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Robertson & Andow - 1

WORKING PAPERHuman-mediated dispersal of emerald ash borer:

Significance of the firewood pathway

DESIREE R. ROBERTSON1 & DAVID A. ANDOW2

University of Minnesota, Department of Entomology,1980 Folwell Ave., Room 219, St. Paul, Minnesota 55108, USA.

e-mail: [email protected]; [email protected]

IntroductionInvasive alien species can have devastating consequences on native biodiversity, ecosystem andevolutionary processes (Lovett et al. 2006, McNeely 2001, Mooney and Cleland 2001). In the UnitedStates alone, economic costs associated with environmental damages and losses as a result of invasivespecies have been estimated to total between $120-137 billion per year (Pimentel et al. 2005, Pimentelet al. 2000). Forest products are particularly affected with losses attributed to exotic insects andpathogens reaching approximately $4.2 billion per year in the U.S. (Pimentel et al. 2005). One exoticinsect currently threatening North American forests is the emerald ash borer, Agrilus planipennisFairmaire (1888) (Coleoptera: Buprestidae) (Fig. 1), a wood-boring beetle native to Asia that infestsash trees (Fraxinus sp.). First discovered in Michigan in 2002, emerald ash borer (EAB) has sincespread to Indiana, Illinois, Kentucky, Maryland, Missouri, Minnesota, New York, Ohio, Ontario,Pennsylvania, Quebec, West Virginia, Virginia, and Wisconsin (www.emeraldashborer.info).

Potential costsassociated with theinvasion of EAB areextremely high. Lossesof infested urban ashtrees in United Stateshave been projected torange from $20-60billion, excludingreplacement costs(Federal Register2003), althoughadditionalconsiderations suggestthat this numberunderestimates the truecosts. In Ohio alone,total costs to urbancommunities includelandscape loss, treeremoval, andreplacement costs,which have been estimated at $7.6 billion, or $665,000 per 1000 residents (Sydnor et al. 2007).Additionally, forest ash species are commercially important for a number of wood products such as

Figure 1. Emerald ash borer adult and insects commonly mistaken for EAB.


baseball bats, cabinets, furniture, crating, cardboard, and paper (Cappaert et al. 2005, Poland andMcCullough 2006) and loss of ash saw timber nationwide based on stumpage value has been estimatedat $25.1 billion (Federal Register 2003). The total undiscounted compensatory value of the over 7.5billion ash trees estimated to occur in U.S. forests has been valued at over $280 billion (FederalRegister 2003). Although not yet quantified, the ecological costs related to EAB invasions could alsobe severe: ash species are major components in a variety of forest types in the Upper Midwest andeastern half of United States (Burns R.M. and Honkala 1990, Eyre 1980) and play an important roleproviding food and/or shelter for a variety of wildlife (Cappaert et al. 2005, Poland and McCullough2006). Similarly, while difficult to assess in monetary terms, ash trees have certain aesthetic value,the loss of which could negatively impact forest-based recreation. Finally, EAB threatens the culturalheritage of some indigenous communities of the Great Lakes region who utilize ash species in theconstruction of traditional baskets, snowshoes, hunting and fishing decoys, and canoe paddles (Reo2005).

Emerald ash borer was likely first transported to the United States in crating, pallets, or othersolid wood packaging material (Cappaert et al. 2005). Since its discovery in 2002, federal and/or statequarantines have been established where it has been detected to regulate the movement of live ashsaplings and trees, wood packaging material, ash logs, lumber, stumps, branches, wood chips, and allhardwood firewood. In this paper, we present the evidence that firewood may now serve as theprimary mode for new infestations of EAB, describe the specific pathways that firewood may take andthe actors involved in moving it, and review evidence suggesting human behaviors that may increaseor decrease the risk of spreading emerald ash borer via firewood.

Emerald ash borer life history and pathology (Figs. 2, 3).

In North America, emerald ash borers feed exclusively on ash trees (Anulewicz et al. 2008).Although adults feed on foliage, it is the larval stage, which feeds on the inner bark (phloem) orvascular cambium of ash trees, that inflicts the greatest damage (Cappaert et al. 2005, Haack et al.2002, Poland and McCullough 2006). High density larval feeding can disrupt nutrient and watertranslocation, girdling the tree and eventually causing tree death (Poland and McCullough 2006).

Movement of ash wood that harbors living EAB larvae, pupae, or young adults contributes tothe human-mediated spread of EAB. Larvae are known to occur in stems and branches as small as 2cm in diameter and mature trees with diameters of 1.5 m or greater (Cappaert et al. 2005, Haack et al.2002). Moving live saplings and trees, such as nursery stock can certainly spread EAB, but so candead wood that has been harvested from infested trees. Early instar larvae live just underneath barkwhile late instars excavate pupal cells about 1.5 cm deep into the sapwood (xylem) or outer bark(Cappaert et al. 2005, Haack et al. 2002, Poland and McCullough 2006). Consequently, woodcontaining bark and the first 1 inch of sapwood poses the greatest risk for spreading EAB. Yet earlylarval stages require a living host to complete development (Petrice and Haack 2006). Once logs arecut, the moisture content and nutritional quality of the phloem gradually decreases (Haack and SlanskyJr. 1987), thereby reducing larval survival (Petrice and Haack 2006). However, if wood is cut lateduring immature development, the survival probability increases and EAB are likely to completedevelopment and emerge as a viable adult (Petrice and Haack 2006). Furthermore, for populationswith multiyear development, EAB can emerge from logs for two years after infested trees have beencut (Petrice and Haack 2007).


