Chapter 21

Integrated Pest Management for Greenhouse Crops

Developing Economic Thresholds

A decision to use a pesticide should be made only when a pest population has reached or exceeded an economic threshold (ET), a fundamental concept in integrated pest management. The economic threshold (sometimes called an action threshold) is the pest density (number of pests per unit area) at which control efforts are triggered so as to prevent pest populations from reaching the economic injury level. Thresholds should be quantitative (numerical) to be useful. For example, they could be based on the average number of pests per trap each week, the percentage of plants or leaves found to be damaged or infested during visual inspection, or the number or size of weeds for a given area. The economic injury level (EIL) is the pest population density that causes losses equal to the cost of control measures. To justify using a control method, it is necessary to set the ET below the EIL (See Figure 21.3). Otherwise, producers lose money—first from the damage caused by the pest, and then by the cost of the control method. Setting the ET below the EIL triggers the appropriate control method before pests reach the EIL. The principal components used derive ETLs as the damage caused by a single or set number of pests; the cost of controlling the pest; and the value of the crop. Typically, a destructive insect feeding on a high valued crop results in a low economic threshold value. A low valued crop, combined with a less destructive insect results in a higher ETL.

Treatment Thresholds

Thresholds vary with crop, stage of plant development, cost of control methods, type of pest, and time until harvest and market. The amount of pest presence or damage that can be tolerated is determined by many factors, including the type of pest and damage, crop species and cultivar, stage of plant development, time until harvest or sale, and market conditions. For instance, the action threshold for whiteflies on greenhouse tomatoes may be quite high compared to whiteflies on a poinsettia crop. Ornamental plant consumers expect that the blooming plants be insect and disease free for a proper display in their home; the tomato consumer, however, does not care if whiteflies dwell on the tomato vine in the greenhouse, but the fruit must be clean and blemish free.

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