106 Condra et al.: Elm Resistance to Insect Pests Figure 6. Abundance of K. alni leaf mines on elm species and cultivars. See Figure 4 legend for sample sizes and statistical dif- ferences. of EEFW mines differed within and between elm species and parentages (Figure 7). American elm cultivars, as a group, were more resistant than Asian or hybrid cultivars (ANOVA, linear contrasts, P < 0.01). There also was significant variation within elm species and parentages; e.g., U. americana ‘Jefferson’ and U. japonica × wilsoniana ‘Morton Accolade’ had particularly low numbers of EEFW mines in both 2008 and 2009 (Figure 7). Life History of Orchestes alni and A. aristata in Kentucky EEFW almost certainly overwinters as an adult. The weevils were first observed on susceptible cultivars on April 9, 2009, corre- sponding to accumulation of 88 Centigrade degree-days at base 10°C from January 1 (UK AG Weather Center 2009). Many mat- ing pairs and the start of adult feeding damage were seen on young leaves of early-flushing cultivars at that time (Figure 8). There was no clear relationship, however, between timing of leaf flush and resistance to EEFW–the phenological ratings on April 9 had aver- aged 3 (dormant buds) and 1 (leaf tip emergence) for susceptible ‘Homestead’ and ‘New Horizon’, respectively, whereas resistant ‘Jefferson’ and ‘Morton Accolade’ were rated 2 (green tip buds) and 5 (>1 leaf fully expanded), respectively. Resistance to A. aristata also appeared not to be driven by bud break timing: sus- ceptible ‘Valley Forge’ and ‘Princeton’ were both rated 2 (green tip buds) on April 9, whereas resistant ‘Prairie Expedition’ and ‘Jefferson’ were rated 2 and 4 (one expanded leaf), respectively. European elm flea weevils oviposit in the underside of the mid-vein of young leaves leaving a visible scar. No EEFW mines ©2010 International Society of Arboriculture Figure 7. Abundance of European elm flea weevil, O. alni, leaf mines on elm species and cultivars. There were significant differ- ences among cultivars (ANOVA, P < 0.001) with the following sig- nificant 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. Small numbers of weevils were first observed at the study site in 2007. Data here are number per 100 leaves (2008) or per ten 30 cm shoots (2009). were apparent on April 23, but five days later (April 28) they were noticeable on many of the trees. The legless larvae are cream-colored with a dark brown head capsule, somewhat flat- tened, and deeply segmented (Figure 2). Larvae with three dis- tinct sizes of head capsules were found in the weekly samples, suggesting three instars. Mines collected on April 28 (n = 20), contained first or second instars but a few third instars were present by early May. All dissected mines (n = 200) contained a single larva. In a sample of 180 mined leaves, 95% (171) had a single mine and the numbers having 2, 3, or 4 mines were 6, 2, and 1, respectively. All of the mines originated in the mid- rib except on the one leaf with four mines, on which one started in the midrib and the rest originated in lateral veins. EEFW pupated in the mine. Pupae, which are yellowish with an obvious snout, were first observed on May 15. New adults be- gan emerging May 23–29, and large numbers were feeding on the leaves by early June (Figure 8), but unlike in early spring, no mat- ing pairs were observed at that time. By early July, they had riddled leaves of the more susceptible cultivars with dozens of feeding holes (Figure 2; Figure 8). Cultivars with relatively few mines had little adult feeding, and vice versa (Spearman rank correlation 0.89, P < 0.001; Figure 8b). Numbers of weevils on the elm foliage declined in mid-late June and by early July they had nearly disappeared from the
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