Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Establishing Action Thresholds

The presence of a few pest insects or mites and some amount of arthropod damage commonly is unavoidable and can be tolerated. The number of pests or level of damage beyond which management action should be taken is known as the action threshold, a fundamental concept in integrated pest management. When management action should be taken (when a threshold is exceeded) is determined by the total cost of the action (including monitoring), the value of the crop, and the impact on the environment. Few thresholds have been established for flower and nursery crops, in part because of the lack of research in comparison with the large number of crop plants, pests, and growing situations. Specific thresholds or management action guidelines may be developed over the long term by growers who regularly monitor crops, keep good records, and evaluate and summarize outcomes for comparison over time. Consult the chapters "Integrated Pest Management" and "Insects, Mites, and Other Pests" in the Container Nursery Production and Business Management Manual and also Integrated Pest Management for Floriculture and Nurseries for more discussion and illustration of these techniques.

Why Use Thresholds

Pesticides sometimes are applied on a calendar schedule, when pest presence is only suspected, or when pest numbers are already high and difficult to control. Using thresholds can maintain or improve crop quality while reducing the cost and frequency of control measures. Less frequent applications help maintain pesticide efficacy by reducing the development of pesticide resistance. Fewer applications reduce the disruptions to cultural practices that occur during applications and the subsequent restricted entry interval (REI). In addition, fewer applications may improve plant growth and quality by minimizing phytotoxicity and increase profit by reducing costs of pesticide purchases, application labor, and regulatory compliance.

When to Take Management Action

Because crops are grown for profit, action thresholds are based largely on economics. Management action is warranted when the increased revenue expected from improved crop quality or yield will exceed the cost and adverse impacts (such as phytotoxicity, harvest disruption) of the action. The amount of pest damage or presence that can be economically 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. Tolerance to pests can be higher if infested plant parts are not marketed, such as older leaves on seed crops or cut flowers. Thresholds can often be higher if highly effective or quick-acting methods are available for controlling the problem. Conversely, if available controls are slow-acting or only partially effective, or crops are of an exceptionally high value, thresholds may be relatively low. In certain situations, regulations such as quarantines may impose zero tolerance for exotic organisms even when numbers are low or an organism does not directly damage the marketed crop.

Mother stock and new plants should have virtually no pests. If pests are present at the beginning of the production cycle, many arthropods can develop through multiple generations resulting in large populations before plants are shipped. Abundant pests on young plants may require repeated management actions and greatly increase the likelihood of damaged, poor-quality plants.

Action thresholds may be higher for mature plants of certain crops. More mature plants are often better able to tolerate some level of certain types of pests or their damage. It is unlikely that susceptible crops can always be maintained pest-free throughout their production cycle. As crops mature, they may be increasingly likely to become infested and are often more difficult to treat effectively because of the risk of phytotoxicity to colored bracts or flowers, increased difficulty in achieving good spray coverage on larger plants, and pesticide reentry intervals relative to crop harvest or shipping.

If monitoring reveals very low pest abundance or damage near the end of production, it may not be necessary to take management actions because there may be insufficient time for populations to develop to problem levels before the crop is sold.

How to Establish Thresholds

Establish thresholds by regularly monitoring plants in a consistent and systematic manner. Keep good records and judge the acceptability of the finished crop in comparison with records of pest monitoring and management actions and the price received for the crop. Experiment over time to develop thresholds appropriate for your situation and market conditions. Be flexible in adjusting thresholds and adapt monitoring and management methods as appropriate.

Thresholds should be quantitative or numerical to be useful. For example, thresholds could be based on the

  • average number of pests per trap each week
  • percent of leaves or plants found to be damaged or infested during visual inspection
  • number of pests dislodged per beat or shake sample

Quantitative thresholds can be developed for most pest monitoring methods, such as treating when certain conditions are conducive to disease development or when invertebrate pests or damaged plant parts exceed specified numbers or percentages. For example, management action may be warranted for whiteflies early in production when more than about 5 adults per trap per week are captured on one well-maintained 3-by-5-inch yellow sticky trap deployed per 1,000 sq. ft. of production. Thresholds for your situations may be very different.

Text Updated: 03/21