Determining the Need for Management
Part 2, Section 1: Pest Management
Activities Involved in an IPM Program
Determining the need for management
After isolating the cause(s) of crop injury, it is necessary to determine if the amount of injury an potential for further injury warrant a management alternative. Entomologists and economists have developed indicators to help producers decide whether a particular management option is justified based on economic, social, and environmental constraints.
The most common decision rule used in pest management is the economic threshold (Figure 2.1-3). The economic threshold (ET) is the number of pests that, if present in the field at a given time, in all probability will increase and cause economic losses. This threshold contrasts with the economic injury level (EIL), which is the number of insects that cause a dollar loss equal to the cost of a management action. By implementing a management strategy when the economic threshold is reached, a farm manager keeps pest populations from reaching the economic injury level. Therefore, losses associated with an insect population above the economic injury level are prevented. The economic threshold provides a buffer period to allow adequate time to implement a management strategy.
The ET and EIL are not static; they vary as economic and biological conditions change. Factors that can influence these indices include weather conditions, plant growth stage, nutritional health of the crop, presence of other pests, market value of the crop, expected yield of the crop, and the cost and effectiveness of the pest management alternative used. Which of these variables to take into account in estimating ET values depends on how much is known about a pest's biology, its interaction with its host, and the economic environment.
Economic thresholds for the potato leafhopper and alfalfa weevil vary with crop height, cost of an insecticide application, and crop value. Corn rootworm ET values are dependent on whether beetles are being monitored for the first year of corn in a rotation or later years. When little information is available about a pest's biology and its interaction with its host, single simple threshold values are used. Because little is known about the influence these pests have on crop yield and quality, ET values are typically conservative toward using an insecticide treatment. Therefore, simple threshold values tend to err toward protecting the crop when the pest would not have caused an economic loss. Conservative thresholds prevent severe crop damage. Most thresholds established for field and forage crop insects are single simple thresholds.
Figure 2.1-3 shows how economic thresholds and economic injury levels can vary as the crop matures. In the case of alfalfa, lower numbers of potato leafhoppers are required to cause economic damage on the second cutting than the third cutting, because a higher percentage of the insects population is female. The more rapid population growth in the second cutting is a direct result of the offspring produced by these females.