Evaluation location: Kentucky, United States
Composite score of resistance to Japanese beetle (Popillia japonica), European elm flea weevil (Orchestes alni), European fruit lecanium (Parthenolecanium corni), European elm scale (Gossyparia spuria), Kaliofenusa ulmi, Agromyza aristata, pouch gall aphids (Tetraneura nigriabdominalis), cockscomb gall aphid (Colopha ulmicola), woolly elm aphid (Eriosoma americanum), woolly apple aphid (Eriosoma lanigerum), cottony maple scale (Pulvinaria innumerabilis), and Oedophrys hilleri.
The main study site was an open, grassy area on Maury silt loam soil adjacent to the Arboretum and State Botanical Garden of Kentucky (Lexington, Kentucky) (38°1’N, 84°30’W; elevation 302 m). Five replicates of each accession, were planted in a randomized complete block in rows spaced 7.6 m apart, with 7.6 m between trees within rows and about 50 m between replicates. Trees were planted in spring 2005 or 2006. The trees were obtained from nurseries as bare-root transplants and ranged from 1.5 to 2.4 m height at time of planting. They were staked, watered as needed, and mulched over grass that had been killed with glyphosate herbicide. Leaf characteristics (mean area and number per shoot) were previously summarized (Condra et al. 2010) to standardize insect feeding by leaf area.
Three species of sap-feeding scale insects colonized trees at the study site. Their numbers were assessed by inspecting each tree on multiple dates. For some of the taller trees, observers stood on a step ladder or truck tailgate to reach and sample sufficient numbers of representative branches. European fruit lecanium, Parthenolecanium corni (Hemiptera: Coccidae), was abundant enough to provide data from three growing seasons (2010–2012). Five twigs were randomly selected from throughout the canopy of each tree; then beginning at the previous year’s node the number of adult scales was counted on a 30-cm long section of each twig. Data were standardized to number of scales per 1.0 m of twig length. Counts were taken over 2–3 days in early July in 2010 and 2011, and in June 2012. European elm scale, Eriococcus spuria (Hemiptera: Eriococcidae), was abundant enough to census in 2011 and 2012. Adult scales were counted on five 30 cm-long twigs per tree as previously described. In addition, because E. spuria also infested bark of the trunk and scaffold limbs, two observers standing on opposite sides of each tree inspected bases of those branches and main trunk of each tree to 2 m height, and counted scales spotted in 30 seconds, taking care not to count particular scales more than once. Counts from the two methods were pooled for analysis. Sampling dates were the same as for P. corni. Cottony maple scale, Pulvinaria innumerabilis (Hemiptera: Coccidae), was first noticed on the trees in 2012. Two observers slowly circled each tree on June 7–8 and together counted all of the distinctive white scales through the canopy. Oedophrys hilleri (Coleoptera: Curculionidae), a mottled gray, leaf-notching weevil was found feeding on elms at the study site in 2010. Adult populations on each tree were sampled on July 11–12 and August 11 in 2011, and on August 8, 2012. The first sampling was with a gasoline powered leaf blower reversed for suction and fitted with a paint strainer in the intake tube. Each tree’s canopy was sampled for 60 seconds; samples then were transferred to bags and frozen before counting. For subsequent samples, researchers used a beating sheet (71 cm × 71 cm, BioQuip, Rancho Dominguez, California, U.S.), striking eight branches (four each in lower and upper canopy) with a stick and counting dislodged weevils. Two species of foliage-distorting woolly aphids were abundant enough to evaluate in 2012. Spring feeding by woolly elm aphid, Eriosoma americanum (Hemiptera: Aphididae), causes developing leaves to swell and their edges to curl downward. The aphids feed and reproduce within the leaf rolls. Spring feeding by woolly apple aphid, Eriosoma lanigerum (Hemiptera: Aphididae), results in unsightly rosette-like clusters of deformed leaves at the ends of shoots. Both aphids also cause damage by sucking sap from the host, and by producing honeydew. Incidence of each pest’s damage was assessed in early June by two observers who counted all individually-rolled leaves (E. americanum) and rosettes (E. lanigerum) on each tree. Elm cockscomb gall aphid, Colopha ulmicola (Hemiptera: Aphididae), is a relatively minor pest that induces elongated, raised, irregularly-toothed galls on the adaxial surface of leaves. The gall’s shape and reddish color at maturity account for the common name. The leaf galls harden and turn brown after the aphids depart. The number of galls on each tree (to 2.5 m height) were counted by two observers on July 11–12, 2011, and on June 7–8, 2012.
Averages for each insect on each accession were calculated and then ranked on a 1–5 scale based on pest density or extent of damage. The most susceptible accession(s) always received a 5 rating, and those that sustained no damage or infestation by a particular insect received a 0 rating for that pest. For Japanese beetle defoliation, 0–5 ratings corresponded to <10, 22–20, 21–30, 31–40, 41–55, and >55% cumulative leaf loss, respectively. Tree ratings for the leafminers O. alni, A. aristata, and K. ulmi, and for T. nigriabdominalis aphid pod galls were based on numbers of mines or galled leaves per 100 leaves (2006–2008) or per ten 30-cm shoots (2009–2012), with 1–5 ratings generally assigned to accessions ranked in successive quintiles of the frequency distribution. For those pests first reported on in this paper, cultivar ratings were based on number of European fruit lecanium and European elm scale per 1.5 m of twig length, or whole tree counts for O. hilleri, elm cockscomb gall aphid, wooly elm aphid, wooly apple aphid, and cottony maple scale, as described herein. For susceptibility/resistance rankings, the study authors considered four of the insect species, Japanese beetle, European elm flea weevil, European fruit lecanium, and European elm scale, as “major” pests because, at least in Kentucky, their impact on heavily infested trees and/or problems associated with their honeydew can be serious enough to warrant control. Ratings for those four pests were doubled and then added to the sum of ratings for the remaining eight pests to get overall scores upon which the elm accessions were ranked for relative susceptibility to insect pests.