162 DISCUSSION Overall, Quercus spp. has the highest total pest rating, which is the mean of all seven pest categories, and Acer spp. has the second highest. This does not mean that Quercus spp. or Acer spp. are necessarily more pest prone or poorer choices for planting along streets or in landscapes. Considering trees at the genus level means some genera, such as Quercus and Acer, are each represented by dozens of species, but genera such as Liriodendron, Platanus, and Zelkova are each represented by one. Thus, when respondents consider all the species in a large genus, it is likely they can think of a species on which bark sucking insects are import- ant, a species on which leaf sucking insects are important, and another on which defoliators are import- ant. A genus with one or two species could seem rela- tively pest free by comparison. The provenance of trees is another important con- sideration when interpreting the total pest score for each genus. Zelkova spp. is represented primarily by a single species, Z. serrata, imported from Asia. Exotic trees often host fewer herbivore species, at least ini- tially, than native trees (Southwood 1961; Burghardt and Tallamy 2013). Quercus and Acer, for example, are represented primarily by native species that may host hundreds of herbivore species (Southwood 1961; Tallamy and Shropshire 2009). A few of these herbi- vores, such as scales or caterpillars, may become pests under urban conditions (Coffelt et al. 1993; Meineke et al. 2013; Dale and Frank 2014; Dale and Frank 2018). Quercus and Acer also contain exotic species with few pests, such as sawtooth oak (Quer- cus acutissima), trident maple (Acer buergerianum), and Norway maple (Acer platanoides). Key plants in an IPM program are those that have a disproportionate amount of pests or damage or those that are particularly valuable (Raupp et al. 1985). In this sense, some native Quercus spp. and Acer spp. may constitute key plants that are more pest prone than other species. Consumers and municipalities often have goals of increasing the number of native species planted as a conservation measure to preserve arthropod and bird populations. Greater use of native species combined with their higher herbivore abun- dance suggests that researchers and practitioners should prioritize developing IPM tactics for the most common and pest prone native trees. From the inven- tories used in the survey, these include species such as Q. phellos, Q. palustris, Q. falcata, and A. rubrum. ©2019 International Society of Arboriculture Frank: Survey of Street Tree Pests A more detailed survey of species within these genera would be required to ascertain key plants on which to focus research and management. A future survey should also include susceptibility to abiotic conditions and pathogens. Key pests are those that frequently or persistently cause severe damage and account for a disproportion- ate amount of control effort or expense (Smith and Van den Bosch 1967). Galling arthropods and mites appear to be the least important pest categories among the tree genera in this survey. Pests in both of these categories can be very destructive on some tree spe- cies under certain conditions and can be more diffi- cult to manage due to concealed or cryptic life stages and fewer available pesticides (Kropczynska et al. 1988; Eliason and Potter 2000b; Eliason and Potter 2001; Szczepaniec et al. 2011). Thus, galling insects or mites may constitute key pests for some arborists. The other four pest categories have similar ratings, though leaf sucking insects is highest. Most tree gen- era had leaf feeding scales, aphids, or lace bugs iden- tified as important in the survey. These and bark sucking insects qualify as key pests on susceptible species due to their persistence and density. Phloem feeding insects in this group have the added problem of producing honeydew (Dreistadt and Dahlsten 1988). Defoliators were also important overall. As key pests, they are often sporadic rather than persistent, but are very damaging to their hosts and adjacent plants with damage and frass that is apparent to the public (Schultz and Sivyer 2006; Eirich 2008; Frank 2014). This leads to high demand for expensive man- agement practices when defoliator outbreaks occur (Coffelt and Schultz 1990; Eirich 2008). Similarly, borers may occur sporadically but with severe conse- quences, such as rapid tree death. The importance of this will depend largely on the number of susceptible trees (Raupp et al. 2006). For example, emerald ash borer is a key pest that is devastating municipalities in Northern and Central states that have a high propor- tion of ash trees. In Southern regions, where ash trees are less common, the effect of emerald ash borer on individual trees is still severe, but consequences for municipalities are less. The prominence of Pinus spp. increases the risk from southern pine beetles. The diversity of plants and pests in urban land- scapes makes IPM education and implementation difficult. This survey provides an overview of the most important pest categories on the most common
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