Efficient Monitoring for an Urban Ipm Program

Methods

The town of Owatonna, Minnesota (pop. 17,000) was selected for the monitoring study. Selected homes were inventoried for all their woody plants. A landscape map was made of each homesite and every woody plant was located on the map. Each individual plant was then examined for any pests or environmental injury. Once a map was completed, visits were made on a biweekly basis to determine any changes in plant condition and pest population.

The homes were randomly selected by a single stage cluster sampling technique (4). Thirty clusters were selected within the area and then at the intersection marking the cluster every other house in a westerly direction was selected until three houses were at each cluster. Homeowners were encouraged to participate in the monitoring study by being given a copy of the inventory and a summary of the monitoring information.

To study the influence of certain plants on the length of monitoring time, homesites were paired with homes having similar number of plants. Size of homesite was approximately .25 acres for all sites. The data were analyzed by the Wilcoxon signed rank test (11).

Results and Discussion

A total of 2136 plants were located on the 90 homesites in Owatonna, Minnesota. American arborivitae (Thuja occidentalis) was the most common plant, making up slightly more than 20 percent of the total (table 1). The second was juniper (Juniperus spp.) at 8.3 percent and third was white spruce (Picea glauca) at 5.6 percent. This list differs greatly from one prepared for Maryland (10). The predominance of evergreens is a reflection of the Minnesota winters and points out the importance of regionally developed plant lists.

Of the 10 most common plant species, the most problem prone was Tatarian honeysuckle (Lonicera tatarica) (table 1). The pest problem affecting 95 percent of the plants was witches’ broom caused by the honeysuckle aphid (Hyadaphis tataricae). This insect, since its introduction into the Midwest several years ago, has caused a tremendous amount of aesthetic damage on an otherwise relatively pest-free plant. If this same survey had been conducted in 1975, this plant would have been considered relatively pest-free. The second most problem prone plant on this list was green ash (Fraxinus pennsylvanica). While plant bugs (Tropidosteptes spp.) infested 88 percent of trees it is not considered a serious problem by most homeowners, especially once informed about the minor influence it has on tree health. Hence, this problem is not one that warrants much concern. However, in the extremely wet spring of 1986 the green ash in the area suffered severe defoliation from ash anthracnose (Gloeosporium aridum).

More important than the position in the pest problem ranking is the number of serious or life threatening problems that occur on the plants. For this list, serious is defined as a persistent problem that produces an objectionable aesthetic injury (table 2). While tatarian honeysuckle is still on this list, so are hawthorn (Crataegus spp.) because of cedar-hawthorn rust (Gymnosporangum globosum) and honeylocust (Gleditsia triacanthos var. inermls) because of nectria canker (Nectria cinnobarina). Homesites containing species from this list require additional time for monitoring.

This is a part of the key plant concept (10). Key plants are those ornamental plants that have serious, persistent pest problems. These plants can represent a considerable investment in monitoring expense. Using paired homesites, there was a significant difference (P< .01) in monitoring time between homesites containing fewer than 8 key plants and those containing more (table 3). Hence, an IPM service may find that knowing the number of key plants on a client’s homesite can be an accurate indicator of monitoring time required for the site. The length of monitoring time was closer correlated to the number of key plants (r² = 0.68) than it was to the total number of plants (r² = 0.41).

Monitoring can be segregated into four categories based on function: detection surveys, biological evaluations, loss or damage survey and pest control evaluation (5). Detection involves the identification of the presence and kind of pest. This type of monitoring should be conducted at least once a year on all plants, preferably in the spring. While some plants are relatively pest free, annual inspection alerts the scout to any new problems.

Pest control evaluations are conducted to determine the effectiveness of a particular treatment tactic. This survey is a necessary part of any IPM program. Pest control evaluations are not conducted at any regular interval but when necessary.

Biological evaluations are directed at collecting population size and distribution information. Loss or damage surveys are utilized to determine the impact of a pest on a plant. These two categories, the biological evaluation and damage survey, are the backbone of a commercial monitoring service. Many IPM services monitor on a biweekly basis (6). However, this schedule may not be the most efficient use of monitoring time. Instead a service should know the key pests of the plants and intensely monitor during those time periods biological evaluations or damage surveys would be pertinent to that particular pest. Since homesites contain different plant species, the monitoring interval and frequency will differ among the homesites.

A monitoring calendar was developed to determine the monitoring interval and frequency for a homesite. The calendar was developed from the list of key plants and their pests. Each pest was then identified as to the most appropriate time for biological evaluation and damage survey. The number of monitoring visits required for each of the homesites was then tabulated. These data were compared to a standard biweekly monitoring schedule. This standard schedule, which typically run from mid-April until mid-September, would require approximately 12 visits. By utilizing a calendar developed for the key plants in an individual homesite, monitoring visits were reduced an average of 40 percent. This is only meant as an example. This system could possibly result in more visits rather than less. The important use of this system would be to produce a more accurate estimate of time required to monitor a particular homesite. This would improve the ability of an IPM service to price the monitoring aspect of a particular job.

Summary

Monitoring is a vital part of an urban IPM tree service. Unfortunately, it is not a service that clients consider critical, hence they are generally reluctant to pay for it. To reduce monitoring cost it would be better to keep it in line with the cost associated with a particular homesite rather than have a flat charge. An accurate method of determining yearly monitoring costs is to determine the number of key plants contained at the homesite. This will enable an IPM tree service to determine the length of time to monitor at a given site and also the frequency of the visits.