162 Sargent et al.: Emerald Ash Borer Within an Intensively Managed Quarantine Zone detection was regressed (one focus for each of three years) from the nursery each year against time (year). The slope of this rela- tionship represents the maximum detectable spread of EAB from the current study’s data. Regression analyses were performed us- ing the linear regressions procedure of Systat 11 (Systat 2004). RESULTS AND DISCUSSION There was a highly significant positive relationship between the distance of the leading edge of EAB advancement from a central location and the time since the initial introduction of the beetle in Maryland (F1,11 = 144.64, P < 0.007) = 128.06, P < 0.001). The linear relationship y = 1.00 x, where y equals distance from the nursery in km and x equals time since the initial detection explained approximately 92% of the variation in the data (Figure 2). Similarly, the maximum dispersal distance was positively related to time (F1,2 (y = 1.37 x) (Figure 2). The regression of maximum distance on time explained approximately 98% of the variation in the data. In the intensively managed quarantine zone in Maryland, the leading edge of EAB moved on average about 1 km each year and the fur- thest dispersers traveled about 1.37 km. Regarding the direction of dispersal, results concur with those of Smitley et al. (2008) in that no obvious pattern was observed with respect to cardinal di- rection (Figure 1). Further investigations should focus on the role of other factors including topography, prevailing winds, and veg- etative cover in shaping patterns of emerald ash borer movement. Findings indicated the leading edge of EAB advanced 1 km on average with a maximum distance of 1.37 km, are in agreement with those of Haack and Petrice (2003) who reported a maximum dispersal distance of 1.5 km for EAB. The reason estimates for the current study are somewhat greater than those generated by McCullough et al. (2004) are likely related to disparities in the relative availability of hosts found in the two studies. McCullough et al. (2004) studied movement of EAB from a central location to surrounding ash trees in a forest comprised of native trees that included hundreds of ash. As beetles emerged from infested logs and dispersed, they readily encountered and colonized suitable hosts. Most beetles colonized trees within 100 m of their point of emergence. In Maryland, EAB had little opportunity to colonize ash proximate to the infested nursery for the following reasons. Subsequent to the initial detection of EAB in Maryland in August 2003, more than 1,000 ash trees were removed within a circu- lar 0.8 km eradication zone around the nursery. Beetles emerg- ing from any ash tree that escaped the initial eradication attempt faced a relatively long flight to locate a suitable host. Moreover, if the initial eradication attempt in 2003 or subsequent attempts in 2006 and 2007 were completely successful, then the only pos- sibility for detections other than a relatively small number of sen- tinel trees planted within the eradication zone would have been infestations generated by beetles that escaped expanding eradica- tion buffers of 0.8 km per year. Finally, the analysis provides a liberal estimate of EAB movement. In 2006, only four foci were detected and included in the analysis. However, in 2007 and 2008 only the foci at the leading edge of the infestation and most dis- tant from the nursery were included in the data set. Excluding infestations near the nursery likely inflates the estimate of the average advancement of the leading edge; however, it does not Figure 2. Rates of spread of emerald ash border in kilometers from time of initial detection in 2003. Average rate of spread of the leading edge (solid line) was based on the four most distant grid cells with EAB detections for each year in 2006, 2007, and 2008. Maximum rate of spread (dashed line) was based on the single most distant grid cell with an EAB detection in each year. Fitted lines are linear least square regressions. affect the estimate for the maximum rate of spread. It is suggested that the maximum rate of spread is the more informative of the two estimates in assisting planning decisions by urban foresters. These results provide insight into the relative merits of an ag- gressive quarantine and eradication program for EAB. Siegert et al. (2008) estimated the spread of EAB in southeastern Michigan to be 6.5 km each year during the initial phase of infestation. Smit- ley et al. (2008) estimated the rate of spread of canopy thinning to be 10.6 km each year. They suggested that combinations of natural aerial dispersal by the beetle and anthropogenic dispersal enabled by transport of ash trees or, more importantly, infested firewood or wood products underlie the rapid spread of EAB in southern Michigan. They warn that this trend is likely to continue through- out the Midwestern United States. The study authors were unable to exclude the possibility that human-assisted dispersal via the transport of trees, firewood, or wood products was responsible for some of the movement detected in EAB in our study. However, we believe that the concentration of detections in relatively close proximity to the initial site of infestation and the rather uniform annual rate of spread are consistent with the notion of aerial dis- persal as a primary mode of movement by the beetle. It is entirely possible that eradication and educational activities in Maryland reduced the rate of spread of EAB many-fold compared to oth- er locations where such efforts were not attempted or feasible. Finally, a caveat for future attempts at eradication programs for EAB: if destruction of hosts is considered, the eradication zone must exceed the 0.8 km employed in the Maryland infestation. Acknowledgments. Support for this project was provided by the USDA- APHIS-PPQ Center for Plant Health Science and Technology. We thank Eric Rebek for comments to a previous draft of this manuscript and sug- gestions made by two anonymous reviewers. ©2010 International Society of Arboriculture
July 2010
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