Arboriculture & Urban Forestry 45(5): September 2019 nationally but was ranked lower in the South (Wu et al. 1991). Elms are less common now due to Dutch elm disease, but new resistant American elm geno- types and Asian species such as Siberian elm (U. pumila) and lacebark elms (U. parvifolia) are becom- ing more common in urban plantings. Although elm leaf beetle was identified as an important elm pest in this survey, recent research suggests European elm flea weevil (Orchestes alni) is the most frequent bee- tle to damage elm leaves in many regions (Condra et al. 2010; Griffin and Jacobi 2016). European elm flea weevil was first discovered in North America in 2007 and has since become widespread (Anderson et al. 2007). European elm flea weevil larvae mine leaves, whereas elm leaf beetle larvae skeletonize leaves. Determination of which pest species is present will inform the best course of management. Sawflies can be important defoliators, primarily of evergreen tree species, that reduce tree growth and survival (Kulman 1971). Only one sawfly species, Redheaded pine sawfly (Neodiprion lecontei), was identified as a pest in this survey. Leaf and Stem Gallers Galls, such as horned oak gall (Callirhytis cornigera), become important pests in some situations (Johnson and Lyon 1976). Galls have not been ranked highly or even appeared in previous pest assessments. How- ever, their consequences for plant vigor and manage- ment can be severe (Eliason and Potter 2000a; 2000b; 2001). Quercus spp. appear to have the most conse- quential gall pests of the tree genera in this survey. Gallers were essentially tied with defoliators as the most important pest category of Quercus spp., with horned oak gall and gouty oak gall (Callirhytis quer- cuspunctata) mentioned most frequently. Trunk and Twig Borers Important trunk boring insects are primarily cole- opteran and lepidopteran species. Borers are often difficult to control, and the consequences of borer attacks can be rapid and severe. Thus, borers have ranked highly in previous pest assessments. For example, “borers”’ ranked as the first and third most important street tree pest in the Southern and North Central U.S.A. and fourth nationally (Kielbaso and Kennedy 1983; Wu et al. 1991). Several exotic borers that are devastating pests have arrived since these surveys were conducted, including emerald ash borer 161 (Agrilus planipennis), Asian longhorned borer (Ano- plophora glabripennis), and walnut twig beetle (Pityophthorus juglandis). Borers were the most important pest category of Fraxinus spp. due to emerald ash borer, which is spreading through the South. Borers of Pinus spp. are primarily Scolytine beetles. The most serious pest of Pinus spp. is southern pine beetle (Dendroctonus frontalis), which attack trees en masse, especially when trees are stressed by drought or injury. South- ern pine beetle is an example of how the importance of pests can change due to higher temperatures, as it has spread north as far as Massachusetts where it did not occur before the last decade (Dodds et al. 2018). In the south, flat-headed apple tree borer (Chrysobo- thris femorata) and ambrosia beetles, such as Xylosandrus crassiusculus, are pests of some tree species especially when stressed by drought, flood- ing, winter damage, or injury (Frank and Ranger 2016; Ranger et al. 2016; Frank et al. 2017). Mites Spider mites can be important pests of some urban tree species. Spider mite abundance can erupt in response to environmental conditions such as imper- vious surface cover, heat, drought, or dust (Ehler and Frankie 1979; Rigamonti and Lozzia 1997; Sperry et al. 2001; Kropczynska et al. 2002; Meineke et al. 2016), or in response to anthropogenic inputs such as insecticides (Szczepaniec et al. 2011), fertilizer (Prado et al. 2015), or road salts (Fostad and Pedersen 1997). Thus, they are often innocuous until some environmental conditions or disturbance occur. Due to their sporadic outbreaks, mites are not always ranked highly in urban tree pest assessments (Kiel- baso and Kennedy 1983; Wu et al. 1991). However, spider mites were among the most common and problematic pests in urban landscapes and among the pests for which the most pesticide applications were made (Holmes and Davidson 1984; Raupp and Noland 1984; Braman et al. 1998). Mites were not the highest ranked pests of any tree genera in this survey. Mites ranked highest on Acer spp. and Quercus spp., which can be infested with maple spider mites (Oli- gonychus aceris) and oak spider mites (O. bicolor). These can become abundant after insecticide applica- tions that kill natural enemies and due to excess nitro- gen that improves leaf quality (Frank and Sadof 2011; Prado et al. 2015). ©2019 International Society of Arboriculture
September 2019
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