Pathways involved in the spread of EABNatural dispersal is relatively short. Emerald ash borer is known to disperse locally via short-

distance flight (Bauer et al. 2003, Fraser et al. 2007, Haack and Petrice 2003a, Taylor et al. 2004,Taylor et al. 2007). EAB adults fly in 8 to 12 meter bursts (Yu 1992) but are believed to be capable offlying distances of more than 1 km (Haack et al. 2002). Laboratory studies using flight millsdemonstrated that gravid females can fly 1.7 km in 24 hours (Taylor et al. 2004), with a totalmaximum lifetime distance of 9.8 km flown over 4 days (Taylor et al. 2007). These studies suggestthat natural dispersal results in local spread of a few kilometers, and do not explain “jump” dispersal ofmuch larger distances (BenDor and Metcalf 2006, BenDor et al. 2006).

Figure 2. Emerald ash borer biology. A: Adults emerge from ash trees from early May through the end of Augustleaving 3-4mm width D-shaped exit holes (Brown-Rytlewski and Wilson 2005, Haack et al. 2002). Afterapproximately 5-7 days of extensive feeding on ash foliage, mating begins (Poland and McCullough 2006). B: Femaleslay approximately 50-90 eggs during their lifetime (3-6 weeks), deposited individually in bark crevices along the trunkand lower portions of major branches (Haack et al. 2002, Poland and McCullough 2006). C: Eggs hatch within 2weeks at which time larvae begin feeding on the inner bark (=phloem or vascular cambium) of ash trees from mid-summer through autumn, leaving serpentine-shaped galleries packed with frass (Cappaert et al. 2005, Haack et al. 2002,Poland and McCullough 2006). D: After 4 instars, completed in October or November, most EAB overwinter asprepupae in cells excavated about 1 cm deep into the sapwood (=xylem) or outer bark (Cappaert et al. 2005, Haack etal. 2002, Poland and McCullough 2006). E: Pupation occurs the following April thru May, with adult emergence 3weeks later (Cappaert et al. 2005, Poland and McCullough 2006). In low-density populations or colder regions,multiyear development may occur with larvae overwintering in the cambial region and then completing developmentthe following summer (Cappaert et al. 2005, Haack et al. 2002, Poland and McCullough 2006).


Figure 3. Infested ash trees along a boulevard in Minnesota. Signsand symptoms of infestation are difficult to detect during the firstyear, but in later stages, tree canopy dieback, epicormic sprouts alongthe main trunck, and bark cracks over larval galleries may be visible(Cappaert et al. 2005, Haack et al. 2002).

Most newly infested ashpopulations are most likely theresult of long-distance “jump”dispersal. Hence it is concluded thatEAB movement must be associatedwith human movement of EABinfested materials such as ashfirewood, nursery stock, saw timber,or wood packaging material(BenDor and Metcalf 2006, BenDoret al. 2006, Iverson et al. 2008,Muirhead et al. 2006). In one studythe probability of new infestationwas positively related to the size ofhuman population centers implyingthat human activities are a majorfactor in the spread of EAB(Muirhead et al. 2006). Similarly,dispersal modeling showed thenatural rate of EAB spread was tooslow to account for the observedspread of EAB (BenDor andMetcalf 2006, BenDor et al. 2006).Although wood packagingmaterials, ash nursery stock, andsaw-timber may have contributed tothe spread of EAB in the past, theirrelative importance may have beendiminished by increased complianceand enforcement of regulations,shifting market demands of certainash products, and the availability oftreatment options.

Wood packaging material (dunnage, pallets, crates), which are typically constructed of low-grade lumber and used in shipping worldwide, are the suspected source for the original introduction ofEAB to North America (Cappaert et al. 2005, Haack et al. 2002, Poland and McCullough 2006), andmany other invasive pests (Doyon et al. 2004, Haack 2006, Haack and Petrice 2009). There arecurrently regulations for wooden pallets that are manufactured in or shipped through quarantined areasor internationally (Federal Register 2003, IPPC 2002). However, recent studies have found that eventhe international standard ISPM 15 heat treatment regimen may not be effective against EAB(McCullough et al. 2007). While there is no evidence yet that pallets have contributed to the furtherspread of EAB in North America, the nationwide adoption of stronger regulations for wood packagingmay reduce the risk of such occurrences.

Increased enforcement after early violations involving ash nursery stock, the authority toinspect and certify nurseries and a decrease in supply of ash stock may lessen the importance ofnursery stock as a pathway for spreading EAB. Federal regulations completely prohibit the movementof ash nursery stock from quarantined areas (USDA–APHIS 2008a). Nonetheless, ash nursery stock


from Michigan was identified as the source of new infestations in Maryland and Virginia (VA–DF2007, MD–DA 2004). However, such occurrences were early in the quarantine process (April of 2003)and the Michigan grower responsible for the violation was prosecuted and heavily fined (LM 2004).The nursery industry is highly regulated at the state level and most states perform routine inspectionsof nurseries at least annually to ensure stock is free of plant pests. In addition, ash nursery stock hasprobably declined. The first 6 counties quarantined for EAB in Michigan, experienced a loss of over$2 million in annual nursery stock sales during the first year of regulation (Federal Register 2003).Indeed, as a result of EAB infestation the industry providing ash nursery stock has collapsed in Ohio,Indiana, and Michigan (Bell 2005) and in regions not yet infested, many nurseries are participating involuntary moratoria on the importation of ash stock (IA–DNR 2008). Furthermore, it has beenreported that many nurseries have cut back on planting ash or adjusted their business practices becauseof potential restrictions and the declining demand for ash (Brewer 2004).

