Abstract
Early detection and immediate girdling plus prompt removal (within 20 work days) of diseased elms saved more elms at a lower cost than sanitation practices in which diseased elms were just removed promptly or were allowed to remain standing into the dormant season. A 5-year case history demonstrated savings of 25 percent in total cost and an additional 163 elms per thousand.
Research to improve community Dutch elm disease control programs has shown that an intensive sanitation program results in fewer elms lost to the disease than a sanitation program in which diseased elms are left standing into the dormant season (Barger 1977). Intensive sanitation is a program of frequent surveys (at least three during the growing season) of the elm population with each survey followed by prompt removal of diseased elms within 20 work days after symptoms are observed. A 3-year case history of such a sanitation program demonstrated a substantial improvement in control: a saving of 92 more elms per thousand while costing 25 percent less (Cannon, Barger, and Worley 1977).
Removal of diseased elms, directly after expert diagnosis, controls Dutch elm disease by disrupting root-graft transmission of the fungus Ceratocystis ulmi (Buism.) C. Moreau, by removing the pathogen reservoir from the elm population, and by removing breeding sites of the bark beetle vectors. Scolytus multistriatus (Marsham) and Hylurgopinus rufipes (Eichhoff). The importance of each of these factors may vary at different times in different communities.
Despite intensive sanitation methods, elms may die from Dutch elm disease via transmission of the fungus through root grafts. In rows of closely spaced elms where root-graft transmission of the fungus from diseased to adjacent elms is possible, immediately girdling of diseased elms could limit the spread of the fungus (Fig. 1). The efficacy of this technique in controlling Dutch elm disease was shown by Barger and others (1982). By following a rigorous program of girdling elms immediately upon expert detection of the disease and promptly removing those trees, they significantly reduced the disease rate.
A diseased elm being girdled to disrupt root-graft transmission of the fungus to adjacent elms.
This may seem to be a drastic method of disease control because girdling kills the tree. This thought is reinforced if the tree is an elm that exhibits only the earliest symptoms of Dutch elm disease. However, communities trying to maintain large numbers of street-side elms may find it worthwhile to sacrifice those diseased elms to protect the remaining elm population.
Here, we discuss the cost of girdling diseased elms, and the extent to which this technique can improve sanitation-program performance. Using a strictly financial approach, we assessed the costs of survey and disease detection, girdling diseased elms, and tree removal in terms of the municipal budget. We realize that this approach excludes significant portions of the Dutch elm disease picture. The value of elms, alive and well, in city neighborhoods greatly overshadows the cost of the disease control programs presented here. The physical, biological, and social benefits of saving elms are much greater than any monetary savings to be gained.
Survey and Girdling Analyses
In a large-scale pilot test involving 7,000 city-owned street-side elms, Barger and others (1982) tested 3 kinds of sanitation practices. We classified these practices by the time of tree death or removal after detection of the disease: (1) delayed (conventional sanitation) — a survey to detect diseased elms in August followed by removal of those trees during the fall and winter months; (2) prompt (invensive sanitation) — surveys in June, July,and August, each followed by removal of diseased elms within 20 work days after symptoms were observed; (3) girdling-plus-prompt — surveys in June, July, and August in which each diseased elm detected was immediately girdled and subsequently removed within 20 work days.
The performance and cost of surveying for diseased elms as described for delayed and prompt tree removal were reported by Cannon and others (1977). The performance and cost of girdling diseased elms for the girdling-plus-prompt sanitation technique were determined using gross job time studies (Worley et al. 1965).
The number of trees surveyed and the number and diameter at breast height (dbh) of diseased trees detected and girdled were tallied during the three surveys. Girdling time began when the chain saw was started to make the first of the two cuts shown in Figure 2a and continued through the process of chopping out the wood between the two cuts (Fig. 2b) until after the third cut was completed as shown in Figure 2c.
Girdling process: (a) initial parallel cuts made with a chain saw to girdle a diseased elm, (b) sapwood removed from between the initial saw cuts, and (c) a third cut made into the wood to assure vessels are severed.
Girdling Performance and Costs
The average time to complete the girdling process shown in Figure 2 was 3.7 minutes per elm. However, smaller elms ranging from 10 to 20 inches (25.4 to 50.8 cm) dbh were girdled in 2.5 minutes. Elms, regardless of size, that had convoluted boles were girdled in an average of 5 minutes. Statistical analyses showed that these time estimates are significantly different from each other. These statistics are presented in detail in the Appendix.
