Arboriculture & Urban Forestry 40(6): November 2014 Considering native borers together (Sesiidae and Cerambycidae), only water stress signifi- cantly influenced borer abundance (interaction: χ2 = 0.49, P = 0.48; water stress: χ2 0.009; ash species: χ2 = 6.91, P = = 0.80, P = 0.37; Figure 3E). Because so few roundheaded borers were found in surviving trees, the results for all natives in surviving trees mirror those of clearwing borers only, with neither ash species nor water stress hav- ing a significant effect on native borer abundance (interaction: χ2 = 1.07, P = 0.30; ash species: χ2 = 0.03, P = 0.85; water stress: χ2 = 0.10, P = 0.75). higher in green compared to Manchurian ash and in water-stressed trees (interaction: χ2 water stress: χ2 For all borers together, borer abundance was = 0.96, P = 0.33; = 10.62, P = 0.001; ash species: χ2 = 16.14, P < 0.0001; Figure 3F). Considering surviving trees only, the effect of water stress was unimportant, and borer abundance was only affected by ash species (interaction: χ2 0.44, P = 0.51; ash species: χ2 = 0.32, P = 0.57; water stress: χ2 = 16.24, P < 0.0001). = Phenological Observations for EAB in Maryland All EAB individuals discovered while debarking trees in April and May 2010 were in the pre-pupal, pupal, or adult stages, with no evidence of indi- viduals in early stages of larval development. All evidence of EAB detected while debarking trees two years aſter the initial exposure (February and March 2011) was from dead individuals that apparently had died much earlier. No evidence for a two-year EAB lifecycle was apparent in this study. DISCUSSION Urban canopies can be greatly reduced by EAB where ash is commonly planted (Poland and Mc- Cullough 2006; Raupp et al. 2006; Kovacs et al. 2010; Sargent et al. 2013). Along with biological control and chemical treatment of ash, the deployment of resistant woody plant material is a key component of integrated pest management for this insect (Raupp et al. 1992; Herms 2002; Herms and McCullough 2014). In the search for resistant ash stock, Manchu- rian ash has repeatedly been identified as highly re- sistant to EAB attack (Figure 3A; Rebek et al. 2008; Herms and McCullough 2014), and where planted, may be the only ash species remaining through the EAB invasion. If Manchurian ash were to be highly 339 susceptible to North American insects, however, its survival and utility in breeding programs would be compromised. In this study, Manchurian ash was indeed attacked by the suite of indigenous North American borers, but the frequency of borer attack and the abundance of borers in trees was never higher in Manchurian than in green ash trees. These results indicate that Manchurian ash may withstand attack by EAB and the suite of North American borers and be an appropriate source of resistance material for breeding programs and a replacement for North American ash trees in urban settings. Through evolutionary time, plants develop vari- ous mechanisms for resistance against key insect herbivores. The defense free-space hypothesis proposes that native plants that have not evolved to defend against exotic herbivores can be over- exploited (Gandhi and Herms 2010a; Raupp et al. 2010). These results provide support for the defense free-space hypothesis as it pertains to EAB. Across levels of plant water stress, EAB was 6.7 times more abundant in green ash compared to Man- churian ash (Figure 3A). These findings corrobo- rate those of several other authors regarding host preferences of EAB (Liu et al. 2007; Wei et al. 2007; Rebek et al. 2008; Pureswaran and Poland 2009). North American borers, on the other hand, did not strongly prefer native or exotic ash species, with similar frequencies and abundances in green and Manchurian ash (Figure 3E). North American bor- ers, therefore, may not be advantaged by defense free space in Manchurian ash. Experiments have shown that Manchurian ash has a distinct chemical profile likely involved in plant defense (Eyles et al. 2007; Cipollini et al. 2011; Whitehill et al. 2012). These anti-herbivore mechanisms may be general defenses against borers and prevent North Ameri- can herbivores from successfully exploiting Man- churian ash. Although there is mounting evidence that exotic herbivores frequently find defense free space in their invaded range (Desurmont et al. 2011; Nielsen et al. 2011), it is clear that not all such herbi- vore-plant interactions lead to herbivore outbreaks (Schierenbeck et al. 1994; Tallamy 2004; Cappuc- cino and Carpenter 2005). The inconsistent pattern of herbivory between native and exotic plants and insects in this experiment indicates a need for con- tinued experimental approaches and investigations into mechanisms underlying patterns of herbivory. ©2014 International Society of Arboriculture
November 2014
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