The availability of treatment options to eliminate EAB and the enforcement of regulatoryactions may have lessened the importance of saw-timber movement as a pathway for EAB. Fullyprocessed and treated high grade lumber, such as dimensional boards of the quality typically used inconstruction, generally comes from the inner portions of logs and probably does not present much riskfor spreading EAB. Compliance agreements with mills inside EAB quarantined areas require ash logsbe either fully debarked with an additional 1/2 inch of sapwood removed, fumigated, heat treated, orkiln dried before they can be transported from quarantined areas (USDA–APHIS 2008a). However,raw ash timber (logs that have not undergone any processing and have bark) hauled to sawmills couldharbor EAB life stages. Indeed, an outlier infestation in Indiana was attributed to the transport of rawtimber logs to a sawmill from infested areas in Michigan before quarantines were in place (Kilbane2004, PU 2004). Compliance agreements are required to transport raw ash timber logs from aquarantined area to a non-quarantined area. These agreements provide that logs are 1) moved to afacility approved under an appropriate USDA compliance agreement, and 2) are not moved during thepotential EAB emergence period (April 1 to September 30) (Federal Register 2003). It has beendemonstrated recently that portable debarking machines can eliminate most EAB larval and pupalstages that occur in raw logs (D. McCullough, Michigan State University, East Lansing, personalcommunication, 27 January 2009). Field debarking could potentially allow logs to be safely moved toother locations with minimal risk of EAB spread.

In contrast to the pathways above, firewood remains an important dispersal pathway for EAB.Firewood moves through many formal and informal pathways, making it difficult to identify, track,and regulate. Unlike pallet, nursery, and timber industries, there is no national organization forfirewood producers and dealers and consequently no representative or central clearinghouse for theindustry to turn to for regulatory information. Firewood consumers may not be as well informed aboutinvasive species associated with firewood and may be less likely to be concerned with purchasingproducts that are certified pest-free. Additionally, firewood use is still quite widespread and demandhas even increased in recent years due to rising oil prices (Sharp 2008) and a crashing economy.Moreover, although sawmills, timber companies, and pallet manufacturers are likely to be able toinvest in costly EAB treatments, these options may be impractical or unaffordable for many firewoodproducers, especially smaller businesses (Seeland 2007).

Another factor making firewood an important pathway is the ability of EAB to survive infirewood under varying conditions. Haack and Petrice (2003b) tested the survivability of EAB infirewood stored in direct sunlight versus shade and concluded that EAB were able to survive andemerge from all treatment combinations. Another study found that no single treatment (splitting offirewood, storage condition) was 100% effective in preventing adult emergence from firewoodalthough that adult emergence densities were significantly lower if logs were cut early during larval


development or split and stored untarped (Petrice and Haack 2006). Furthermore, it has beendemonstrated that EAB adults can emerge from firewood harvested from infested trees 2 summersafter it is cut (Petrice and Haack 2007)

Firewood movement and new infestationsFirewood is frequently transported long-distances from where it is harvested (Buck 2008). An

individual could potentially move firewood several hundred miles from a quarantined area in just oneday, and there have been several instances where firewood with current or prior EAB infestation hasbeen intercepted outside of quarantined areas. For example, park rangers inspected firewood arrivingat Michigan state parks associated with 8,096 campers. Of these, 1,654 campers were found to havecome from EAB quarantined areas and 114 campers possessed ash firewood with signs of infestation(State of Michigan 2006). Also, in an April 2008 survey of firewood confiscated from people travelingover the Mackinac Bridge in Michigan, 11 of 186 pieces of wood were found to have prior EABinfestation (Haack et al. 2008). In Canada, the Canadian Food Inspection Agency fined four campers$400 each for transporting firewood outside of EAB regulated areas, some of which was infested withEAB (CFIA 2004). In 2005, EAB larvae were found living in bundled stacks of ash firewood on thepremises of a commercial firewood dealer located in an area regulated for EAB (USDA APHIS EABIncident Report 2006). Prior to detection of EAB, the firewood had been shipped to distribution hubsin 6 different states, potentially spreading EAB.

There are also cases where confirmed new infestations of EAB have been linked to thetransport of firewood. Based on personal observations by Animal and Plant Health Inspection Service,Plant Protection and Quarantine (APHIS PPQ) EAB regulatory staff, many of the infestation epicentersoccur in parks & campgrounds in IN, MI, MO, OH, & WV. These epicenters have been assumed to bethe result of firewood transport by campers because of their association with campsites (Buck 2008).In Michigan, 78% of outlier sites with known origins were believed to be the result of firewoodmovement because of their close proximity to state parks, recreation areas, lakes withcampgrounds and/or cottage communities (M. Philip, Michigan Department of Agriculture,Lansing, personal communication, 18 September 2008). In another example, although an outlierinfestation near a sawmill in Indiana was attributed to the transport of timber logs from infested areasin Michigan, the infestation likely spread from the mill in pieces of slab wood, outer layers containingbark (Kilbane 2004). Slab wood is often discarded in piles near sawmills where it is picked up as aninexpensive alternative to cord firewood.

A new infestation of EAB in Tipton, MI was determined to be the direct result of infestedfirewood (McCullough et al. 2003). Infested firewood from southeastern Michigan had been piledalong a drainage ditch in the spring of 2002. The following year, EAB galleries were found in treesgrowing in the vicinity of the firewood pile and more than 70% of EAB galleries occurred on treesgrowing within 100 meters of the pile (McCullough et al. 2003).

