Arboriculture & Urban Forestry 39(5): September 2013 mottled gray, slow-moving, and notch leaves from their mar- gins, whereas EEFW are reddish brown, active jumpers, and leaves upon which they have fed appear as if riddled with tiny shot holes (Figure 1). EEFW adults feed upon and lay eggs in expanding leaves in April; its larval mines are initiated in late April and completed by mid to late May. The resultant adults cause extensive foliar damage in late May and June, but by July they have nearly disappeared from tree canopies (Condra et al. 2010). O. hilleri adults are most abundant in June and July, as cited here as well as by Brown and Mathews (2010). Their larvae presumably feed on elm roots (see above) but whether or not it impacts tree vigor is unknown. Woolly elm and woolly apple aphids overwinter as eggs in bark crevices on elms (Johnson and Lyon 1988). Nymphs hatch in spring and migrate to the new growth. Feeding by the maturing female woolly elm aphid and her parthenogenically- produced offspring causes a tight leaf edge curl on individual leaves within which the colony develops. Woolly apple aphid feeding induces rosettes of twisted, dwarfed leaves to form at growing tips of shoots (Figure 1). In late spring, a winged gen- eration is produced that flies to summer hosts: Amelanchier spp. for woolly elm aphid, Malus or other rosaceous hosts for woolly apple aphid. Those aphids give rise to another brood of nymphs, 241 all females, which migrate to and feed on roots of their summer hosts, and in the case of wooly apple aphid, on aboveground tis- sues too. Winged females from a sexual generation produced in early autumn fly back to elms to lay overwintering eggs. Both Eriosoma species were found only on U. americana, especially ‘Princeton’. Although neither is likely to reduce vigor of estab- lished elms, the leaf deformities are unsightly enough that highly susceptible cultivars might be unsuited to high-profile sites. Elm cockscomb gall aphid has a complex life cycle. It induces galls on elm in spring, progeny vacate them to feed on grass roots in summer, and then winged forms return to elms in autumn to lay over- wintering eggs around buds (Patch 1910). Although the galls may attract attention, they likely have negligible impact on tree health. Another pest, elm leaf beetle (Xanthogaleruca luteola), which in Kentucky was common until about 10–20 years ago (authors’ observations), was conspicuously absent at the study site. Its populations have also declined else- where in the Ohio Valley region (e.g., Gibb et al. 2005). In summary, although none of the elms in the National Elm Trial are immune to insect injury, they differ in susceptibility to key pests that, if present, could potentially compromise the sustainability of landscape or street trees. Awareness of such differences should help urban foresters and landscapers Figure 6. Rolled leaves with woolly elm aphids and rosettes of deformed leaves caused by feeding of woolly apple aphids were restricted to U. americana. Within American elms, there were significant cultivar differences in susceptibility to each pest (Kruskal-Wallis, P ≤ 0.05). Figure 7. Elm cockscomb gall aphid was only found on American elms, among which its abundance differed each year (Kruskal- Wallis, P < 0.001). ©2013 International Society of Arboriculture
September 2013
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