The cost of the individual jobs comprising Dutch elm disease control programs of 39 municipalities were compiled by Cannon and Worley (1976). Survey costs in 1972, our base year, averaged 20 cents per tree per survey. Tree removal cost averaged $125. We have updated these figures by correcting for inflation each year with the producer price indexes; in terms of 1980 dollars, survey costs would be 42 cents per surveyed tree and tree removal would cost $265. Our data showed that at 42 cents per tree it would cost $62 per hour to detect diseased elms with our rigorous survey technique (Cannon et al. 1977).
Because girdling has not been a part of on-going Dutch elm disease control programs, there is little municipal experience on which to base costs of this technique. From the point of view of controlprogram operation, however, girdling is an extension of the survey process. Instead of marking a tree for later removal, the survey team proceeds to girdle the tree. Therefore, we consider the hourly cost of survey to be applicable to the girdling operation. When we used the hourly cost of survey and the average time of 3.7 minutes to girdle an elm, the 1980 cost of the girdling process averaged $3.82 per girdled tree.
Elm Losses
In 1973, prior to this study, the test areas had fair performance records with elm losses averaging about 5 percent per year. For the 5-year study, we tabulated annual elm losses (Barger et al. 1982) following the three diseased-elmremoval strategies (Table 1).
Comparison of three diseased-elm-removal practices based on a 5-year study by 1,000-tree units.
We demonstrated that the girdling-plus-prompt removal strategy was significantly better than either prompt or delayed removal. After an initial high disease rate, elm losses the second year in the girdling-plus-prompt removal group were less, but not significantly less, than those sustained under the prompt-removal treatment. However, by the third year, 1976, a statistically significant difference was generated. If the experiment had ended in 1975, this improved performance would not have been detected. Only through Barger’s conscientious, sustained effort were we able to realize that this strategy pays off by reducing elm losses. The beneficial effect of girdling, though insignificant at first, builds over time.
Program Budgets
What did it cost, in terms of an annual budget, to achieve this improvement? An example of how the cost of a girdling-plus-prompt-removal program might compare with those of prompt or delayed-removal programs is given in Table 1. Our 5-year case history is presented to illustrate the budget for each program. We used the girdling performance and costs developed earlier in this paper and updated survey and tree-removal costs from Cannon and Worley (1976) and Cannon and others (1977). Individual tree-removal costs were increased by 20 percent for the girdling-plus-prompt and prompt programs, because crews return again and again to the same areas to remove diseased trees.
The cost figures in Table 1 are historical costs based on the year in which they were incurred. These costs may be adjusted to a common year or updated with the method shown by Cannon and Worley (1980).
After 5 years, the cost of the delayed-removal program was almost $80,600 per thousand original elms, the prompt-removal program $65,200, and the girdling-plus-prompt-removal program $60,200 (Table 1). Tree-removal costs were 98 percent of the cost of the delayed-removal program, 96 percent of the cost of the prompt-removal program, and 94.5 percent of the cost of the girdling-plus-prompt-removal program. Survey costs were 1.5 percent, 4 percent, and 4 percent of the respective costs of these programs. The girdling technique cost 1 percent of its program total. The total cost of the girdling-plus-prompt-removal program was only 75 percent of the cost of the delayed-removal program. By girdling diseased elms, the cost of the promptremoval program was reduced by 8 percent.
More Elms Cost Less
More elms can be saved for a longer time by incorporating the girdling technique into a promptremoval sanitation program. The results of the three diseased-elms-removal strategies were superimposed on the records in Figure 3 that show the length of time in which save-the-elms goals can be achieved with fair (5 percent annual loss) or good (3.5 percent annual loss) program performance levels (Cannon and Worley 1976). Prompt removal offered an immediate and sustained gain in numbers of elms saved. Girdling-plus-prompt-removal showed a smaller increment in improvement towards good performance. Comparison of the percentage of increase in elms saved (Fig. 4) shows that, for all but the first 2 years, girdling-plus-prompt-removal was superior to prompt removal alone. The 5-year total (Table 1) reveals that 3 percent more elms were saved with girdling-plus-prompt-removal.
Number of elms surviving three sanitation practices judged against control-program performance levels.
Percentage increase in elms saved by prompt or girdling-plus-prompt removal compared to the delayed removal program.
Let us assume that this reduction in disease rate cannot be further improved and that the limits of this technique have been reached. Then, if the present disease rates were to persist, in about 2 more years 50 percent of the original elms would have been lost to the disease in the areas with delayed removal. It would take 8 more years before the elm population would be reduced to this level in areas with girdling-plus-prompt-removal. This highlights the improvement offered by this technique.