General firewood pathway (Fig. 4)The general firewood pathway comprises four groups of people who handle firewood (=actors):

1) producers, who initially harvest and transport firewood to 2) distributors, who package firewoodand then ship it to 3) retailers, who sell it to 4) end users, who move firewood to its end-point locationwhere it is burned.

Two pathways can be identified: a formal firewood economy and an informal firewoodeconomy (Fig. 4). These 2 pathways are not exclusive – actors in each may interact with one anotherresulting in the movement of firewood between the 2 pathways – and the four roles may be blurred assome actors operate simultaneously as producers, distributors, retailers, and even end-users.


The potential distances traveled by the various actors in the different pathways can be great.While much attention has focused on the end-user, in many cases, the responsible pieces of firewoodhave been traveling for an extended period of time over long distances before even reaching the enduser. For example, in the formal economy firewood could be harvested in Ohio, shipped to adistributor hub in Illinois, and then distributed to a retail outlet in Wisconsin where it is finallypurchased. In the informal sector, firewood might be harvested in a rural area but brought into ametropolitan region to be sold door to door.

To reduce the risk of invasive species spread through firewood movement, it is important toidentify the types of movement that truly are the highest risks. Some of the more important aspects toconsider in evaluating risks of these different kinds of movement are the geography and economics ofvarious aspects of the industry, business practices, and human behavior.

The formal firewood economy (Fig. 4)The formal firewood economy is comprised of the largest companies involved in handling

firewood and has the greatest geographic reach. Producers harvest and process logs and then sell thefirewood to national distributors who package it for wholesale distribution to national and regional“big box” stores (e.g., Home Depot, Menards) and other chain stores. Because of the large scale ofsuch operations, actors in the formal firewood economy are usually easily identified and thus easier totarget for regulation. This ability to subject and enforce regulations perhaps mitigates somewhat thepotential spread of EAB and other invasives through this pathway. However, the long distances and

Figure 4. Firewood pathway. Formal pathway indicated by solid line; informal pathway indicated by dashed line.P=Producers of firewood (including harvesters); D=Distributors and brokers; R=firewood retailers; E=firewoodend-users.


large quantities of firewood moved through the formal pathway mitigates against complacency andunderscores its potential importance in spreading invasive species.

Producers: Large commercial enterprises such as large logging operations, land clearingcompanies, and sawmills are usually the main producers of firewood in the formal firewood economy.Some of these enterprises may be located in areas under state or federal regulation or under quarantineand may sell firewood to distributors located 100s to 1000s of miles away. For example, a landclearing company located in Ohio and regulated for EAB, sold logs infested with live EAB larva to anIndiana supplier approximately 70 miles away. These logs were then packaged and sent to distributorcenters in Indiana, Ohio, Illinois, North Carolina, Michigan, and Wisconsin. On average, thedistributor hubs were roughly 250 miles away from the origin of the wood in Ohio, and there was atotal of 1,489 miles of transport route from the supplier to the distributor centers (calculated fromUSDA APHIS PPQ EAB 2005 incident report).

Distributors: Brokers that package and sell firewood to retailers are distributors. Packaging mayinvolve the binding of firewood pieces into fully enclosed shrink-wrapped bundles (Buck 2008).Firewood is sometimes “seasoned” implying it has been dried or aged for enhanced burning, or“treated” for pests, either by heating, spraying with insecticides, and/or removing bark. While heattreatment, fumigation, and/or kiln drying per USDA specifications reduces the risk of EAB spread,seasoning and/or subsequent packaging does not ensure firewood is pest-free. Emerald ash borer canemerge from firewood after 2 years (Petrice and Haack 2007) and USDA regulatory staff haveobserved emergence of EAB from shrink-wrapped firewood bundles (Buck 2008).

There are sometimes several distributors involved before the firewood reaches a retail location.Larger producers (harvesters) may sell to a national distributor who may either distribute firewooddirectly to retail outlets or to the retailer’s own distribution centers. As already noted, the distributioncenters themselves are often several hundred miles from the origin of the wood. The addition of asecondary distributor increases the distance that firewood is transported. Large national distributorsoften cover several thousand miles along their transport route to retail distribution centers (Buck 2008).The distribution centers in turn service many different stores in several states, adding additionalmileage. For example, one national distributor shipped pallets of firewood to 42 “big box” distributioncenters in 35 states, with an average distance of about 250 miles to the distribution centers (Buck2008). In another example, a Michigan regional distribution center shipped palletized firewoodbundles to 67 different stores in 4 different states, for an average of approximately 100 miles to eachstore (Buck 2008).

Retailers: In the formal firewood economy, retailers include “big-box” stores, grocery stores andsupermarkets, gas stations, and convenience stores. Firewood sold in such stores is often packaged andmarketed as “fast-starting”, “long-burning”, or “premium” and tends to cost more than woodpurchased from other sources (at $5/bundle and bundle = 0.9 ft3, bundled firewood costs $5.56/ ft3; at$200/cord and cord = 128 ft3, bulk firewood costs $1.56/ft3). Consumers purchase this firewood forconvenience, and most likely in small quantities for immediate short-term uses at home and inrecreational settings, such as picnicking and camping.

