Arboriculture & Urban Forestry 40(6): November 2014 In addition to herbivore resistance, urban plants must also tolerate extreme environmental condi- tions, such as drought stress (Cregg and Dix 2001; Raupp et al. 2010). Cregg and Dix (2001) found that green ash in the urban environment experienced greater drought stress and suffered greater borer damage compared to those in more natural settings. Indeed, the increase in borer attack in stressed trees is one of the most consistent patterns emerging from studies of herbivory in response to plant stress (Koricheva et al. 1998; Huberty and Denno 2004; Raupp et al. 2010). In the current experiment, the frequency of attack and the abundance of borers in experimental trees increased with tree water stress. This pattern held for both green and Manchurian ash, but there was no evidence for an interactive effect between ash species and water stress. Thus, under conditions of water stress, the relative suscep- tibility of green and Manchurian ash to borers did not change. This was true in tests for individual borer groups as well as for all borers together (Figure 3F). These results are from an experiment conducted with potted ash trees, and the results may be limited, in some regards, because of the use of such small trees and the short duration of the experiment. In the hot summers of this experiment (e.g., 60 days above 32.2°C), the ash trees in pots almost certainly experienced more drought stress than trees in natu- ral forests. Additionally, the three original water stress treatments did not lead to a perfect gradient in stress. However, the trees in the low watering treatment were clearly more stressed than those in the full and half water treatments (Figure 2). Insects known to respond to stress, such as bark beetles and redheaded borers, did indeed attack stressed tress more frequently and achieve higher abundances (Figure 3). Clearwing borers, unexpectedly, did not respond to stress. If only the spring-flying Podosesia syringae were present at the exposure site, how- ever, oviposition and much of larval development would have been completed before water stress treatments began and thereby limited its ability to differentially detect and respond to stressed trees. Additionally, population abundances for clearwing moths may have simply been too low to detect treat- ment effects. Borer attack rates in this study were generally low, with a mean abundance of four bor- ers or fewer per tree (Figure 3F). Further studies should examine the susceptibility of mature Man- 341 churian ash to native borers, where and when bor- ers are more locally abundant, to verify that the results reported here are generalizable under higher pest pressure and for larger Manchurian ash trees. The use of small, potted trees may have also con- tributed to the findings that EAB completed devel- opment in only one generation at the study sites. EAB was initially thought to have one generation per year, but some reports indicated a two-year lifecycle, especially in outlier populations (Cappaert et al. 2005; Marshall et al. 2009; Duan et al. 2010; Siegert et al. 2010). In a study in Michigan, U.S., Duan et al. (2010) reported that over half of the wild population of EAB and 100% of an experimentally established cohort required two growing seasons to complete development. Girdling, however, promotes faster development rates (Tluczek et al. 2011). In the cur- rent study, tree water stress did not promote faster development, and tree size cannot be ruled out as a contributing factor. However, the warm Maryland climate may also be important. In forests near the common garden experiment, most natural trees of all sizes showed evidence of a one-year lifecycle, despite evidence for a two-year generation interval at other locations in Maryland (D.E. Jennings, pers. comm.). These results are consistent with findings from China, where EAB is known to have either a univoltine or a semivoltine lifecycle, depending on location (Wei et al. 2004; Wei et al. 2007) and with studies from other Agrilus spp., which require two years to develop in Canada but one in southern New England, U.S. (Tluczek et al. 2011). Together, these findings suggest that EAB may exhibit more rapid population growth and faster spatial spread at the southern part of its invaded range. Although ash mortality due to EAB infesta- tion varies among cultivars and species of North American ash, these differences have little impact on the ultimate fate of Fraxinus americana, F. pennsylvanica, and F. nigra in natural forests and urban settings, all of which typically experience >99% mortality (Rebek et al. 2008; Herms and McCullough 2014; Klooster et al. 2014). Manchu- rian ash, on the other hand, persists well in Asia even where EAB reaches outbreak densities on introduced North American ash (Wei et al. 2004; Baranchikov et al. 2008). Reports from Asia, sug- gest that EAB is only a problem when Asian trees such as F. mandshurica and F. chinensis, are highly ©2014 International Society of Arboriculture
November 2014
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