Title: Principles of IPM: Economic Thresholds and Injury Levels

Learning Objective: To understand the concepts involved in the development and use of economic thresholds.

Materials: Presentation board (chalk board or flip chart) or slide projector, chalk or pen and lesson fact sheet and activities handout.

Time Needed: 3 hours

Power Point Presentation: Lesson 4 (computer file: lesson4.ppt)

Opening: Open by discussing that total eradication of a pest is virtually impossible and is usually undesirable because it can spell the demise of the pests natural enemies and can upset the broader economic balance. It’s usually better to determine the level of pest presence or pest related damage you can tolerate without harm to your health and plants. Determining these levels or thresholds goes hand and hand with field monitoring and is the back-bone of IPM


Principles of Economic Thresholds and Injury Levels

A. Types of Thresholds

1. Definition of threshold: A boundary, where something starts or ends.

IPM Thresholds: Set levels a pest population must reach before

treatment to control the pest can begin. These are often expressed

numerically and indicate the population levels that will cause economic or

aesthetic damage.

2. Characteristics (especially of economic thresholds)

- can change throughout the season at different stages of crop


- can vary from variety to variety

- must be constantly revised to account for new pests, new varieties,

new management practices, new marketing standards and variation in

commodity prices

- can be developed by the grower to suit their IPM needs

3. Types of IPM thresholds

a. Economic Thresholds (action thresholds)

i. Definition: The pest density at which some control should be

exerted to prevent a pest population from increasing further and causing economic loss.

ii. Examples:

- bean leaf beetles in soybeans: "When defoliation reaches 30 percent

(before bloom) and there are 5 or more beetles per foot of row".

- black cutworms in corn: " apply a postemergence rescue treatment

when 3 percent or more of the plants are cut and the larvae are still


- leaf miners in melons: chemical treatment is recommended if an

average of 15 to 20 un-parasitized larvae per leaf are found.

b. Damage Thresholds

i. Definition: The maximum damage a crop can sustain without yield loss

- Generally used for plant diseases. Since disease pathogens are too

small to be easily seen, counting their numbers is impractical, so an

estimate is made of the amount of damage caused by them.

ii. Examples:

- counting diseased leaf petioles for soybean pod and stem blight

- estimating the percentage of whole plant infection caused by fungal

leaf blights in corn

c. Economic Injury Thresholds

i. Definition: The lowest pest density at which economic damage occurs.

Where the cost of the control measure is equal to the loss likely to be

inflicted by the pest. If pest populations reaches this boundary,

economic damage will occur. This threshold is above the economic

threshold, the economic threshold most be achieved first, before this

threshold can be reached.

ii. Example:

- beet army worm on melons: if army worms begin feeding on fruits

d. Aesthetic Thresholds

i. Definition: The level at which a pest causes an undesirable change

in the appearance of something, typically ornamental plants. This

threshold can be used by homeowners, in parks and other public areas

and can be highly personal to the user or users.

ii. Example:

- boxelder bug: a harmless insect, except its tendency to aggregate en

masse on the exterior of homes makes people fearful of it.

e. Degree Day Models

i. Definition: Models used to predict the emergence of the first

generation of an adult pest, typically after winter. These models are

based on the time of the year and temperature.

ii. Use in Hawaii and the Pacific: These Models are not readily usable in

these areas due to:

- overlapping generations of pests and no distinct first generations

- fairly uniform temperatures year round


B. How Thresholds are Developed

Thresholds, mainly thresholds used by commercial growers, can be developed

from the following factors:

1. Amount of physical damage related to various pest densities; How large the

pest population can grow before it causes a certain level of damage.

2. Monetary value and production costs of the crop at various levels of physical


3. Monetary loss associated with various levels of physical damage

4. Amount of physical damage that can be prevented by the control measure

5. Monetary value of the portion the crop that can be saved by the control measure

6. Monetary cost of the control measure

7. History of the field

8. Determining the danger of the pest at different stages of the crops development

9. The pest distribution

10. How much aesthetic or economic damage can be tolerated

11. Establish a treatment level that keeps the pest population small enough so it

does not cause an unacceptable level of damage.

12. Ultimate destination of the crop, what the standard of the end consumer is.

13. Ability to control the pest rapidly and effectively

C. Importance of Thresholds

1. The main importance is for decision making on scheduling of control and

control methods

2. To establish the optimal amount of control which can be used to minimize risk

of economic damage and environmental hazards


Agricultural Plant Pest Control, Chapter One: IPM on University of Nebraska Web site: http://ianrwww.unl.edu/ianr/pat/training.htm

Flint, M.L. 1990. Pests of the Garden and Small Farm. University of California Pub. # 3332. 276 pp.

Metcalf, R.L. and W. Luckmann. 1975. Introduction to Insect Pest Management. John Wiley & Sons, New York, NY. 587 pp.

Olkowski, W., S. Daar, and H. Olkowski. 1991. Common-Sense Pest Control. Taunton Press, Newtown, CT. 715 pp.

Watson, T.F., L. Moore, and G. W. Ware. 1976. Practical Insect Pest Management. W. H. Freeman and Co., San Fransisco. 196 pp.



Example of threshold settings.

Development of thresholds can be accomplished by close monitoring of pest levels and by keeping accurate records over many different growing seasons. These records need to record pest population and yield. A growers thresholds can be based on published thresholds, but must be evaluated and suited to the growers particular environment and crop conditions. For example, if the published Sweet Potato White Fly threshold is found to be 20 adults per leaf, a grower can monitor their field and control white fly if found in numbers of 5, 10, 20, 25 adults per leaf on different fields or parts of the farm. Then keep track of the effect of these different populations on yield.


On the following graphs, determine the pest population for the economic threshold level. Also show on the graph, the area of the economic injury level:

Example A: Leafminers in cucumbers.


Monitoring Method: Placing white Styrofoam or plastic pans at the soil level below the plants throughout the field to collect larvae as they drop and pupate. The pest population measurement level is pupae/pan/day.

Example B: Tomato fruitworm:


Monitoring Method: The critical period for monitoring begins at flowering and lasts until the green fruit stage. Counts of fruitworm eggs are made from the leaves below the inflorescence (flowers). Thirty leaves are sampled randomly through out the for viable eggs. The pest population measurement level is eggs per leaf.


Example C: Thrips in cucumbers:


Monitoring Method: Monitoring should begin in the early stages of the crop. A sampling of 50 randomly chosen leaves in a small area, around 0.5 acres. The pest population measurement level is number of Thrips per leaf.

Example D: Melon fly in tomatoes:


Monitoring Methods: Monitoring is done on both young and mature fruit. Twenty-five fruits are checked randomly in the field for damage, such as a water soaked appearance in mature fruit and distorted appearance in young fruit. The pest population level is number of damaged fruits