Large retailers often receive huge shipments of bundled firewood, often without much knowledgeof its origin. Federal regulations require firewood originating from quarantined areas to be affixedwith the federal identification on the package label or invoice, certifying that the wood has gonethrough a process to reduce the risk of carrying a regulated pest as it moves out of a quarantined area.Despite this, there have been several instances where firewood has been sold at retail outlets in


violation of these regulations. In 2005, 51 units of firewood originating from EAB quarantined areaswere sold into commerce at 19 retailers in Illinois, Indiana, Michigan, Ohio, and Wisconsin, before arecall and stop sale order took effect (USDA APHIS PPQ EAB 2006 incident report). In Minnesota,APHIS PPQ inspections found a large retailer selling firewood originating from EAB infested areas ofNew York in violation of federal regulations (USDA APHIS PPQ 2006 MN Firewood DealerInspections).

Compliance by retailers appears to be straightforward. Retailers must not buy firewood unless theyknow its origin and only accept firewood from a quarantined area if it has the proper federal shield.Additionally, a list of approved firewood producers and distribution centers in compliance with federalregulations can be accessed through the USDA APHIS website. However, retailers have had littleexperience tracking, confirming and documenting the source of firewood, and there is littleinformation regarding the attitudes of retailers regarding regulation. When it was discovered thatfirewood was being sold at a chain of home improvement stores in Wisconsin in violation of an EABquarantine, APHIS ordered a recall of the wood within 24 hours (Egan 2007). However, a week latergovernment inspectors found some of the firewood still offered for sale in some of the stores (Egan2007).

One potential issue is that retailers may assume firewood they obtain through their vendors ordistributors is in compliance with regulations and therefore may not be as concerned or even aware ofthe details of the regulations. Similarly, end-users purchasing firewood through this recognized“formal” route may also assume that the firewood is pest-free, and therefore may have no reservationsabout transporting it some distance. This underscores the importance of regular compliancemonitoring by regulatory personnel to ensure that the firewood being delivered to and sold at retailestablishments is in compliance.

The informal firewood economy (Fig. 4)Although much regulatory activity has focused on the various actors in the formal economy,

the informal firewood economy may be just as responsible, if not more, for the transport of firewoodand resulting spread of EAB. The informal firewood economy is much more complex than the formaleconomy, and its actors are less easily identified and their roles much less distinct. Generally, themiddleman or distributors are eliminated, with producers selling firewood to small retailers or directlyto end-users.

Producers:Small regional suppliers. Small-scale suppliers may sell firewood wholesale or directly to

end-users. Some interact with the “formal” economy by selling to smaller retail stores or buying woodfrom larger companies. They include small regional companies selling wholesale to local stores,single-family operated businesses, and weekend dealers selling firewood at the end of their driveways,door-to-door, or at roadside stands (discussed further below as retailers). These producers may harvestfirewood themselves from their own woodlots, or through stumpage or cutting rights from landowners.Some suppliers may also obtain scraps from timber sales, lumber mills, tree care companies, or recycleused wood pallets. Except for wood pallets whose origins may be unknown, regional suppliers usuallyharvest and sell firewood within a 200-mile radius and deal with quantities less than 1000 cords(calculated from (USDA APHIS PPQ 2006 MN Firewood Dealer Inspections).

Although smaller suppliers tend to transport firewood shorter distances and in smallerquantities than their larger counterparts in the formal economy, certain characteristics increase the riskof spread. Many of these businesses are short-lived, and may not be officially registered as a business.These circumstances make such suppliers difficult to identify and contact, making regulation difficult


to enforce. As small businesses, they may have fewer financial resources available to treat firewood toreduce survivability of pests and this perhaps leads to negative attitudes towards regulations offirewood in general. In a survey of Pennsylvania firewood dealers, only 4% said their customerswould be willing to pay more for treated firewood, even if they perceived such treatment meant betterprotection against invasive species (Lowenstein et al. 2008). The majority of Pennsylvania dealerssurveyed also rated the feasibility of using treatment options such as heating, bark removal, orfumigation as highly unlikely (Lowenstein et al. 2008). Similarly, in a Minnesota survey, firewooddealers responded overwhelmingly against requiring state licensing for selling firewood and a majoritydid not support regulations requiring kiln drying (Seeland 2007).

Smaller suppliers may not be as aware or as concerned about the risks of transporting firewoodand associated pests. In Pennsylvania, 16% of firewood dealers surveyed reported that they were notconcerned at all regarding the threat posed to US forests by invasive species moved in firewood(Lowenstein et al. 2008). Smaller suppliers may view the threat of invasive species as a problemmostly originating outside of state borders, rather than within. This is evidenced by support ofregulations requiring specific storage and seasoning conditions and time for firewood originatingoutside of Minnesota but a lack of support for those same regulations for firewood originating withinthe state (Seeland 2007). Nonetheless, it is common for smaller dealers to cross into nearby states toobtain wood. In Minnesota, many dealers working along the eastern portion of the state get their woodfrom Wisconsin (USDA APHIS PPQ 2006 MN Firewood Dealer Inspections). EAB has been detectedin western Wisconsin (Bergquist 2009) and firewood from this area is a potential source of newinfestations in Minnesota.

Major demand centers for small scale dealers tend to occur in metropolitan areas. For example,the Twin Cities metro area of Minneapolis-Saint Paul burned 22% of firewood consumed statewide in2008 (Barzen et al. 2009). Although residents in rural areas consumed a larger total volume offirewood than metro residents, most firewood is harvested in rural areas. Accordingly, urban andsuburban centers are major importers of firewood whereas rural areas are major exporters. Therefore,this is a potentially important route of transport of EAB infested firewood into new cities from ruralareas.

