336 Martinson et al.: Water Stress and Geographic Origin Influence Borer Infestations in Ash Family Sesiidae These native Lepidoptera include banded ash clearwing borer (Podosesia aureocincta), which flies in late summer, and ash/lilac clearwing borer (P. syringae), which flies in late spring and early summer (Johnson and Lyon 1994). Characteris- tic signs of clearwing infestation are large, messy wounds with frass that has been expelled from gal- leries and remains on or around the tree. Galleries are deep in the sapwood and are not packed with frass (Herms 2007; Kalisch and Baxendale 2010). Family Cerambycidae Roundheaded borers, including the native redheaded ash borer (Neoclytus acuminatus), leave frass- packed but non-serpentine galleries that extend from just under the bark into the sapwood (Herms 2007). Redheaded ash borers typically complete development in one year, with adults emerging in the spring, though there is evidence that a partial second generation can develop within a growing season (Johnson and Lyon 1994). All galleries clas- sified as those of roundheaded borers were consid- ered to be natives, as non-native cerambicids are unknown from ash in locations used in this study. Family Curculionidae Bark beetles leave surface wounds and easily identifi- able, ornate, fan-shaped galleries just under the bark. Because identification was performed at the family level and both introduced and native bark beetles feed on ash in eastern North America, the geographic ori- gin of bark beetles in this experiment was unknown. Statistical Analysis All statistical analyses were performed in R (version 3.0.1, R Development Core Team 2013). Sepa- rate contingency table analyses were used to test whether the frequency of trees infested with each type of borer (EAB, Sesiidae, Cerambycidae, Cur- culionidae) was independent of ash species and watering treatment. Where expected values in the contingency tables were too low and χ2 tests inappropriate, likelihood ratio G-tests were used (g.test function version 2.1, P. Hurd 2001). Aſter evaluating borers separately, the frequency of trees infested by native borers (Sesiidae and Ceramby- cidae), as well as by any borer (including EAB), was evaluated with contingency table analysis. ©2014 International Society of Arboriculture Generalized linear models were used to assess whether borer abundance, measured as the num- ber of borers per tree, depended on the single and interactive effects of ash species and watering treatment. For count data, such as the number of borers per tree, a Poisson error structure is oſten appropriate but requires the variance to be equal to the mean. In this data set, the variance was greater than the mean (e.g., for EAB abundance, variance = 1.87, mean = 0.89), making the Poisson structure inappropriate (Crawley 2007). Negative binomial models fit the data better and were used throughout (glm.nb function in the MASS package, Venables and Ripley 2002). Separate models were developed for EAB, Sesiidae, Ceram- bycidae, Curculionidae, all natives, and all borers together. For each borer, the significance of model terms was evaluated with likelihood ratio tests. Because of substantial tree death that had the potential to influence the frequency and abundance of borers in this experiment, analyses were conducted first on the full data set of 96 trees and then on the subset of trees surviving the first summer (75 trees). RESULTS Watering Treatments Tree water stress, measured as chlorophyll fluores- cence, did not differ significantly before the start of the experiment among trees assigned to differ- ent treatments (ANOVA for treatment effect, F2 = 1.12, P = 0.34). Once begun, watering treatments successfully led to differences in tree water stress among treatments (F2 A slight statistical interaction between treatment and ash species (F2 2A). Tree water stress did not differ significantly between the two ash species (F1 = 22.16, P < 0.001; Figure = 1.68, P = 0.20). = 2.89, P = 0.06) was driven by the low chlorophyll fluorescence of dying trees. Comparing trees in different watering treatments, trees in the low watering treatment exhibited low- er chlorophyll fluorescence, that is, greater stress, compared to those in the full and half watering treatments (Tukey’s post hoc tests, Padj < 0.0001 In the first two weeks of water restriction, four trees died, and by the end of the first sum- for both comparisons). Trees in the full and half watering treatments did not differ significantly in chlorophyll fluorescence (Padj = 0.75; Figure 2A).
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