Actually, if the technique of girdling diseased elms was used, wherever appropriate, in a community-wide program, we might expect the disease rate, despite annual fluctuations, to decrease over time to some lower level. As fewer elms are lost to Dutch elm disease, control efforts will offer greater protection to the surviving elms. This will be especially true if consistent on-going control efforts are applied to the entire elm population.
We indicated earlier that the 5-year cost of girdling-plus-prompt-removal was about $20,000 less or 25 percent less than the cost of delayed removal (Table 2). Our experience summarized in Tables 1 and 2 enables us to put the cost savings, associated with doing a better job of disease control, on a per original-tree basis.
Savings in cost of control based on a 5-year study of 3 tree removal strategies, by 1,000-tree units.
Beginning with 1,000 trees, the first 5 years of control cost about $81 per tree for the delayed-removal program. Only $1.28 was for survey costs whereas $79 was for the tree removal costs. As we intensified our control efforts by prompt removal, the total bill was $65 per tree; a savings of $15. Tree removal costs were reduced dramatically to $63 per original tree; a savings of $6. But survey costs doubled to $2.55 per tree. Girdling-plus-prompt-removal decreased costs even further. The total cost of this alternative was $60 per tree, which saved $18 over the delayed or $7 over the prompt-removal alternatives. Major savings of $6 per tree in tree removal costs were realized by spending slightly more for surveys, including 70 cents per tree for girdling. Each added increment in control effort has lessened the total cost as well as saved elms. Thus, it is possible to have more elms for less annual cost.
Statistics of girdling time for categories of elms observed to have Dutch elm disease.
The Sanitation Picture
Sanitation is the mainstay of most successful community control programs. Barger (1977) demonstrated that a big improvement in saving elms can be made by frequent surveys followed by prompt removal of diseased elms. Additional elms can be saved if diseased elms are girdled as soon as symptoms are observed and then promptly removed (Barger et al., 1982). However, the increment of improved performance over that of prompt removal is smaller than that of a prompt removal program over delayed sanitation (Fig. 3). We would expect subsequent improvements in sanitation performance to be of even less magnitude than that obtained by girdling.
Sanitation techniques have evolved toward earlier detection and earlier removal of diseased elms. Girdling such trees is the most efficient and cost-effective technique to date for treating street-side elms on a community-wide basis. Further improvements await technological breakthroughs in detecting diseased elms and limiting the spread of the fungus.
Summary
A control technique, such as girdling-plus- prompt removal, that can reduce the number of elms to be removed and that costs less than removing those trees will fit well into a municipal budget and release funds for other urgent tasks. We found that a sanitation program of early detection and immediate girdling of diseased trees, followed by prompt removal, saved money as well as elms:
More elms at less cost
Appendix
We classified the elms girdled in this study by whether or not the bole of the tree was convoluted at the point of girdling and by 2 diameter classes — 10 to 20 inches (25.4 cm to 50.8 cm) and 21 to 38 inches (53.3 to 96.5 cm) dbh. There were no elms less than 10 inches or more than 38 inches dbh. One-third of the elms were convoluted (Table 3). Of the remaining two-thirds, 42 percent were in the smaller and 58 percent in the larger diameter class.
We found that if an elm was convoluted, there was no statistically significant difference between the two dbh classes as to the time required to girdle the tree. Significant differences were found between the following three groups: (1) convoluted elms took an average of 5 minutes to girdle (Table 3); (2) the larger nonconvoluted elms took 3.5 minutes: (3) the smaller nonconvoluted elms to an average of 2.5 minutes to girdle. The overall average girdling time was 3.7 minutes.
If the representation of these categories of elms in the street-side trees is known, then an accurate financial estimate could be made based on the girdling time for each category. Because it is not likely that managers would have such information about the elm population of the urban forest at the outset of a control program, we chose to apply our findings to the average situation using the 3.7 minutes average girdling time.
To the time needed to survey a unit a 1,000 trees, we added 3.7 minutes for each tree girdled. If 10 elms were found to be diseased, then 37 minutes were added to the survey time. If the elms were easier to girdle, that is they were in the smaller category dbh, then only 25 minutes need be used. If the elms were convoluted, then 50 minutes would be more appropriate. Although we have used the average of 3.7 minutes per tree throughout for our cost comparisons, the following equation can be used to compute costs for the other two categories of elms:
Adjusted girdling cost = (cost based on 3.7 min/tree) × (k1 or k2)
where
k1 = 0.67 for the smaller easler-to-girdle elms.
k2 = 1.35 for the convoluted more-difficult-to-girdle elms.
- © 1982, International Society of Arboriculture. All rights reserved.
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