104 canopy; their counts were then averaged. Once mines appeared, 20 mined leaves (two to three per tree) were collected weekly, stored in 80% EtOH, and dissected to track larval and pupal de- velopment. Those leaves were taken from higher up in the cano- py than where the mine counts were taken for assessing cultivar resistance. Samples of mined leaves were examined to determine frequency distributions for mines per leaf, and larvae per mine. The hypothesis that phenological asynchrony between bud break, leaf flush, and onset of adult activity might account for cultivar re- sistance to EEFW was evaluated by comparing timing of bud break of two highly susceptible cultivars, ‘New Horizon’ and ‘Homestead’ with two resistant ones, ‘Morton Accolade’ and ‘Jefferson’. The same hypothesis was tested for A. aristata by comparing bud break timing of highly susceptible ‘Princeton’ and ‘Jefferson’ with more resistant ‘Valley Forge’ and ‘Prairie Expedition’. To keep other factors as con- sistent as possible, three of the four cultivars selected for testing the phenological asynchrony hypothesis with EEFW are hybrids (‘Jef- ferson’ is an American), and all four cultivars tracked for A. aristata are U. americana. Two independent observers rated the overall bud development of all trees of those cultivars on April 9, 2009, the first week weevils were observed at the site—as 1 (dormant bud), 2 (swollen bud), 3 (green tip bud), 4 (one fully emerged leaf), or 5 (two or more leaves). Ratings were averaged within trees and cultivars. The hypothesis that elm cultivars resistant to EEFW mining also resist adult feeding was evaluated in early July by which time the wee- vils had chewed numerous holes in leaves of susceptible cultivars. Twenty leaves were randomly sampled from each tree on July 1, and taken to the laboratory where the characteristic feeding holes were counted. Counts were standardized to holes per 10 cm2 of leaf area. Statistical Analyses Percentage defoliation by Japanese beetles, and numbers of Euro- pean elm flea weevil mines and adult feeding holes were analyzed by two-way analysis of variance (ANOVA) and pre-planned single degree of freedom linear contrasts between groups of elm species or parentages within each year. Percentages were arcsine square root-transformed and mine and hole counts were log-transformed to meet assumptions of normality and homogeneity of variances. ANOVA assumptions could not be met for pouch galls and the other two leaf miner species because all zeroes were document- ed for some cultivars, so the nonparametric Kruskal-Wallis test was used. Relationship between counts of EEFW larval mines and adult feeding holes was tested by Spearman rank correla- tion. Statistix version 9.0 (Analytical Software 2008) was used for all analyses. All data are reported as original means (± SE). RESULTS Pest Resistance Evaluations The different elms varied in susceptibility to Japanese beetles (Figure 3). Straight species and cultivars having Japanese ori- gin (U. parvifolia and U. propinqua) were less susceptible than U. americana or U. wilsoniana which are native to North America and central China, respectively. Hybrid elms, as a group, were more heavily defoliated than any of the straight species (ANOVA, linear contrasts, P < 0.01) although ‘New Horizon’ and ‘Frontier’, two cul- tivars having Japanese parentage, were consistently less susceptible than the other hybrid elms (Figure 3). However, ‘Morton Glossy Tri- umph’, the other hybrid with some Japanese (U. japonica) parent- ©2010 International Society of Arboriculture Condra et al.: Elm Resistance to Insect Pests age, was among the most susceptible cultivars. There was no over- all relationship between Japanese beetle injury (three-year average) and pubescence rating (Spearman rank correlation 0.30, P = 0.19). Mean pubescence rating also did not differ between the six most and six least resistant cultivars (Wilcoxon signed rank test, P = 0.33). There was no difference in Japanese beetles’ consumption of ‘Prairie Expedition’ compared to the mean for other American elm cultivars in the supplemental feeding trial (ANOVA, linear con- trasts, P = 0.98). Mean (± SE) leaf area eaten of ‘Prairie Expedi- Figure 3. Cumulative leaf area loss by late July from Japa- nese beetle feeding on elms. Cultivar abbreviations are listed in Table 1. There were significant differences among cultivars (ANOVA, P < 0.001) with the following significant differences (all P < 0.01) indicated by linear contrasts in each year: 1) U. parvifolia + propiqua < americana; 2) U. parvifolia + propiqua < wilsoniana; 3) U. parvifolia + propiqua < all hybrids; 4) U. americana < all hybrids. For 2006, “X” denotes cultivars that had not yet been planted. tion’ compared to the other four cultivars was 1.80±0.43 cm2 ver- sus 1.82±0.18 cm2, respectively, indicating that despite the heavy feeding that it incurred in the summer it was planted, ‘Prairie Expedition’ is not inordinately susceptible to Japanese beetles. Club-like pouch galls (Figure 1) induced by the aphid Tetra- neura nigriabdominalis were the only abundant insect-induced
May 2010
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