Determining the basis for emerald ash borer resistance
by Darla French, FNR Graduate Research Assistant
Ash trees (genus Fraxinus) are important to North America. There are approximately eight billion ash trees nationwide. Besides being fundamental aesthetic elements of nearly every city and suburban landscape, ash trees have high stumpage value and are an important commercial lumber species to the hardwood, furniture, and tool-making industries. In addition, ash trees serve as significant Native American cultural resources and play an integral part in the ecology of North America. It is difficult to determine the economic importance of ash trees because so much of their worth is from their social and cultural value rather than its market price; however, an estimated dollar value is close to $300 billion annually. Sadly, the ash-tree industry in North America is now in grave danger. Why? A tiny newcomer to the North American landscape: The emerald ash borer.
The emerald ash borer (EAB, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), is a jewel beetle native to Southeast Asia. In its native range, this metallic wood borer acts primarily as a secondary or opportunistic pest, mainly attacking weak or dying trees of two Asian ash species, Chinese and Manchurian ash. The life cycle of EAB includes larval, pupal, and adult stages, but it is the larval stage that is most damaging. After hatching, the larvae bore through the bark into the cambium layer, where they preferentially feed on phloem, the tissue layer located just inside the bark of the tree that conducts food from the leaves to the roots. This can lead to girdling (a circumferential discontinuity in the phloem), which starves the tree. The effects of girdling are not obvious until well after adults emerge, usually a few months later, but in some cases the following summer. By the time it is apparent that the trees are dying (usually three to five years after infestation), the adults have long since moved on to infest other trees, either by natural (short-range) or human-facilitated (often long-range) dispersal.
The emerald ash borer adults pictured above, were caught by the IDNR in Huntington County and transported to Huntington Nursery, where they were released on one pair of enclosed green ash trees for subsequent infestation. photo by Darla French
In North America, the EAB is considered an invasive, exotic species. It is thought to have been inadvertently introduced to this continent via shipping materials unloaded in Michigan, probably sometime in the early 1990s. Since its discovery in Detroit in 2002, it has spread as far east as Maryland, as far south as Kentucky, as far west as Wisconsin, and north into Canada. All North American ash trees, regardless of species, age, and health status, are vulnerable to EAB infestation. This insect seems to attack and subsequently kill North American ash trees indiscriminately; this is unusual in light of its behavior in Asia. We can conclude from this observation that our North American ash trees are somehow completely susceptible to EAB infestation, whereas those ash trees native to Asia are not. But what is the biological mechanism by which EAB results in death of its host, and why are our North American trees susceptible to this insect while the Asian species are resistant?
The EAB is a relatively new insect to North America, so there are very few published reports on this devastating beetle. A literature search revealed a total of about 30 published papers, a few of which are from China and the rest have been published since the discovery of the insect in Michigan in 2002. One might wonder at first about the relative lack of information on this insect even in its native range, but this really is not surprising. In contrast to the destruction it has caused in North America in just a few short years, in its native range EAB is not problematic. To date, most U.S. research has focused on efforts of controlling or curbing the spread of EAB, including mass trapping, chemical treatment of individual ash trees, detection methods such as test trapping, and various visual screening and monitoring methods. Biological controls, including parasitoid wasps, are also currently being investigated. So far, none of these approaches has been particularly successful. We currently lack the information about this insect and its effects on ashes that we need to effectively control it.
In the photo above cages were removed in the fall of 2009 from each pair of trees. The trees were harvested, and trunk sections were frozen within 60 seconds of harvest. In the photo above with the tree cage, Darla French screens branches of one of the infested trees for EAB larvae. photo by FNR/Associate Professor Rick Meilan
Through my research, I am attempting to determine why North American ash species are so markedly different from their Asian counterparts with respect to EAB susceptibility. The main objective of my work is to compare the metabolomic profiles of North American and Asian ash species. If we can determine that the difference is due to compounds produced by one species but not by another, we might be able to exploit those differences to engineer ash trees with elevated resistance to EAB. Seedlings of susceptible North American ash species (white, green, and black) are being grown in a greenhouse alongside resistant Asian ash species (Chinese and Manchurian) and two European ash species that have unknown resistance levels. Seedlings will be challenged with the compound methyl jasmonate, via a root drench, to simulate insect feeding; some will be left untreated to serve as controls. At various intervals post-treatment, a range of plant tissues will be sampled and subjected to chemical analyses. Resulting chemical compound profiles for each species group will be compared in an attempt to identify one or more compounds that may be responsible for resistance in the Asian ash trees.
In photo to the right, Vince Burkle (Indiana DNR) releases emerald ash borer adults on one pair of enclosed green ash trees, while Darla French (Purdue University, Graduate Research Assistant) looks on. photo by Rick Meilan
A second objective of my research is to compare the transcriptomic profiles of infested and uninfested green ash trees via high-throughput sequencing in an attempt to determine which genes are turned on or turned off in response to EAB feeding. Green ash trees were planted in cages on property owned by Huntington Nursery (Huntington, IN) in summer 2009. In collaboration with the Indiana DNR, one pair of trees was subjected to infestation by introducing EABs into the cage in which these trees were housed; the cage surrounding the control trees protected them from infestation. The trees were harvested in early fall 2009. Messenger RNA was isolated from phloem of each tree and submitted to the Purdue Genomics Facility for subsequent high-throughput sequencing. The resulting transcriptome profiles are being compared to identify genes that show an altered expression pattern in response to EAB infestation. Once candidate genes have been identified, a set of molecular experiments will be conducted to investigate the function of various genes in the jasmonate pathway, which mediates a plant’s response to insect attack, in tissue harvested from methyl jasmonate-challenged seedlings.
This photo shows Vince Burkle (IDNR) screening last season’s EAB infested logs, used as a source for EAB adults to use in our study, for larval galleries and adult exit holes. A gallery can be seen on the peeled log near Vince’s left foot. photo by Darla French
For more information on EAB view Purdue Extension Emerald Ash Borer. For more ground-breaking research articles see FNR Compass 2010. Darla French is studying under Associate Professor Rick Meilan.