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UC Pest Management Guidelines


Monitoring field of flowers.

Floriculture and Ornamental Nurseries

Establishing Treatment Thresholds

(Reviewed 3/09, updated 3/09)

In this Guideline:


The presence of a few pest insect or mites and some amount of damage usually can be tolerated. The number of pests and level of damage beyond which treatment should be taken is known as the treatment threshold, a fundamental concept in integrated pest management. 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 crops, pests, and different growing situations. Specific thresholds or control action guidelines may be difficult to determine, but thresholds can be developed over the long term by growers who regularly monitor crops and keep and evaluate good records.

Why Use Thresholds. Pesticides sometimes are applied on a calendar schedule, when pest presence is only suspected or when populations are already high and difficult to control. Using thresholds can maintain or improve crop quality while reducing the frequency of pesticide applications. Less frequent applications help maintain pesticide efficacy by reducing the development of pesticide resistance. They also reduce disruptions to cultural practices that occur during applications and reentry intervals. In addition, fewer applications may improve plant growth and quality by minimizing phytotoxicity. Finally, they increase profit by reducing costs of pesticide purchases, application labor, and regulatory compliance.

When to Treat. Because crops are grown for profit, treatment thresholds are based largely on economics. Control action is warranted when the increased revenue expected from improved crop quality or yield will exceed the cost and adverse impacts (such as phytotoxicity) of control. 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. Tolerance to pests can be higher if infested plant parts are not marketed, such as older leaves on seed crops or cut terminal 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, thresholds may be relatively low for that pest. In certain situations, regulations such as quarantines may impose zero pest tolerance even when populations are low and pests do 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 crop production cycle, pests can develop through many generations before plants are shipped. Abundant pests on young plants may require repeated management actions and greatly increase the likelihood of damaged, poor-quality plants.

Treatment 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 be maintained pest-free throughout their production cycle. As crops mature, they are 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.

If monitoring reveals very low pest problems near the end of production, it may not be necessary to take control 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 systematically monitoring plants, keeping good records, and judging the acceptability of the finished crop in comparison with pest monitoring and control records. Experiment over time to develop thresholds appropriate for your situation. 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, the percent of plants or leaves found to be damaged or infested during visual inspection, or the number of pests dislodged per 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, control action may be warranted for whiteflies when more than about five adults per trap per week are captured on one well-maintained 3-by-5-inch (7.5-by-12.5 cm) yellow sticky trap deployed per 1,000 square feet (90 sq m) of production. Thresholds for other situations may be very different from this.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Insects and Mites
J. A. Bethke, Entomology, UC Riverside
Acknowledgment for contributions to Insects and Mites:
K. L. Robb, UC Cooperative Extension, San Diego County
H. S. Costa, Entomology, UC Riverside
R. S. Cowles, Connecticut Agricultural Experiment Station, Windsor, CT
M. P. Parrella, Entomology, UC Davis

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