Arboriculture & Urban Forestry 36(3): May 2010 galls on the study trees. The galls, formed on the adaxial side of expanding leaves, were green at first, becoming bright red as the aphids developed, and later withering to brown after the aphids have matured. Individual pouch galls (n = 36) dissected on May 8, 2009, contained a mean of 4.7±0.6 aphids (range: 1–12). Out of 91 galled leaves, the numbers having 1, 2, 3, 4, or >4 galls, were 75, 4, 5, 2, and 5, respectively. Some Tetraneura pouch galls were found on all species but they were particularly abun- dant on the U. pumila × U. japonica hybrids ‘Morton Plainsman Vanguard’ and ‘New Horizon’ (Figure 4). Overall densities of pouch galls were much higher in 2009 than in 2006–2008. Small numbers of comb galls induced by the aphid Colopha graminis also were present but their numbers were too low for analysis. Three different elm-infesting leaf miner species were found on the study trees (Figure. 1; Figure 2). Agromyza aristata (formerly Agromyza ulmi), a leaf-mining fly, was most abundant on U. amer- icana but also found on U. wilsoniana and several of the hybrid elms (Figure 5). In contrast, Kaliofenusa (= Fenusa) ulmi Sunde- 105 vall (Hymenoptera: Tenthredinidae), a European sawfly that en- tered North America before 1898 (Smith et al. 1971), was relative- ly abundant on several of the hybrid cultivars, but either absent or uncommon on U. americana, and on the Asian species (Figure 6). Populations of A. aristata and F. ulmi were decimated by the April 2007 freeze that killed young leaves of most cultivars. By 2009, the latter species had rebounded but A. aristata densities remained much lower than in 2006 (Figure 5). EEFW, the invasive leaf- mining weevil first noticed on the elms in 2007, had become the most abundant and destructive leaf miner at the study site by 2008. Mines of these three species are readily distinguished: A. aristata forms a winding serpentine-blotch mine that typically begins at the leaf margin, K. ulmi causes large blotch or blisterlike mines, and EEFW forms a shorter linear-blotch mine that begins in the mid- vein and widens at the leaf margin, usually near the tip (Figure 1). EEFW had the broadest host range among the three species of leaf miners at the study site. It was found on all species and hybrids, including 19 of the 20 cultivars evaluated. Abundance Figure 4. Abundance of aphid (T. nigriabdominalis) pouch galls on elm species and cultivars. There were significant differences among cultivars in each year (Kruskal- Wallis nonparametric ANOVA, P < 0.001). For 2006, “X” denotes cultivars that had not yet been planted. An April freeze killed the first flush of leaves of most cultivars in 2007. Data are number per 100 leaves (2006, 2008) or per ten 30 cm shoots (2009). Figure 5. Abundance of A. aristata leaf mines on elm species and cultivars. Cultivar abbreviations are listed in Table 1. See Figure 4 legend for sample sizes and statistical differences. ©2010 International Society of Arboriculture
May 2010
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