Economic Effectiveness Of Operational Therapeutic Pruning For Control Of Dutch Elm Disease

Materials and Methods

The communities participating in these studies were Falcon Heights, and Inver Grove Heights, Minnesota. In 1 983 Falcon Heights lost 68 of its 840 elms to Dutch elm disease; Inver Grove Heights lost 56 of its 2600 elms. Candidate trees were identified during routine biweekly inspections for Dutch elm disease. Trees were considered pruneable if, based on foliar symptoms, the fungus was not yet thought to be in the main stem of the tree. The locations of candidate trees were given to tree removal firms under contract to the appropriate city. Contractors were selected by lowest bids for a projected number of hours of pruning, and a projected number of trees to be removed. Trees were pruned as soon as possible after detection, often within 24 hours. Contractors were instructed to remove the diseased branch at the main stem of the tree. When possible, a sample was collected for confirmation of the presence of the Dutch elm disease fungus, and the distance between the main stem and the vascular discoloration was measured. All samples processed were positive for Dutch elm disease.

Pruning cost was determined from contractor billing for pruning. Removal cost was determined from the contractors bid price, based on tree diameter at 1,4m (dbh).

Pruned trees were examined during routine inspections, and at least once annually for the purpose of this study.

Results

During this study, 32 trees were pruned (Table 1). Of these, 25 are still alive. Pruning costs averaged $33.60 per tree, and removal cost for the 7 trees that died after pruning was $781 (Table 2). The removal cost of the trees that had been pruned would have been $5351. The average distance between the main stem and the last detectable stain was 8.5 feet. One tree with 5 feet of stain-free wood did not survive.

Discussion

This study reports the effectiveness of a pruning component within an operational Dutch elm disease control program. The trees pruned were selected during routine inspections for Dutch elm disease. Pruning was done by commercial contractors, at competitively bid prices. Under these constraints, it was usually possible to prune trees within 24 hours of detection. By pruning trees, 25 trees were saved, and $3495 that would have been spent on tree removal was saved. Although several more trees may die, with an average removal cost of $167 each, and savings may decrease somewhat, pruning will still provide substantial savings. Our study, and the results are quite similar to those reported by Himelick and Ceplecha (1976).

Several urban foresters have suggested that administrative costs of a pruning program are much greater. This is in fact the opposite. By securing contractor services based on hourly rates for pruning and dbh class for tree removal, there is only one bidding process per year, and only one contractor to deal with. This greatly reduces the response time for pruning, and also for tree removal. On numerous occasions, root graft barriers were not necessary, because trees were removed before the fungus colonized the root systems of the diseased tree. Contractors were notified by phone when trees were to be pruned, and would often do the pruning first thing the next morning, and then work elsewhere. This rapid response is essential for a pruning program.

Others have raised the issue that pruning trees only delays the inevitable removal of the tree. They ask "Why spend money to prune a tree, when it will have to be removed anyway? Better to spend the money now to remove it and be done with it.” This argument really doesn’t really hold. Based on an average pruning cost of $33.60 per tree, a removal cost of $167 per tree, if the difference between pruning cost and removal cost were invested at 10% interest, it would take less than 3 years to recover the pruning cost. Thus, if trees did not become infected during the next 3 years, the community would be making money in addition to having the trees to enjoy. This illustrates the importance of an effective sanitation program. Good sanitation is the most important component of a Dutch elm disease program. If the infection rates are high, secondary opportunities such as pruning and injection are of very limited value.

Thoughout this discussion, the effectiveness of pruning has been discussed in terms of dollars saved. Trees were saved, incidental to reducing costs. For many citizens, saving trees is of primary importance. Trees increase property values, reduce heating costs, and can add greatly to the aesthetic appeal of the community. Homeowners are willing to pay for these benefits, as indicated by a survey which found 61-82% of homeowners willing to pay $10 or more in increased taxes for a shade tree program (Reynolds et al. 1981). Once a community has established an effective sanitation program, pruning is a viable and feasible control measure which can protect both financial and elm resources.