Tree care workers. Tree care specialists and arborists are an important source of firewood inthe informal economy by providing firewood to small dealers or directly to end-users. Sixty percent offirewood dealers surveyed in Pennsylvania indicated their firewood originated from tree servicecompanies (Lowenstein et al. 2008). Trees in urban settings are typically subjected to harsh growingconditions, making them more vulnerable to pest infestations and diseases (Ward and Johnson 2007).Tree care workers are likely to be the first to encounter infested or diseased trees as part of their normalwork. Felled trees may be left for the property owner for use as firewood or removed and sold directlyto other end-users or firewood dealers. Because firewood originating from an infested ash is likely toharbor EAB, the transportation of this firewood out of metropolitan areas may be a significant route ofspread. However, tree care professionals may be more aware of invasive species issues, receivespecialized training in dealing with pests, and thus take greater precautions to prevent the spread ofEAB. Be that as it may, an owner of tree care company in Michigan was found guilty of illegallytransporting firewood out of a quarantined area (MI-DA 2004).

Retailers:Retailers in the informal firewood economy may include single-family operated businesses and

small-volume weekend dealers selling firewood at the end of their driveways, door-to-door, or atroadside stands. Twenty-one percent of people surveyed by The Nature Conservancy indicated theypurchased firewood from door-to-door dealers, while 13% said they bought from roadside stands (TNC


2007). These dealers may travel to neighborhoods in metropolitan areas to sell their wood, oftentransporting firewood by the truckload. Many small retailers may only be identified by yard signs,posted flyers, or newspaper and internet ads (Buck 2008). Additionally, some businesses are seasonal,temporary, and not registered or licensed in any way. The transient nature of, and the inability toreadily identify small dealers make them difficult to contact and therefore regulate (Buck 2008).Another issue is that small dealers may not be concerned that firewood movement could spreadinvasive species, as discussed above. The recent rise in oil prices during the summer 2008 may havestimulated more activity by firewood retailers (Sharp 2008). Nationwide economic hardship due to the2008 recession may be another factor encouraging entry into firewood sales.

End-users:End-users are the people who burn firewood. Firewood movement associated with activities of

end-users can result in new infestations of EAB (Bratkovich et al. 2008, Buck 2008). Almost half offirewood users may move firewood at least occasionally (TNC 2007) to burn at vacation homes, whilecamping, hunting, or attending large spectator events (Buck 2008). Firewood may also be transportedwhen someone moves to another home. For instance, the APHIS PPQ EAB program documented onecase involving a person transporting an entire household, including firewood, from Michigan toIndiana (Buck 2008). Firewood end-users may not be aware or may not consider forest pests andpathogens to be much of a serious threat. For instance, in one survey of firewood users, only 25% ofrespondents felt that non-native forest insects and diseases was a serious threat to local trees, comparedto 30% who thought the threat was somewhat serious and 31% who thought it was not serious (TNC2007). Additionally, end-user movements are not tracked or regulated. Hence, it is important toidentify the specific activities associated with using firewood and characterize the likely patterns offirewood use and movement.

Firewood is most commonly used at home in fireplaces, wood-burning stoves, or backyard firepits (TNC 2007). Many people use firewood as a home heating source (33% of those surveyed inMinnesota), but most people burn it for pleasure during recreational activities (63%) (Barzen et al.2009). However, with the exception of metropolitan areas, home heating still accounts for the largestvolume of firewood consumption (Barzen et al. 2009). Firewood consumption and the number ofhouseholds burning firewood for heat has been linked to the cost and availability of fossil fuels(Barzen et al. 2009). For example, the recent rise in oil prices during the summer of 2008, increaseddemand for firewood as households sought out cheaper, alternative fuel to heat their home (Sharp2008). As firewood supplies dwindle, dealers may be forced to sell lower quality green firewoodwhich has an increased likelihood of harboring live EAB larvae. Also, dealers may have to travelfarther to obtain firewood, increasing the potential distance EAB is moved.

People who purchase firewood for home use generally buy it in bulk (half a cord or more), andat least once a year (TNC 2007). However, many people also harvest their own firewood. Nearly halfof residents surveyed from the Northeast and upper Midwest said they cut their own firewood (TNC2007). Similarly, 58% of Minnesota households reportedly harvest their own firewood (Barzen et al.2009). End-users may harvest firewood on public property, such as a national or state forest, or onprivate land. Private landowners may cut down dead or dying ash trees for use as firewood. Thelandowner is likely to stockpile the firewood at the home residence, but may also share some firewoodwith neighbors and relatives. In Minnesota, 10% of households reported that they obtained firewoodfor free, as a gift, or had firewood leftover from previous years (Barzen et al. 2009). Similarly, asurvey of Wisconsin campers found that 10% received firewood from friends or relatives (Peterson andNelson 2007).


End-users who purchase in bulk or harvest their own firewood may be more likely to transportit to another location. Instead of purchasing small quantities of firewood for a recreational trip, theymay choose the option of taking firewood from their home supply. In fact, 42% of people who cutfirewood themselves, reportedly moved it from one location to another (TNC 2007). Similarly, 24% ofvisitors to Wisconsin state parks brought firewood that they cut themselves (Peterson and Nelson2007). End-users harvesting firewood may have little awareness of invasive species issues andprobably are not trained to recognize signs and symptoms of pests or pathogens. Additionally,firewood harvesting by end-users is not well monitored and is difficult to regulate. Accordingly,firewood movement by end-users may be a particularly important route of EAB spread.

