100 Veilleux et al.: Rapid Removal of Symptomatic Trees Reduces Dutch Elm Disease Infection Rates visible in late June or July (Manitoba Conservation 2006), a time when overwintered females would be expected to have already selected dead or moribund trees for brood galleries and com- pleted most of their oviposition (Kaston 1939; Finnegan 1957). Thus, delaying removal of previously healthy but now diseased trees until autumn or winter might be considered acceptable, as it could be assumed that there would be little oviposition in such trees in the year of symptom appearance. This assumption appears to be incorrect: in winter, last season’s brood galleries in newly symptomatic trees showed evidence that adult beetles had emerged (Robbie-Draward 1995); and in newly symptom- atic trees, O. novo-ulmi spore-bearing new adults that were ready to leave brood trees have been found in September and October (Oghiakhe and Holliday 2009). These findings suggest that there is a risk that autumn/winter removal could allow many spore- bearing beetles from newly symptomatic trees to transmit the pathogen to healthy elm trees the following spring. However, it is not known whether H. rufipes adults that emerge in September and October can survive winter, and if so, what contribution such spore-bearing adults might make to DED transmission. The ob- jective of the current study was to investigate in an operational setting whether rapid removal of symptomatic trees confers suffi- cient benefits to compensate for the additional logistic difficulties imposed by such a regime. To do this, pairs of communities were selected that were operating autumn/winter removal as part of the provincial DED management program. From 2004 to 2010, one of each community pair operated a rapid removal regime, while the other continued autumn/winter removal; and the an- nual DED prevalence under each removal regime was compared. METHODS In 2004, having received formal consent from each, 14 of the communities involved in the Manitoba provincial Dutch elm dis- ease program were selected to take part in this study. Selected communities (Figure 1) were paired on the basis of geographic proximity, population size, and area under DED management (Table 1). Before the study, tree removals in all communities were performed in autumn/winter. One of each pair of com- munities was assigned to the rapid removal treatment, and this regime was practiced in these communities from 2004 until the end of the study in 2010. Surveys for DED were conducted in a similar manner in all communities and usually started when- ever the first DED symptoms became visible, typically in late June. The first survey was usually completed by the first week in July, and in communities with rapid removal, symptomatic trees identified in the first survey were removed before the be- ginning of August. A second survey was conducted and gener- ally completed by the end of the first week in August. In com- munities with rapid removal, trees that were first identified as symptomatic in the second survey would usually be removed during autumn and winter. All symptomatic trees in communi- ties with autumn/winter removal, along with hazard trees in rapid removal communities, were removed during autumn and winter. Aerial imagery was used to select a visually homogeneous, treed urban area within each community as a study area. These study areas were approximately square and their area averaged 0.98 km2 , although there was considerable variation (Table 1). In each study area in May and June 2008, the U. americana of diameter at breast height ≥5 cm were counted. At the be- ©2012 International Society of Arboriculture ginning of the period, buds of U. americana had not yet burst, but by the end trees were in full leaf. Consequently a variety of characteristics, including bud arrangement, bark color and texture, general tree shape, and leaves and flowers when pres- ent were used to distinguish U. americana from trees of other genera, and from its congener U. pumila. Mostly, walking along public access routes and in parks was enough to observe and identify U. americana, but in doubtful cases, binoculars were used or permission was obtained to walk in private backyards. The counts of U. americana in each study area in 2008, to- gether with records of removals of DED infected and hazard trees for the study areas from Manitoba Conservation, were the basis of estimates of U. americana numbers in other years between 2004 and 2010. To estimate the number of U. ameri- cana in each area in June 2007, the numbers of symptomatic and hazardous U. americana removed between June 2007 and June 2008 were added to the 2008 count. Similarly, sequential addi- tions of numbers of removed trees to the estimated standing stock were used to estimate U. americana numbers in June of years 2004–2006; sequential subtractions of removed trees from count- ed or estimated standing stock were used to estimate U. ameri- cana numbers in June 2009–2010. Annual prevalence of DED for each study area for each year was calculated as the number of symptomatic trees detected in summer surveys expressed as a percentage of the estimated number of U. americana present in June of that year. For each study area, Manitoba Conserva- tion provided data on the number of basal sprays applied for con- trol of H. rufipes and on the number of illegally stored elm logs. Figure 1. Map of south-central Manitoba, Canada, showing major rivers and the location of the study communities.
May 2012
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