Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), a secondary pest of stressed or declining ash (Fraxinus) trees in its native Asia, is the most destructive forest insect to ever invade North America. To date, tens of millions of ash trees have been killed in 19 states and two Canadian provinces. North American ash species are generally less resistant to A. planipennis than Asian species. Fraxinus quadrangulata Michx. and Fraxinus americana L. were originally abundant in two woodlots in southeast Michigan, but survival rates of the two species following A. planipennis invasion differed profoundly. Overall, 63% and 71% of F. quadrangulata trees were alive at the sites. In contrast, only 16% (all <11 cm diameter) of the F. americana trees survived at one site and all were dead at the second site. Urban trees are often planted in confined spaces and may experience varying degrees of environmental stress. Fertilizer and paclobutrazol (PB), a gibberellin inhibitor, may be applied to enhance tree vigor. Effects of fertilizer and PB were assessed on the physiology and growth of North American (F. americana and F. quadrangulata) and Asian (Fraxinus mandshurica Rupr.) ash trees in a plantation established in 2006. Fraxinus quadrangulata had 33% less radial growth, 13% more root biomass and 25% lower foliar nitrogen than F. americana or F. mandshurica. Ratios of pre: post treatment radial growth in control and fertilized trees were 33 and 43% higher, respectively, than PB trees. Aboveground growth of F. quadrangulata and F. mandshurica were reduced by PB treatment while F. americana was unaffected. Fertilized F. quadrangulata trees had higher relative chlorophyll content and nitrogen concentrations than control or PB trees, but other species did not respond to fertilizer. Suitability of the ash species for adult A. planipennis feeding and survival and larval density was also determined. Survival of adult A. planipennis caged on F. quadrangulata was lower and beetles consumed less leaf area than beetles caged on other species. North American Fraxinus nigra Marsh. and Fraxinus pennsylvanica Marsh. trees were heavily colonized by A. planipennis larvae. In contrast, F. quadrangulata and F. mandshurica were rarely colonized, while F. americana were moderately colonized. Results confirm North American F. quadrangulata has resistance to A. planipennis similar to F. mandshurica. Systemic trunk injected insecticides are often used to protect ash trees from A. planipennis, but wounds and injury are a concern. We examined basal trunk sections from 22 F. pennsylvanica and 24 F. americana trees macro-injected with a low or medium rate of emamectin benzoate in 2008 only or 2008+2009. Only 12 of the 233 injection sites had evidence of injury and there was no sign of pathogen infection. Confocal laser scanning microscopy (CLSM) examination showed xylem discoloration was not indicative of tissue damage. Cross-sectional area of earlywood lumen was 55% larger in F. quadrangulata than in F. pennsylvanica trees. Interspecific differences in xylem anatomy likely influence efficiency of trunk injections. Phenolic compounds and reactive oxygen species (ROS) are produced by plants in response to injury, stress or insect damage. Autofluorescence indicative of phenolics was detected in F. quadrangulata and F. mandshurica phloem, but not in F. pennsylvanica phloem. Fluorescence of ROS was detected in all the phloem of all three ash species, but F. mandshurica produced three times more than the other species. A continuous layer of sclerenchymatous cells was observed in F. quadrangulata and F. mandshurica and may affect A. planipennis larvae.
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