Campers. Camping is one recreational activity that has come under particular scrutiny as aroute of spread of EAB. Campfires remain a crucial aspect of the camping experience, serving ascenters of conversation and sociability for campers (Reid and Marion 2005). A survey of visitors toWisconsin state parks found that 89% of campers said that campfires were extremely important, and93% had a campfire on their most recent trip (Peterson and Nelson 2007). As summarized by Reid andMarion (2005), campers may be willing to accept some degree of campfire-related forest damage (e.g.,litter-filled fire rings, charred rocks and tree roots) based in part on the importance of campfires to theircamping experience. Studies have also found that campers are strongly opposed to campfirerestrictions and may even be willing to violate regulations to have a campfire (Reid and Marion 2005).

Approximately 14,500 cords of firewood were burned while camping in Minnesota from 2007-2008 (Barzen et al. 2009). In a survey of state park visitors to Minnesota, firewood was the 3rd mostfrequent item brought to the park after tents and lawn furniture (MN–DA 2005). People who havefirewood stockpiled at their home residence may be more inclined to raid their personal stock forrecreational trips. In Wisconsin, 27% of state park campers moved firewood in bulk to and from theirhomes, more than 3 times a year, and at an average distance of 55 miles (Peterson and Nelson 2007). Asurvey in Minnesota found that 65% of campers brought firewood from outside the park for a previouscamping trip (Hanson and Albers 2006). In another survey of visitors to Minnesota state parks, 41%reported they brought firewood from their home supply (MN–DA 2005). However, when the surveywas repeated 3 years later after the implementation of new regulations requiring that firewood broughtinto a park be from an approved vendor, only 18% reported bringing firewood from home (MN–DA2008). It is not clear if the reported decrease represents an actual change in camper behavior, or if therespondents were avoiding incriminating themselves as the surveys were conducted at the parks(MN–DA 2008).

Campers are motivated to bring firewood to their campsites for a variety of reasons. As alreadymentioned, those with a home supply of firewood, are more likely to transport it to another location,but there are many possible reasons. The convenience of the home supply may be an important factor.Visitors to Wisconsin state parks indicated they brought firewood because they felt the wood soldinside the parks was priced too high (Peterson and Nelson 2007). There were also concerns regardingthe quality, supply, and availability of firewood sold at parks (Peterson and Nelson 2007). Somecampers indicated they had firewood leftover from a previous camping trip (Peterson and Nelson2007). Similarly, 3% of campers said they transport leftover firewood from one campground to thenext, while 19% take leftovers back home (Peterson and Nelson 2007).

Firewood movement by campers is likely contributing to the spread of EAB as is evidenced bythe number of infestation epicenters in and near parks and campgrounds (Buck 2008). Many camperstravel long distances on vacation, and some may be arriving from infested areas. One Minnesotasurvey found that about 4% of campers were visiting from out-of-state, and of those, 35% broughtfirewood (Hanson and Albers 2006). Another survey in Minnesota found that some campers came


from EAB quarantined or regulated areas, including one person who brought firewood from their homesupply (MN–DA 2005). Many National Park Service lands are also frequented by out-of-statecampers arriving from quarantined areas (Buck 2008).

Younger campers may be more likely to transport firewood than older campers. In theNortheast and upper Midwest, many more (66%) of 18-29 year olds surveyed said they movefirewood compared to those in older age brackets which steadily decreased from 45% (30-39 yearolds) to 14% (those 65+) (TNC 2007). Younger people may move firewood because they are lessaware of firewood regulations and the problems associated with invasive species. Only19% of 18-29year olds had heard a message urging the public not to move firewood while 36% of 30-39 year oldsand 40% of those aged 65+ had heard a message (TNC 2007). When people are aware of the risksassociated with moving firewood, they are less likely to move it; 29% of respondents who had receivedinformation about firewood and invasive species reported that they moved firewood compared to 44%of those who had not heard a message (TNC 2007). Similarly, 90% of respondents said they werewilling to use only local firewood immediately after they heard a message about the risks posed by thespread of invasive species via firewood (TNC 2007).

There is also evidence suggesting that people with lower levels of education are more likely tomove firewood than those with more education. In the Northeast and upper Midwest residentialsurvey, 44% of those with a high school education or less reported that they move firewood ascompared to 39% of those with some college, 36% with a college degree, and 29% with post graduateeducation (TNC 2007). It may be that those who have achieved higher levels of education are alsomore informed about invasive species issues and the risks posed by firewood movement and aretherefore less likely to move firewood. However, because financial earnings are positively correlatedwith educational level (Day and Newburger 2002), the actual relation may be that those with lowerincomes may be more likely to move firewood. People with lower incomes may be motivated bymany of the economic reasons mentioned above to harvest firewood themselves or seek out cheapfirewood sources. Similarly, transporting previously acquired firewood will be less expensive thanbuying it inside or near the recreation area where it typically costs more.

