WOOSTER, Ohio -- Fertilization makes trees grow faster but doesn't necessarily make them healthier. In fact, contrary to popular belief, fertilization may actually reduce a tree's ability to ward off pests, said an Ohio State University scientist.

Dan Herms, an entomologist with the Ohio Agricultural Research and Development Center in Wooster, said trees and all plants face a tradeoff between growth and defense. Devote more energy to growth and there's less for the production of natural defensive chemicals.

"It's similar to the 'guns or butter' decision faced by a government, where increased defense spending comes at the cost of decreased domestic spending, and vice versa," Herms said. "Plants face the same types of tradeoffs.

"The conventional wisdom that fertilization makes trees more resistant to insects is clearly not supported by a large body of evidence and also by the experiments that we're conducting here," he said.

The finding can help people improve the health of trees and the environment by knowing when and why to fertilize and by reducing the need for insecticides, Herms said. He based it on a review of published research and on his own research on landscape trees. It was published in the journal Phyton.

Tree-care professionals, homeowners, gardeners and farmers can put this knowledge to use, he said.

For example, if a homeowner wants to have fast-growing trees, then fertilization is called for, but it can be done with the understanding that it might increase insect problems. The homeowner -- assuming that he or she is willing to accept this tradeoff -- will know to monitor for the appearance of outbreaks and can deal with them sooner and more effectively, Herms said.

On the other hand, if rapid growth isn't important or if the homeowner doesn't want to worry a lot about pests, then he or she may choose to not fertilize or to fertilize less. The resistance of the trees won't suffer, and fewer or no insecticides may be needed.

"It's a matter of knowing the objectives of the landscape and having a clear understanding of the impact of the cultural practices," Herms said. "In many cases there's the thinking that fertilization will make a tree more resistant to insects, and that's a viewpoint that we need to get away from."

Instead, he suggests a new definition of tree health -- not rapid growth, but a more balanced allocation of resources. Trees in the wild are an example, he said.

"The natural tree environment is a nutrient-limited environment with predictable drought stress in summer and the presence of insect and disease organisms," he said. "The tree's response to this is a moderate rather than a rapid growth rate with higher levels of defensive compounds and higher levels of storage reserves than what might exist in a heavily fertilized, rapidly growing tree."

In essence, a plant's resources are fixed; they're determined by the photosynthesis rate, and fertilization doesn't have much effect on this rate. So instead of changing the plant's resources -- its "income" -- fertilization only changes where the plant spends its money: in this case, more on growth, less on defense. Pest resistance falls.

However, trees that are severely nutrient-stressed -- they're obviously yellow, or "chlorotic" -- are an exception. Fertilization can increase their photosynthesis rate and their "income" and thus can increase their insect resistance as well.

Herms conducts research on the physiology, health and growth of landscape trees. He is also looking at how different mulches may be used to enhance soil fertility and to establish nutrient cycling regimes that are similar to natural forest systems.

"We're trying to fill in the knowledge gap regarding the effects of cultural practices and management practices on tree health," he said. "We're trying to look at the urban and suburban landscape as an ecosystem and to develop management programs that are ecologically sound yet maintain the aesthetics of the landscape."

Source: Dan Herms, Department of Entomology, OARDC, (330) 202-3506, herms.2@osu.edu.

Written by: Kurt Knebusch, Section of Communications and Technology, OARDC, (330) 263-3776, knebusch.1@osu.edu.