102 Veilleux et al.: Rapid Removal of Symptomatic Trees Reduces Dutch Elm Disease Infection Rates in 2004 was not a significant (F1, 68 = 0.15, P = 0.7) covariate. removal communities was also evident in 2004, before the ef- fect of the switch to rapid removal could have had an effect. The initial linear mixed model analysis showed that the ef- fect of rapid removal on annual prevalence for 2005–2010 was significant (F1, 68 = 8.82, P = 0.004), and that disease prevalence Nevertheless, because the pattern of disease prevalence in 2004 was similar to that which followed, the 2004 disease prevalence was retained as a covariate while the potential influence of oth- er factors was explored. The response of annual prevalence to removal regime was robust and not greatly influenced either in magnitude or significance level by the addition, individually, of the following covariates to the initial model: DED managed area, area of study area, number of basal insecticide applica- tions made from 2004 to 2009, number of basal applications made from 1999 to 2003, whether a basal application had been made in the previous year, whether a basal application had been made two years previously, number of illegally stored elm logs detected in the study area, number or square of the number of U. americana trees counted in the study area in 2008, and den- sity or square of the density of U. americana in the study area in 2008. Consequently, the final model chosen did not include any of these covariates. The final model included pair and year as random effects and had removal regime as the only fixed effect; removal regime was highly significant (F1, 69 = 10.70, P = 0.002). Overall, during the period 2005–2010, the rapid re- moval member of a pair had a lower annual prevalence of DED than its corresponding autumn/winter removal com- munity in 80% of the 40 possible comparisons. In this pe- riod, the average annual prevalence in rapid removal com- munities was 1.5 ± 0.2%, and that in autumn/winter removal communities was 3.1 ± 0.4%. The best estimate of the ben- efit of rapid removal was a reduction in annual prevalence of 1.56% with 95% confidence limits of 0.84% and 2.29%. DISCUSSION The paired experimental design and the exploration of the poten- tial influence of measurable covariates give confidence that the significant effect of timing of removal was not an artifact asso- ciated with some other variable. However, researchers were not able to eliminate all potential nuisance variables: possible effects of degree of isolation from wild sources of Dutch elm disease inoculum could not be removed either by the design or analysis. All the study communities lie within the intensely farmed re- gion of Manitoba, where most natural stands of U. americana occur in riverbanks and other moist areas that are unsuitable for agriculture (Scoggan 1957). There is no coordinated DED man- agement of riverbank elms except where these trees lie within the provincial DED management areas, and so riverbank forests can be a source of DED inoculum, a habitat for vector beetles, and a potential corridor along which DED can be transmitted between communities. As the DED epidemic has progressed in Manitoba, fewer U. americana remain in riverbank communities, and many of the study communities are now quite isolated from external sources of infection. An exception is Selkirk, which is adjacent to a large area of wetland containing many diseased trees. It was not possible to derive an indicator for each community that would indicate the amount of inoculum from wild elms. Selkirk received the rapid removal regime and there were three rapid ©2012 International Society of Arboriculture removal communities with rivers, compared with two autumn/ winter removal communities with rivers. Consequently, if lack of isolation from wetland elms were to increase DED prevalence, the authors would expect elevated prevalence in rapid removal communities, rather than the reduction that was observed in this study. Thus, the authors conclude the effect of isolation from wild sources of inoculum could not have produced the observed difference between rapid and autumn/winter removal, but rather could have reduced the magnitude of the effect that was observed. This is the first long-term experimental study to show that DED prevalence can be reduced by rapid removal in regions where the dominant vector is H. rufipes, and the insect over- winters only in the adult stage. Under these circumstances, a major effect of rapid removal is to prevent the emergence of spore-bearing H. rufipes adults from brood galleries in newly symptomatic trees. By comparing experimental plots in Detroit, Michigan, U.S., Barger (1977) demonstrated that tree removal within 20 working days of symptom detection results in sig- nificantly lower DED prevalence than does removal during the following autumn and winter. In Detroit, both H. rufipes and S. multistriatus are important vector species and both overwinter as larvae and as adults. Consequently, emergence of spore-bearing adults from brood galleries can occur at most times of the grow- ing season. Rapid removal would also reduce the likelihood of the pathogen transferring among neighboring trees through root grafts. In the current study, the rapid removal regime was applied only to trees in which symptoms appeared early in the season, yet this was sufficient to produce a consistent reduction in DED prevalence, perhaps because trees that exhibited symptoms late in the summer became attractive for brood gallery construction after the main period of H. rufipes oviposition. However, it can- not be assumed that DED-infected trees become targets for brood gallery construction only after visible DED symptoms appear, as O. novo-ulmi induces infected trees to produce volatiles that are attractive to H. rufipes (McLeod et al. 2005), and volatile production could precede the appearance of visual symptoms. All communities were practicing an integrated disease man- agement program for DED throughout the data collection phase of this study, and so annual prevalence was quite low in all com- munities. A similar program in Winnipeg resulted in an annual prevalence of about 2% over the first 30 years since DED was de- tected, compared with an estimated annual prevalence of 18% in similar cities with no management program (Domke 2005). Not- withstanding the generally low prevalence, there are considerable benefits from using rapid removal as a component of the manage- ment program. Over a 10-year period, for areas similar to these study sites and the same U. americana density, the study authors estimate that the 498 trees/km2 would decline to 427/km2 rapid removal regime, compared with 365/km2 under a if autumn/winter removal were employed. The current average cost of removal of a U. americana tree as part of Manitoba’s provincial DED man- agement program is about $350, and the cost of planting and ini- tial maintenance of a replacement boulevard tree for two years in Manitoba is $600, giving a total average removal and replacement cost of $950 per tree lost to DED. Using the trunk formula meth- od (Council of Tree and Landscape Appraisers 1992), the esti- mated value of a boulevard tree in an urban environment in Mani- toba is about $9,600, when accounting for its aesthetic and other contributions. Thus, in 10 years, the retention of an additional 62 trees/km2 through rapid removal would provide savings/km2 in
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