Expensive user fees may be another reason that campers with lower incomes are more likely tomove firewood. User fees are an important factor people consider when selecting public recreationsites (Schroeder and Louviere 1999). When user fees increase, the number of amenities expected bycampers, including firewood, also increases significantly (More et al. 1996), visitors may not need tobring firewood. However, low income campers are less likely to choose sites with high user fees(Burns R.C. and Graefe 2006). One survey found that families earning <$30,000 per year were morelikely to reduce their participation or go elsewhere as a result of increases in user fees than higherincome earners (More and Stevens 2000). A study by McFarlane (2004) concluded that high user feestend to exclude lower income campers who choose sites that are free or the least costly. Additionally,people using more developed campsites (which have firewood) and higher user fees were found tohave a higher income than people at less developed, less expensive sites (McFarlane 2004). Iffirewood is not available within or near a recreational area, campers will have to bring it from anotherlocation if they want a campfire. In fact, 100% of campers surveyed at a state park in Minnesotawhere firewood was no longer sold, indicated they brought outside firewood into the park (Hanson andAlbers 2006). This suggests that unmanaged campgrounds, sites considered “primitive”, or otherrecreational areas where firewood is not sold may be especially vulnerable to EAB infestation as aresult of firewood movement associated with campers.

Seasonal property owners and renters. Visitors to cabins and vacation homes are frequentusers of firewood and may burn significant amounts for recreational or heating purposes. In


Figure 5. Firewood stockpiled outside a cabin in northwest Wisconsin. Firewood on right half of shelter originatefrom Minneapolis-St. Paul, Minnesota. Firewood on left half and outside the shelter was harvested from cabinproperty. (Courtesy of Dr. Melvin Baughman, University of Minnesota).

Minnesota, over 100,000 cords of firewood were burned at secondary or recreational residences during2007-2008 (Barzen et al. 2009). Although there is little data regarding firewood movement by thisgroup, it is known that some people take firewood from their primary residence to their vacation home(Fig. 5). Owners or renters may buy firewood in bulk or bring it from their primary residence andstockpile it at the secondary residence. Conversely, people may bring firewood that is stockpiled attheir vacation home back home to their primary residence. Visits to a vacation home are seasonal andinfested firewood may remain stockpiled for long periods of time before it is burned, therebyincreasing the possibility for EAB larvae to complete their life cycle and emerge. Additionally,because firewood use occurs on private property, it is difficult to track, much less regulate theindividual actions associated with this group. Because volumes are high, movement of firewood byseasonal property visitors may be an important route to spread EAB.

Other end-users. Although there is even less data regarding firewood use and movementamong other kinds of recreational end-use, they may be important in spreading EAB via firewood.Heavy users of firewood include frequent outdoor recreation participants (TNC 2007), includinganglers and hunters. Anglers may burn firewood as part of a fishing trip. If they are fishing from aboat, they could transport firewood around lakes or even up and down rivers, potentially spreadingEAB along the way. For instance, a new infestation of EAB was discovered near a boat ramp along


the Mississippi river in Wisconsin (Bergquist 2009). Hunters are likely to bring firewood on a huntingtrip since most hunting seasons occur during colder months and firewood left on site may be too wet toburn (D. McCullough, Michigan State University, East Lansing, personal communication, 18 August2008). Additionally, people engaged in hunting activities may be more likely to cut their ownfirewood (D. McCullough, Michigan State University, East Lansing, personal communication, 18August 2008), a risk factor for moving infested firewood. Hunters might also use an off-highwayvehicle (OHV or ATV) to transport firewood to remote hunting sites and unburned wood may be left atthe site.

There are many additional recreational activities that involve firewood. An unknown numberof recreational vehicle (RVs) users may also be responsible for the spread of EAB. People who useRVs may travel long distances on vacation and visit numerous locations. The additional cargo spaceavailable on an RV may provide ample opportunity to transport firewood. Additionally, largespectator events, such as NASCAR races, balloon festivals, and Native American pow-wows mayattract multitudes of participants who could potentially bring firewood from long distances (Buck2008). For example, during a 3-day firewood “blitz”, APHIS officials made 29 seizures of illegallytransported firewood near a NASCAR speedway (USDA–APHIS 2008b). People may also travel longdistances and bring firewood to participate in large OHV rallies or attend outdoor music concerts.Additional research is needed to evaluate the importance of these and other end-user activities inspreading EAB.

ConclusionsEmerald ash borer may spread by its own natural dispersal processes and by human-mediated

transport. Long distance spread (greater than a few kilometers) has all been human-mediated.Firewood, timber and log production, horticulture, and wood packaging materials are known pathwaysthat have spread EAB in the US since its discovery in 2002. EAB probably arrived to North Americaon wood packaging materials, and although these have not been implicated in its spread within the US,WPM remains a concern and will continue to be regulated in quarantined areas. The risk associatedwith horticulture has probably declined because of increased regulation and declining demand for ashstock. Movement of raw logs may remain an important route of transport, but the greater risksassociated with timber production is the waste materials that enter the firewood economy. Most of theevidence suggests that the major pathway for movement of EAB is firewood. The firewood economyis complex, but the basic structure involves producers, distributors, retailers and end-users in a formaland informal economy (Fig. 4). The relative volumes of firewood associated with the variouscomponents and how these activities relate to the probability of transporting EAB is poorly known.Campers are an important vector of EAB-contaminated firewood, but relatively little is known aboutother end user activities. Without better quantitative data on flows and risks associated with thecomplex firewood economy, it will be difficult to devise effective strategies of education andregulation for reducing the rate of spread of EAB. Given the proclivity of humans to move firewood, itis our recommendation that EAB infested trees be cut and chipped on site to limit the opportunity formovement of EAB infested firewood.

Acknowledgments

We are grateful to James Buck (USDA–APHIS), Robert Haack (USDA–Forest Service), and KathrynKromroy (MN Department of Agriculture), for helpful suggestions and comments regarding thismanuscript. This material is based upon work funded by the Minnesota Futures Grant Program to DA(University of Minnesota, Office of the Vice President for Research).


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