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Photo credit: David Cappaert, Michigan State University, Bugwood.org

Emerald Ash Borer in Oakville, Ontario

Can we save our ash trees?

by Catherine Kavassalis


Downy Woodpecker
Source: Wild Bird Guide AP

On July 24, 2008, inspectors from the Canadian Food Inspection Agency (CFIA) confirmed the presence of the emerald ash borer (EAB) in the Town of Oakville, Ontario. Over the course of the summer of 2008, the beetle was discovered in many locations throughout this community of some 165,000 people and 176,000 ash trees (McNeil 2006).  Ash trees (Fraxinus spp.) including white, red and black ash, make up nearly 10% of Oakville's urban forest canopy. The estimated potential cost to the community is $86 million; the potential cost to the ecosystem is immeasurable.

So what do we know about this invasive beetle that has decimated tens of millions of ash trees across North America over the last decade? 

EAB (Agrilus planipennis Fairmaire) is native to East Asia. In most of its native range: Korea, China, Taiwan and Japan, it is considered a minor pest of predominantly ash, (Fraxinus spp.) but also walnuts (Juglans spp.), wingnuts (Pterocarya spp.) and elm (Ulmus spp.) (Haack, 2002).   Thus, though currently only attacking ash in North America, this borer also poses a potential threat to other genera (Anulewicz, 2008).   Within its native range, trees have evolved chemical defense mechanisms that inhibit feeding. In conjunction with predation, parasitism and disease, this keeps beetle populations in check. Though woodpeckers and predatory beetles actively feed on the borer here in North America,    they are insufficient in themselves and the population is exploding from Virginian to Ontario (Lindell. 2008; Cappaert. 2005). 

Adult beetles are about one centimeter long and a third as wide. They are hairless, metallic green with kidney shaped eyes (Fig A)(images source: Kimoto, 2004). While adults feed on ash leaves, the real damage is done by the burrowing larvae. Their typical one year life cycle begins with females laying 60 to 90 eggs singly in bark crevices from late May through July (McCullough, 2004).  Within 20 days, larvae emerge and bore into the sapwood, creating S-shaped galleries under the bark, (Fig B). They feed on the nutrient rich vascular tissue and grow through the summer (Fig. C), then rest for the winter. After pupating the following spring, adults emerge from the trunk (April to August) by cutting a characteristic D shaped exit hole (Fig. D). Over 100 beetles can emerge per square meter on an infested tree (McCullough, 2007).  

Early detection is difficult. Though foresters are now vigilant for the signs of this phloem-feeder, by the time exit holes, bark cracking over galleries, epicormic shoots, and canopy dieback are readily apparent (Fig.s D, E, F, G)(Kimoto 2004), EAB is generally well established in an area. From hi-tech aerial surveys that use chlorophyll-sensitive spectral analysis (Pontius 2008) to traps that lure adult beetles using the green leaf volatiles produced by ashes (de Groot 2008),  researchers are working to develop early detection tools. If identified soon enough, treatment may reduce the level of infestation and tree loss.

Currently, there are several approaches to stemming the tide of EAB. Quarantine has been the first line of defense. By prohibiting the movement of materials that might be contaminated with the pest and removing diseased wood, it was hoped the insect could be slowed if not stopped. When the first outbreak was discovered in the Windsor Ontario area in 2002, CFIA decided to create a wide ash free corridor and ordered the removal of more than 85,000 trees. Unfortunately, it did not prevent the beetle from spreading (Keiran 2004).  Capable of flying at least 10km, it is likely the pest had already escaped the quarantine zone before action was taken. While CFIA still carefully regulates infested areas, other treatments are being investigated and implemented.

In the U.S., a variety of pesticides (like imidacloprid and emamectin benzoiate) may be used as soil drenches, trunk injections or trunk and foliar sprays (Smitley, 2004; Usborne, 2008).  Systemics appear to provide the best control with the least non-target species impact. Currently only azadirachtin (TreeAzinR), a biosystemic pesticide derived from the Neem tree (Azadirachta indica), has been approved for use in Canada. Injected into the trunk or used as a soil drench, trials have shown that azadirachtin inhibits ash borer (McKenzie 2004; Helson, 2007). The Town of Oakville is planning to use trunk injections on a limited number of high value publically owned trees. Homeowners are advised to do the same. Injections are recommended yearly and are best done on sunny days in July or August (OMAFRA, 2008). It should be noted that yearly injections can potentially injure trees and are not 100% reliable (Cappaert, 2005). Some area companies are offering a lower cost Neem fertilizer soil drench, which may provide some degree of protection (Wilby, 2008).  

Research and field trials of other possible deterrents is ongoing. Various naturally occurring pathogens, bacterial and fungal, of A. planipennis are being investigated (Bauer, 2004). In addition, predators and parasitoids of EAB have been collected from East Asia, and are being tested. Early positive results suggest such predators may provide hope for the future (Gould, 2005; 2007). Meanwhile, botanists are busy trying to develop resistant hybrids (Rebek, 2008);  genetic engineers are contemplating transgenic ash trees (Du 2006),  and seed collectors are gathering ash samaras for preservation in storage banks like the National Center for Genetic Resources Preservation in Fort Collins, Colorado. 

In the short term, there will likely be considerable loss of ash here in Oakville and beyond. We can only hope that we can slow the progression of the beetle down so that natural deterrents can arise and research brings new solutions for communities yet to be invaded. If ash trees are to be saved as a species in North America, a concerted effort is required by all.

References