IPM Committee Tasks: Developing IPM Plan Components: Objectives & Thresholds: Setting Injury & Action Levels

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Setting Injury and Action Levels

Before proceeding with any type of pest treatments, it is important to consider whether the abundance of pests warrants the action. Can a low number of pests be tolerated on a regular basis? If so, at what level of abundance does the pest begin to pose a problem; this quantity is referred to as the “injury level”. The “injury level” is the level below which a specific number of pests or the amount of pest-related damage can be tolerated without suffering an unacceptable medical, economic, or aesthetic loss.

Example 1: One to two flies would probably go unnoticed by the students and teachers within a classroom; however it was determined that five flies or more was enough of a nuisance to distract students. The injury level for this particular facility was five or more flies.

The “action level” as the name suggests, refers to the number of pests that are necessary to initiate some type of treatment. The treatment used is designed to prevent the pest from reaching the injury level. The action level depends largely on pest biology and environmental conditions supporting the pest.

In the example above, 3 to 5 flies would probably be considered the action level.

Example 2; if it was determined that on a playing field, five or more weeds per square yard began interfere with the footing of sports players. The injury level for the playing field is five weeds per square yard. In this situation, the action level in this example would likely be 3-4 weeds per square yard.

Example 3; if a particular facility has decided that it will not tolerate cockroaches within the food preparation area, the injury level would then be one, and the action level would also be one. Remember! Treatments can only be based on the presence of a pest; they cannot be used as a preventative.

Determine Injury Levels First

Before you can determine the action level, you must first determine the injury level. This is the level of damage or the level of the pest population that causes unacceptable injury. The injury level will be higher than the action level.

Three Types of Injury
There are three types of injury in IPM: 

• Aesthetic injury is applied mainly to plants. This is injury that affects the appearance without affecting the health of the plant. There are few indoor pests or pests of structures that cause only aesthetic damage.
• Economic injury refers to pest damage that causes monetary loss, e.g., clothes moths destroying band uniforms or a plant disease that causes the death of a tree.
• Medical injury relates to human health problems caused by pests like rodents, flies, yellowjackets, poison ivy, etc.

Injury Levels Differ Depending on the Pest

The number of pests or amount of pest damage you can tolerate (another way to think of injury level) will depend on the kind of pest and its location. The presence of ants trails winding through an unused outbuilding is an entirely different situation from an ant invasion in the cafeteria. Many thousands of aphids can usually be tolerated on a tree, but one louse or nit on a child’s head cannot.

Don’t Set the Level too Low

One of the major causes of unnecessary treatments for pests is unrealistically low tolerance levels. Obviously, there is little leeway in tolerance for pests that have consequences for human health or the school budget, but for many other pests, the range of tolerance can be very wide. By understanding which kinds of damage are serious and which are unimportant and by simply changing the way we view pests and pest damage, we can avoid many unnecessary treatments. For instance, most trees and shrubs can support substantial populations of caterpillars, aphids, psyllids, or leafhoppers without coming to any harm. Lawns can still be very attractive and functional even though the grass is not all of one kind and there are a number of weeds mixed in (as long as they don’t pose a tripping hazard, of course).

Determining the Injury Level

We all have intuitive, unspecified notions of injury level in various pest management situations, but these may not be accurate. In an IPM program, the aim is to try to make injury levels explicit and accurate. Monitoring is the only way to do this. It also takes knowledge and experience to understand the life cycles of pests, how fast their populations grow, and whether or not their damage will have serious consequences.

Example: Last year a chemical control was used when the aphid infestation in trees was first noticed by a school employee. This year, a monitoring program was initiated. Data collected indicated that 100 to 200 aphids per leaf produced no significant damage to the tree. In fact, the data showed that only when there were over 500 aphids per leaf did leaves start to drop from the tree. This level of aphids also began to elicit complaints about the sticky honeydew raining down from the tree.

Periodically, the injury level should be re-evaluated for each pest and for each site. Changes in weather conditions, plant cultivars grown, horticultural practices, level of IPM experience of employees, building renovations, etc., can affect the setting of injury levels.

Determine Action Levels Based On Injury Levels

The action level is the level of pest damage or number of pests that triggers a treatment to prevent pest numbers from reaching the injury level. The action level will be lower than the injury level. Determining action levels involves making educated guesses about the likely impacts of numbers of pests present in a given place at a given time. In other words, you need to estimate how high you can let the pest population grow before you need to treat to prevent unacceptable injury. The action level must be determined and treatments applied before the injury level is reached.

Example 1: You know from previous observations that the injury level for the shade tree you are monitoring is 15 caterpillars per foot of branch. Current counts show 5 caterpillars per foot. These counts, weather data, and your experience lead you to expect the pest population will exceed the injury level in about two weeks, unless there is a surge in natural enemy activity or the temperature drops. Your choices depend on available time and resources:

1. You can decide to set your action level at 5 to 7 caterpillars and schedule a treatment right away if it will be difficult to check again in a week.
2. Because the trees are extremely valuable and because you see that caterpillars are starting to die from attacks by natural enemies, schedule another visit in one week. At that time, if natural mortality does not appear likely to keep pest numbers below the injury level, there is still time to apply an insecticide. In this case, set your action level at 7 to 10 caterpillars.

Precise pest treatment recommendations to achieve specific results are an essential part of an IPM program. 

Example 2: The action levels used to treat for cockroaches will vary for different sites with different histories and conditions. This level is determined by deciding how many pests can be tolerated by school occupants. Action thresholds are set by the IPM coordinator and the school occupants and should reflect the pest management objectives for the site. The presence of some pests does not necessarily require application of pesticides.

Action thresholds for the control of German cockroaches
Average number  trapped per zone	Action to be taken
0	None caught for 3 months: extend monthly monitoring interval to every 2 months, and replace monitoring stations every 6 months;
1-2	Continue routine use of monitoring stations, check for any sanitary problems;
3-5	Continue to check for sanitary problems, conduct a thorough review of crack and crevice treatment
5-10	Review sanitation, review physical barriers, use bait gel formula, replace sticky traps for monitoring every 2 weeks;

When an IPM program is first implemented for a particular pest/site, guidance on setting the action level may be available from existing school records, from the literature on the pest, through discussions with those who have experience managing the pest elsewhere, or from recollections of the problem in prior years by school staff.

Set Conservative Action Levels in the Beginning

During the beginning phase of an IPM program, it is wise to be conservative when establishing an initial action level. Set it low enough (i.e., low numbers of pests trigger treatments) to insure a wide margin of safety while learning monitoring methods. The initial action level should then be compared with other action levels for the same pest at different sites or locations. This is necessary to determine if the action level is set too high or too low, if treatments were necessary or not, and if they were properly timed.

The easiest way to collect comparative data is to set aside a portion of a school that remains untreated at the time another area is treated, or to monitor two schools where different action levels are applied to the same pest. By monitoring both sites, and comparing records, adjustment of the initial action level up or down can be evaluated.

Avoid “Revenge” Treatments

Sometimes action takes place after the injury level has been reached and the pest population has begun to decline naturally (Figure 3-2). These “revenge” treatments are generally useless at controlling pests, damaging to the environment, and an unnecessary expenditure of time and resources.

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