mealy bugs on plantWithin an IPM program, assessment is the process of determining the potential for pest populations to reach an action threshold. The art and science of this approach is what makes IPM different from other pest management strategies. The process merges biology, ecology, and economics with the results of monitoring and identification of the pests. Is a grower likely to suffer financially? Will the disease vector transmit the infection? Those are some of the questions this process is trying to forecast.

  • early blight on tomatoForecasting. Pest models can help determine if weather conditions will be favorable for the development of diseases and insect pests.  For example, by entering weather facts (such as the number of rainy days and the temperatures for those days), growers can predict outbreaks and spray only when conditions are favorable for diseases. Growers who have kept good records of pests in previous years can use these records to help determine if problems such as weeds, insects, and diseases will reoccur.  They might be able, for example, to apply the most effective herbicides at the proper time for early control of a specific weed problem.
  • Thresholds. Before any pest control action is taken, the IPM approach considers if the pest has exceeded a pre-set threshold; the point at which the pest population or environmental condition indicate that pest control action must be taken. Thus, finding a single pest or even very low numbers of pests does not always result in taking action. Conversely, some pests in a field may be below the threshold while others are above it. Keep in mind that thresholds are crop- and pest-specific, as well as weather-related.

Important terms:

  • Economic threshold: The pest density at which a control tactic must be implemented to avoid an economic loss.
  • Action threshold: A pest or damage level at which control is initiated to avoid significant damage or loss of property.  Usually, a lower level than the Economic threshold.
  • Economic damage: The amount of injury which will justify the cost of control action.
  • Economic injury level: The lowest population density that will cause economic damage.

There are five basic factors considered when setting action thresholds; economics, health and safety concerns, aesthetic concerns, public opinion, and legal requirements.

  1. Economics. Economic thresholds for many pests and crops have been carefully determined. The grower would not treat the crop unless the pest threshold is exceeded and there is risk of economic loss. Once the pest density (number of pests per unit area) has reached threshold, action is justified. The costs of control (i.e. applying a pesticide) will be less than or equal to the estimated losses that the pests would cause if left uncontrolled.
  2. bees inside wallHealth and safety concerns.When the pest creates a health or safety concern, the action threshold is set low. For example, if Lyme disease is prevalent in near a playground, and if the ticks that transmit the disease are found, then this would trigger the action threshold
  3. Aesthetic concerns. When the appearance of something is degraded, then that triggers an aesthetic concern, for example, bird droppings on statues and monuments, or defoliation of landscape plants or high value horticulture crops. Thresholds for horticulture crops are usually set low because damaged goods are difficult to sell.
  4. Public opinion. It may be less obvious, but public opinion will often result in an action threshold being set very low. The pest could be perceived as scary or disgusting, but whatever the reason, social, cultural, or psychological, some creatures are pests because people will not tolerate them. Spiders are a good example. There are also those who are unwilling to accept pests at any level.
  5. Legal requirements. Pest levels are regulated under federal, state, and county health regulations. Therefore, action levels are being set by regulation. For example, there is virtually no tolerance for mice, rats, flies, cockroaches or any other pests in food facilities. If a public health emergency is declared, government agencies may mandate pest control in the event of a rabies or encephalitis outbreak. Building codes and standards may determine action to control termites, rats or flies on private or commercial properties and public areas, including parks or schools.

Predicting Pest Problems

Topics covered in this section


Compiled by Ron Gardner

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