Agrilus planipennis Fairmaire (Coleoptera: Buprestidae; emerald ash borer) is a metallic green beetle native to northeastern Asia that has become a pest to North American ash (Fraxinus spp. L.) [1]. This pest was introduced into the Detroit/Windsor area of Michigan, USA/Ontario, Canada, and quickly dispersed via human assistance, including movement of firewood, nursery stock, and wood packing material [2,3]. In its native range, A. planipennis coevolved with Manchurian ash (F. mandshurica Rupr.) and is a secondary pest in this tree species requiring a primary stressor for successful attack [4,5]. Ash species in North America lack this natural resistance and succumb to attack, regardless of the presence of a primary stressor, often within one to four years after initial attack [4,6]. While black (F. nigra Marsh.), green (F. pennsylvanica Marsh.), and white (F. americana L.) ash are the most susceptible in the introduced range of A. planipennis, all North American ash are susceptible [2,7,8]. Blue ash (F. quadrangulata Michx.) has the lowest susceptibility to A. planipennis attack among North American ash species [9].
In North America, the life cycle of A. planipennis is typically completed within one year [10]. Females and males mature as they feed on canopy leaves. Males identify suitable mates via visual and contact cues, and females feed on foliage for an additional five to seven days after mating before oviposition begins [8,11,12]. Eggs are typically laid in bark cracks and crevices, with larvae subsequently tunneling into the bark to feed on the phloem and vascular cambium of the tree. Phloem consumption creates serpentine-shaped galleries, which severs photosynthate transport leading to eventual mortality. While a one-year life cycle is most common, a two-year life cycle does occur, especially in more northern latitudes, with larvae overwintering in intermediate instars within the phloem [13].
The proliferation of A. planipennis throughout forests in North America has caused the mortality of millions of ash trees, producing devastating ecological and economic impacts [8,14,15]. These impacts have created long lasting changes to North American forest ecosystems, requiring substantial restoration efforts [8,10]. Additional negative impacts include eradication of wood products produced from ash and diminished aesthetics in urban and suburban neighborhoods [8,14]. The cost of removal and replacement of ash trees in urban landscapes has been estimated at $12.5 billion from 2010–2020 [15]. Additionally, the estimated loss by timberlands in the United States is $300 billion [8].
Given the large-scale distribution of A. planipennis in both natural and urban landscapes, management options for control of the pest remain limited. Therefore, a long-term solution to preserving ash will depend on successfully identifying resilient genetic variants of ash. Resistance to wood-boring beetles is typically a function of female host selection and larval survival rate [17]. Therefore, resistance mechanisms can be placed into three general categories: antixenosis, antibiosis, and tolerance [18,19]. Antixenosis traits are aimed at decreasing preferences for feeding and/or ovipositioning, while antibiosis results from traits that negatively affect insect growth, survival, and/or fecundity. Lastly, tolerance is the ability of the host to withstand infestation while remaining relatively healthy compared to other individuals undergoing the same level of attack.
There is evidence of antixenotic traits in the interaction between A. planipennis and hosts. Adults of A. planipennis express variation in both feeding and oviposition host preferences. When given a choice, adult beetles preferentially feed on white, green, and black ash compared to Manchurian, blue, and European ash (F. excelsior L.) [20]. North American ash species receive more eggs compared to Manchurian ash, suggesting a female choice of susceptible hosts in order to increase larval performance [21,22]. Within North American ash species, inter- and intraspecific variation of volatile emissions and oviposition preferences of A. planipennis have been shown to play a role in resistance [23–25]. The bark of blue ash has a phenolic composition that may contribute to its resistance relative to white, green, and black ash [5]. Bark smoothness, as a phenotypic characteristic, may be a limiting factor in oviposition locations and subsequently limits the number of larvae that could attack a tree at a given time [26]. Additionally, variability in ash growth rates have been related to susceptibility to A. planipennis, with trees tolerant of attack having more rapid and constant growth compared to susceptible trees [27].
Antibiosis interactions also exist in larval development. Mechanisms that affect larval performance mainly focus on variation in phenolic and defense protein chemistry [5,28–30]. Previous studies comparing phenolic and lignin profiles of ash species found that Manchurian ash contains unique profiles that may contribute to their resistance to A. planipennis [5,28,30]. Four potential defense-related proteins are expressed more than five-fold higher in Manchurian ash than in other species, and may contribute to resistance [30].
Mechanisms of tolerance are more difficult to quantify and therefore have not been as well studied. Identifying the genetic variants that allow these surviving trees in North America to tolerate infestation would greatly aide in the conservation of ash [29]. Even with severe levels of ash mortality in the introduced range, certain trees have been able to survive after years of repeated exposure [26]. This has led to the identification of trees with differing apparent tolerance levels to A. planipennis attack. Trees classified as tolerant survive in spite of signs of A. planipennis attack and damage [26]. The objectives of this study were to (1) identify ash single nucleotide polymorphisms associated with the tolerance-susceptibility gradient to A. planipennis, (2) identify phenotypic and genotypic relationships between trees relative to this tolerance-susceptibility gradient, and (3) test the hypothesis that tolerance and susceptibility are linked to identifiable genetic markers.