Arboriculture & Urban Forestry 39(5): September 2013 was found on twigs, large branches, and bark of the trunk around the bases of scaffold limbs. The two pests had similar patterns of relative abundance or scarcity across elm species, cultivars, and parentages (Figure 2; Figure 3) and abundance of each dif- fered significantly among the 20 genotypes of elms (Kruskal- Wallis tests, P < 0.001). American elms, as a group, were more susceptible than Asian or hybrid cultivars to both scale spe- cies (Figure 2; Figure 3). Honeydew accumulation on the more heavily infested U. americana cultivars, such as ‘Valley Forge’, ‘Pioneer’, and ‘New Harmony’ promoted growth of sooty mold and attracted ants and wasps. Scale densities also varied among U. americana cultivars in each year (two-way ANOVA, P < 0.01), with ‘Jefferson’, in particular, having relatively few scales in each of the first two years that the respective species were evaluated. By 2012, however, densities of European fruit lecanium had built up to >25 scales per 1.5 m of twig growth, even on ‘Jefferson’ (Figure 2). Among hybrid cultivars, ‘Patriot’ and ‘New Horizon’ had relatively high densities of both scales. Cottony maple scale was first observed on some trees in 2012. Densities were low, with no overall differences among cultivars (Kruskal-Wallis, P = 0.50), but as with the other scales, it was mainly found on American elms (Figure 4). O. hilleri adults infested leaves of all 20 elm cultivars (Fig- ure 4). Although there were cultivar differences in each year, the weevil was not consistently more or less abundant on elms of any particular species, parentage, or provenance. Adults were first observed in the trees in June and became more abundant during July and August when many mating pairs were observed. Leaf distortions from woolly elm aphid and woolly apple aphid were restricted to U. americana (Figure 5). Both Eriosoma spp. were only abundant enough for meaningful comparisons in 2012, when abundance of each differed among U. americana cultivars (Kruskal-Wallis tests; P = 0.05 for rolled leaves with woolly elm aphid; P = 0.04 for rosette-like clusters with woolly apple aphid). Both pests were especially abundant on ‘Princeton’ (Figure 6). Elm cockscomb galls were found only on four Ameri- can elm cultivars (Figure 5) but their occurrence was spo- radic and variable, with detectable cultivar differences only in 2012 (Kruskal-Wallis test, P = 0.04). Cockscomb galls were most numerous on ‘Jefferson’ (Figure 7). Cultivars of the Asian species—U. parvifolia and U. propinqua—were top-rated from the standpoint of insect resistance and nearly pest-free except for EEFW foliar damage, to which nearly all species and cultivars were vulnerable (Table 2). Most U. americana cultivars were relatively susceptible to P. corni and E. spuria, and to leaf disfiguration by woolly elm and woolly apple aphids. Most hybrid cultivars were susceptible to foliar damage from Japanese beetles. Cultivars varied in relative resistance or susceptibility to the remaining types of pests (Table 2). DISCUSSION Diversifying the urban forest is important because invasive and native pests—such as emerald ash borer (Agrilus planipennis), Asian longhorned beetle (Anoplophora glabripennis), hemlock woolly adelgid (Adelges (Phytopthora tsugae), thousand cankers ramorum), and bacterial leaf disease of walnut caused by combined activity of the walnut twig beetle (Pityophthorus juglandis) and a canker produc- ing fungus (Geosmithia morbida), sudden oak death/decline fungus scorch Figure 2. European fruit lecanium relative abundance on elm spe- cies and cultivars. Cultivars abbreviations are indicted in Table 1. There were significant cultivars differences each year (Kruskal- Wallis, P < 0.001). Absence of variance (means of zero) for some cultivars precluded parametric analyses in 2010–11. U. americana cultivars as a group were more heavily infested than those having Asian parentage (U. parvifolia + propiqua + wilsoniana), or hybrid cultivars in 2012 (ANOVA, linear contrasts, P < 0.01). (Xylella fastidiosa)—threaten the health of prevailing tree spe- cies (Raupp et al. 2006). Replacement trees ideally should be relatively pest free to reduce need for insecticide treatments. Evaluations to date at the Kentucky NET site provided resistance- susceptibility data for a range of commercially-available Dutch elm disease-resistant elms to 12 different insect pests. European fruit lecanium and European elm scale are univoltine and of Palearctic origin. Both were especially abun- dant on certain U. americana cultivars, often infesting the same trees. The former is polyphagous, whereas the latter mainly feeds ©2013 International Society of Arboriculture 239
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