236 Potter and Redmond: Elm Resistance to Insect Pests Arboriculture & Urban Forestry 2013. 39(5): 236–243 Relative Resistance or Susceptibility of Landscape-suitable Elms (Ulmus spp.) to Multiple Insect Pests Daniel A. Potter and Carl T. Redmond Abstract. The National Elm Trial is a cooperative project to assess landscape suitability of Dutch elm disease-resistant elms (Ulmus spp.) in various regions of the United States. Researchers evaluated 20 cultivars of American, Asian, and hybrid elms for relative resistance or susceptibility to multiple insect pests in central Kentucky over seven years. Ratings for Japanese beetle, European elm flea weevil (EEFW), and several other pests were previously published. This paper reports data for seven additional pests, including honeydew-excreting scale insects (Parthenolecanium corni, Eriococcus spuria, and Pulvinaria innumerabilis), leaf-distorting woolly elm and woolly apple aphids (Eriosoma spp.), elm cockscomb gall aphid (Colopha ulmicola), and an invasive weevil (Oedophrys hilleri) not previously known to damage elms. Rankings for all 12 of the monitored pests are summarized. Most U. americana cultivars were relatively susceptible to the scale insects and likewise, Eriosoma spp. and C. ulmicola only infested the American elms. O. hilleri is a new state record for Kentucky. Its adults, active in mid- to late summer, chewed notches in edges of leaves. Cultivars of the Asian species U. parvifolia and U. propinqua, including ‘Athena Classic Lacebark’, ‘Everclear Lacebark’, ‘Emer II Allee’, and ‘Emerald Sunshine’ were top-rated for insect resistance. They were nearly pest-free except for foliar damage by EEFW, to which nearly all elms were susceptible. Insect resistance should be considered when re-introducing elms to urban landscapes. The data may help city foresters, landscapers, and others re-introducing elms to urban landscapes to select relatively pest-free cultivars requiring minimal inputs for insect control. Key Words. Cockscomb gall Aphid; Dutch Elm Disease; Eriococcus spuria; Eriosoma spp.; European Elm Flea Weevil; National Elm Trial; Oedophrys hilleri; Parthenolecanium corni; Ulmus spp. The National Elm Trial (NET) is a cooperative effort to evalu- ate Dutch elm disease (DED) resistant, commercially available American and hybrid elms (Ulmus spp.) for horticultural char- acteristics and suitability for use across a range of environ- mental conditions in the United States (Jacobi et al. 2009). It seeks information to support successful return of American and hybrid elms to streets and landscapes. Large replicated field plots were planted in Kentucky and 14 other states. Since 2006, researchers have been evaluating 20 elm cultivars of dif- fering parentage at the Kentucky site for relative susceptibility or resistance to multiple insect pests. Some of the pests cause minor aesthetic damage whereas others have a severe enough impact to potentially reduce sustainability of established trees. Elms, particularly the stately American elm (Ulmus amer- icana) were once a dominant component of the urban forest throughout much of North America (Hubbes 1999). Dutch elm disease, caused by the fungus Ophiostoma ulmi and vectored by elm bark beetles (Coleoptera: Curculionidae: Scolytinae), had already decimated elms in Europe when it was first discovered in the United States in 1930 (Hubbes 1999). By 1977, pandemics of DED had killed most of the original elm population in the eastern United States (Merkle et al. 2007). Efforts to develop hardy DED-resistant American and hybrid elms have been underway for many years (Smalley and Guries 1993; Merkle et al. 2007). Although straight selection by inoculation screening yielded several commercial Ameri- can elm cultivars with partial DED-resistance (Merkle et al. ©2013 International Society of Arboriculture 2007), all of them may become infected (Hubbes 1999). The main sources for DED resistance genes are Asian elm species, such as U. parvifolia, U. pumila, U. propinqua, U. japonica, U. wilsoniana, and others (Smalley and Guries 1993, Hubbes 1999). The DED fungus likely originated in Asia (Brasier and Mehrotra 1995), so genotypes having Asian provenance have had longer to develop resistance. A number of DED- resistant hybrid elms have been developed from combinations of Asian and, to lesser extent, European genotypes (Smalley and Guries 1993). Their growth habit, leaf size, and hardi- ness vary across different regions (Smalley and Guries 1993). Resistance ratings and biological observations from the Kentucky portion of the NET were previously reported for Japa- nese beetle (Popillia japonica), pouch gall aphids (Tetraneura nigriabdominalis), European elm flea weevil (Orchestes alni), and the leaf miners Agromyza aristata and Kaliofenusa ulmi (Condra et al. 2010). Since then additional pest species have colo- nized the tree plots. Researchers report here on relative resistance to seven additional insect pests including three species of honey- dew-excreting soft or felt scales, two leaf-distorting woolly aphid species, aphid cockscomb galls, and an invasive leaf-notching weevil not previously reported as a pest of elms. By late sum- mer 2012, tree loss from drought and other causes had reduced the original five replicates to only 2 to 3 trees of some cultivars; then a construction project that autumn necessitated transplanting trees from a portion of the plot. It is timely, therefore, to sum- marize findings to date from the Kentucky NET and rank the
September 2013
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