A Formula For Higher Pest Resistance in Ornamental Trees

WOOSTER, Ohio — Stronger cultivars. Less fertilizers. Less pesticides. These are the three ingredients in a formula developed by Ohio State University researcher Dan Herms to promote higher pest resistance in popular ornamental trees and shrubs. An entomologist with the Ohio Agricultural Research and Development Center (OARDC) in Wooster, Herms has been studying several factors that enhance or reduce a tree’s ability to ward off insects. One of those factors —perhaps the most misunderstood— is fertilization. “It’s been the conventional wisdom for many years that fertilization increases the resistance of trees to insects,” Herms said. “However, a critical evaluation of evidence and the experiments that we’re conducting here find little to support this. Fertilization can increase the growth and nutrient content of trees, but it’s not helpful for insect resistance.” In fact, Herms pointed out, many studies have found that fertilization affects the resistance of woody ornamentals to insects by boosting the plants’ nutritional quality —increased nitrogen concentration makes them more attractive to pests— and decreasing the production of natural defensive chemicals. “This is not a criticism of the fertilizer companies, of course,” he said. “They are doing what experts have been recommending for a long time. What we are doing is trying to identify the best practices for tree health, and fertilization is not having the effects that we always expected.” All plants face a tradeoff. The more energy they dedicate to growth, the less they can devote to the generation of secondary metabolites (compounds that contribute to plant health and pest resistance) and the accumulation of energy reserves. In a natural environment, trees respond to this tradeoff by having moderate rather rapid growth rates, which allows them to allocate more resources for defense and storage. Fertilization does not only lower the resistance of ornamental trees to insects, Herms noted. Additional studies by Ohio State scientists have found that fertilization is also responsible for reduced tree resistance to other pathogens, such as fungi.  One example is Phytophthora root rot and its effect on rhododendrons. Research by OARDC plant pathologists Harry Hoitink and Steve Nameth shows that the nitrogen concentration in the new growth of rhododendron plants is directly related to Phytophthora dieback susceptibility. For example, lesions on new growth of rhododendrons with high-nitrogen concentrations (greater than 2 percent in young foliage) may extend down to the crown in 10 days or less. Meanwhile, the same infection on a low-nitrogen plant (1.2 percent in new growth) remains very small and may not even be detected. Recent research conducted by Herms and Ohio State plant pathologist Enrico Bonello on red pines reveals that fertilization can also enhance damage caused by the Sphaeropsis shoot blight and canker pathogen, Sphaeropsis sapinea —a pest responsible for extensive damage throughout the world on trees predisposed to disease by stresses. In their experiments, fertilized red pines inoculated with S. sapinea showed larger cankers than non-fertilized trees that were also exposed to the pathogen. In addition, fertilized pines had higher nitrogen content and significantly less secondary metabolites (such as lignin and total soluble phenolic compounds) than their fertilizer-free counterparts.  Herms has also studied the role played by insecticides in tree resistance. He said reliance on chemical pesticides should be limited because they disrupt the effect of many pests’ natural enemies, making trees more susceptible to infestation. Instead, the researcher recommended a more holistic approach to pest management. “Before using insecticides, growers and homeowners need to make sure they really have a pest problem,” Herms said. “They also need to make sure they spray at the right time, when the insecticide is going to be more effective. Spraying ‘to be on the safe side’ is a practice we need to get away from.” A helpful tool to determine the proper time for insecticide applications is Herms’ “biological calendar,” which predicts the emergence of tree and shrub pests based on the blooming of ornamental plants. Nursery managers, landscapers and gardeners are already reaping the benefits of this calendar, making fewer but more effective applications. Another alternative is the use of selective pesticides, such as Bt and insecticidal soaps, that only attack specific pests, leaving non-target organisms unharmed. Producers and gardeners, Herms said, must correctly identify the pest they need to control to make sure these products are going to work for them. Fertilization and pesticides play important roles in ornamental-tree resistance. However, Herms said, the most significant factor is genetics. “The best thing you can do is use plants that have natural resistance to pests,” Herms said. “However, deployment of resistant germplasm has been virtually ignored as a management tool for pests in urban forests and ornamental landscapes.” In spite of this historical trend, Herms said things have started to change in the past few years. As social and regulatory pressures act to limit the use of pesticides in urban areas, host-plant resistance is receiving increased attention from researchers and appreciation from the horticulture industry. One of the most rigorous studies of host-plant resistance was conducted by OARDC entomologist David Nielsen between 1979 and 1999 in Wooster. Nielsen studied resistance of eight species of birch —European, Asian and North American— to bronze birch borer, Agrilus anxius. European white birch was the most commonly cultivated species of birch for a long time, but infestations by bronze birch borer have dramatically curtailed its use in landscapes. The high susceptibility of this birch species to A. anxius has prompted a search for other white birches suitable for landscapes, with borer resistance as the single major selection criterion. At the end of Nielsen’s 20-year study, only the native North American white-barked birch species planted, Betula papyrifera and Betula populifolia ‘Whitespire,’ were still standing, having experienced low mortality rates. Another native species, Betula nigra —which does not have white bark— showed no evidence of borer colonization and virtually no mortality. “Over 75 percent of the native trees survived despite the fact that they endured huge bronze birch borer outbreaks without the benefit of protective pesticides, irrigation or fertilization and were also exposed to substantial droughts in 1988 and 1991,” Herms, who has also contributed to this research, indicated. “This suggests that the two species can be grown successfully in landscapes of the Midwest.” Herms said this research will lead to the release of a bore-resistant white-barked birch cultivar. Over the years, researchers at OARDC’s Secrest Arboretum have also identified pest-resistant cultivars of crabapple —one of the country’s most popular and widely grown landscape trees. These findings have helped green industry professionals and gardeners choose which crabapple varieties to plant, based on resistance, adaptability to Ohio growing conditions and aesthetics. “We’re trying to fill in the knowledge gap regarding the effects of cultural practices and management practices on tree health,” Herms explained. “This will help us develop management programs that are ecologically sound yet maintain the aesthetics of the landscape.” For more information about pest resistance of woody ornamentals, contact Herms at (330) 202-3506 or herms.2@osu.edu. Herms’ biological calendar is available online at http://www.oardc.ohio-state.edu/gdd/. OARDC is the research arm of Ohio State’s College of Food, Agricultural, and Environmental Sciences. Dear Editor: The photographs accompanying this article show North American white birches that have survived bronze birch borer infestations at OARDC's Wooster campus (Credit: Ken Chamberlain).