methods. Ozkan argues that different leaf types merit different dosages and methods.

Adding surfactants may prove to be a valuable step to improve pesticide efficiency. Surfactant manufacturers want to know how their specific products would perform, and the ARS-OSU research team intends to test a wider range of products.

The findings may also help understand the development of weed resistance. They suggest that application factors which limit uptake, rather than the amount of herbicide applied, may contribute to weed survival and the development of resistance.

“To get the most out of a pesticide we need good deposition (on the leaf) in the first place, but we also need to maximize contact time and contact surface area. If we do all three, that’s the road map to success in crop protection,” Ozkan says.

September 2010