Arboriculture & Urban Forestry 36(3): May 2010 tree canopies (Figure 8a). New leaves flushing in July had no adult feeding injury. There was only one leaf-mining generation per year. The observations of the seasonal life history of A. aristata in Ken- tucky are consistent with an anecdotal account by Needham et al. (1928). The flies emerged in April and oviposited in newly expanding leaves. Eggs are usually inserted at or close to the leaf margin. The newly-hatched larva usually mines towards the leaf center, making a narrow linear pale-green track which often runs along a lateral vein and subsequently the mid-vein. The second instar widens the mine and creates lateral turns. After the second molt, the final (third) in- star broadens the mine further to produce a brownish blotch mine (Figure 1). The mines are very noticeable by early May. In a sample of 36 mined leaves, most (89%) had a single mine and the rest had two mines. All mines contain a single larva. In mid- to late-May the mature third instar cuts a semicircular slit, exits the mine, and falls to the ground where it pupates. There is one generation per year. DISCUSSION This study provides new information on relative resistance of com- mercially-available, Dutch elm disease-resistant elms to multiple in- sect pests. Variation in Japanese beetle feeding on Asian elm species 107 Figure 8. (a) Seasonal abundance of O. alni adults on two suscep- tible elm cultivars in 2009 based on two-minute counts performed weekly from before bud break until mid-June, and on two dates in July. (b) Correlation between numbers of O. alni mines and weevil feeding scars showing that cultivars relatively resistant to min- ing also are less-damaged by the adults. Each point represents the mean for a different cultivar. The four most resistant cultivars were Jefferson (fewest mines/holes), Morton Accolade, Emerald Sunshine, and Morton Stalwart Commemoration. The four most susceptible were Homestead (most mines/holes), New Horizon, Emer II Allee, and Frontier. and hybrids, including a few common to this study, was previously documented in trials with detached leaves or leaf disks (Miller et al. 1999; Miller 2000; Miller et al. 2001). Laboratory assays may (e.g., Spicer et al. 1995) or may not (e.g., Ladd 1987) accurately predict differences in Japanese beetles’ response in the field where olfactory and visual cues also mediate host selection (Loughrin et al. 1996; Rowe and Potter 1996; Rowe and Potter 2000). Although Ameri- can elm is traditionally listed as a favored host for Japanese beetles (Fleming 1972), current data indicates cultivars of U. americana, as a group, are less susceptible than many of the hybrids of elm species originating from mainland Asia or Europe. U. parvifolia and U. pro- pinqua, which originate from Japan, generally were less damaged than either American or hybrid elms, possibly reflecting their longer evolutionary interaction with P. japonica. In contrast, hybrids con- taining U. davidiana var. japonica (synonym of U. propinqua), U. pumila, or U. wilsoniana were heavily fed upon. Miller et al. (1999) also reported heavy feeding on complex hybrids of those species. The basis for variation in elm resistance to Japanese beetles is presently unknown. Miller et al. (1999) suggested leaf pubescence on the Asian elms U. glaucescens, U. lamellose, and U. macrocarpa might account for low levels of feeding seen on them in their assays, but those species were not included in the National Elm Trial. Tilia tomentosa L., a resistant linden species, also has pubescent leaves (Potter et al. 1998). Among the more resistant elms in this study, some (e.g., ‘Emerald Sunshine’, ‘New Harmony’) have abundant straight or bent simple hairs (0.3–0.4 mm height) on the leaf underside, whereas others (e.g., ‘Emer II Allee’, ‘Athena’, ‘Everclear’) are glabrous or nearly so. Other factors such as secondary chemistry, leaf toughness, and leaf size (e.g., the three resistant U. parvifolia cultivars all have very small leaves) warrant investigation as possible resistance factors. Bright red pouch galls induced by the aphid T. nigriabdominalis, although unlikely to impact tree vigor, were noticeable enough on some of hybrid cultivars (e.g., ‘New Horizon’) to possibly reduce the marketability of nursery trees. These aphids have a complex life his- tory with a portion of the population alternating between galls on Ul- mus spp., their winter and spring host, and roots of grasses, the summer host (Blackman and Eastop 1994; S. Halbert, pers. comm.). Flying females, called sexuparae, emerge from the grass roots in autumn, go to elms, and deposit tiny males and females (sexuales) on the bark. After mating, those females each lay one egg near a bud. In the spring, the eggs hatch and the nymph initiates a gall. Her all-female offspring are winged and after maturing fly out of the gall to colonize grass roots and start the cycle anew. Part of the population remains on grasses all the time (Blackman and Eastop 1994; S. Halbert, pers. comm.). Kaliofenusa ulmi, introduced from Europe, is best-known of the three leaf miner species at the current study site. The adult sawflies emerge in spring and lay eggs in the upper leaf epidermis (Johnson and Lyon 1988). Eggs hatch in roughly one week and the legless, flattened larvae form blotch-like mines. After several weeks the mature larva drops from the leaf and burrows into the soil where it overwinters. There is one generation per year. A survey of 30 elm accessions in Wisconsin (Guries and Smalley 1994) suggests Eurasian elms (including U. glabra) are highly susceptible, U. japonica and U. carpinifolia are less susceptible, and U. americana, U. parvifolia, and U. pumila are resistant to K. ulmi. Another survey at The Morton Arboretum in Illinois found far more K. ulmi mines on European elm species than on Asian or North American elms (Miller 2000). In the current study, K. ulmi was found almost exclusively on hybrid cultivars, some of which have European parentage (U. glabra, U. carpinifolia), but ©2010 International Society of Arboriculture
May 2010
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