Arboriculture & Urban Forestry 32(6): November 2006 301 Nowak et al. (2001) used tree inventories to estimate num- bers of trees that might be killed and monetary losses result- ing from attack by Asian longhorned beetle. We suggest that losses associated with Asian longhorned beetle and emerald ash borer would be quite dramatic in terms of the proportion of trees killed, those subject to removal and replacement as part of eradication programs, and those that will require on- going protection by one or more insecticide applications if both borers become widespread in the eastern United States. The cities included in this study face the loss or need for insecticide protection of 29% to 70% of their street trees (Table 1). The average percentage of trees at risk was 49.7% (4.0% standard error) This estimate may be conservative. Recently, Morewood et al. (2003) demonstrated that red oak, Quercus rubra, was used for oviposition and supported larval development of Asian longhorned beetle. They recommended that red oak be considered a potential host when surveys are conducted to detect infested trees. Host associations in Asia include several genera in which no current information is available in the United States. The analysis provided here points out an important flaw in the way we think about the biologic diversity and pest resis- tance of our urban forests. Guidelines such as the ‘10-20-30’ rule were developed with the goal of preventing or avoiding the types of catastrophic losses seen with diseases such as DED. What these rules fail to consider is the fact that many, perhaps most, insect pests attack more than one cultivar or species of plant. In recent years, natural forests in the eastern United States have been subjected to attack by hemlock woolly adelgid and emerald ash borer. Although these pests are among the most stenophagous of our invasive species, feeding on Tsuga and Fraxinus, respectively, many exotic insects are far more polyphagous and attack several species, genera, and families of plants. The gypsy moth is known to feed on more than 500 species of plants in families that in- clude trees, shrubs, and vines. The winter moth, Operophtera brumata, recently introduced to New England, has a broad host range that includes oaks, maples, basswood, ash, crabapples, apple, blueberry, and certain spruces. The Asian ambrosia beetle, Xylosandrus crassiusculus, a pest of grow- ing concern in the southern United States, attacks important street trees such as cherry, goldenraintree, sweetgum, oak, elm, crapemyrtle, and magnolia. This borer is reported to attack plants in 46 families (Schedl 1962). As we point out in this article, the Asian longhorned beetle also has a broad host range and attacks and kills trees in several genera and families. Urban foresters that have used rules such as 10% diversi- fication may have created a buffer against only the most specialized kinds of exotic plant feeding insects. For ex- ample, a city that planted 3 different species of ash, each to the suggested 10% level, may now face a 30% loss with the arrival of emerald ash borer. Moreover, when government agencies conduct eradication programs and management de- cisions are made at higher taxonomic levels, specific or sub- specific differences in host susceptibility or adaptation to urban conditions may be meaningless. What steps can urban foresters take to lessen the impact of exotic pests on urban forests? The search for pest-resistant plant material should be accelerated, and this search should include genera and families of plants that are underrepre- sented in lists of currently available stock. At the specific level, clonally propagated material may be at an especially high risk to attack by invaders as a result of its inherently narrow genetic base. Diversification at generic, familial, and, perhaps, ordinal levels would help buffer against insect pests or pathogens that specialize at plant genera and species. The identification of resistant varieties will help in this quest. This means that plant breeders and growers must become a large part of the solution. A priority must be placed on pest resis- tance in breeding programs. Beautiful Fall color or an inter- esting habit will mean little when a borer or blight has killed a tree. Exotic plants may also prove to be part of the solution. Plant species that have a long association with an exotic pest may have evolved clever defenses to resist attack or may be better able to tolerate infestations (Herms et al. 2004b). Care will have to be taken to ensure that introduced plants are noninvasive and do not become pests. The impact of exotic plants on local food webs and community structure is yet to be elucidated and remains a concern. In addition, exotic plants have their own complex of potentially devastating ex- otic pests as evidenced by the eucalyptus pest complex in California (Paine and Millar 2003). Progress has been made in the area of tree selection with respect to Asian longhorned beetle management. The New Jersey Forest Service in cooperation with USDA has com- piled a recommended tree planting list for the Asian long- horned beetle quarantine zone and surrounding areas. This list is reproduced in Table 2 and is available at the following web site: www.aphis.usda.gov/newsroom/hot_issues/alb/ alb_pdfs/ALB-ReplantingList.pdf. With respect to management of emerald ash borer, a list of recommended replacements for ash in the lower peninsula of Michigan can be found in the Michigan State University Bul- letin E-2925 available at the following web site: www. emeraldashborer.info/files/19Alternativetreelisteab.pdf. Con- sideration must be given when selecting trees from this list, because several recommended trees are known to be highly suitable hosts of Asian longhorned beetle. Although many species identified on these lists have with- stood the test of adaptability and longevity in cities, others have not. One of the goals of urban foresters should be to plant and evaluate untested replacement species for traits such as adaptability and longevity in stressful urban sites. This will enhance the likelihood that attempts to diversify ©2006 International Society of Arboriculture
November 2006
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