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How to Manage Pests

UC Pest Management Guidelines

Using a yellow sticky trap in a greenhouse crop.

Floriculture and Ornamental Nurseries

Monitoring with Sticky Traps

(Reviewed 3/09, updated 3/09)

In this Guideline:

Adults of whiteflies, thrips, fungus gnats, leafminers, psyllids, shore flies, winged aphids, and parasites can be monitored with yellow sticky traps. Traps warn of pest presence, hot spots, and migration. Traps provide a relative measure of insect density; comparisons of the number of adults caught among sample dates may indicate whether pest density is changing or remaining relatively constant over the long term. Traps are often a very efficient and important monitoring tool, alerting growers to pests early, before damage is observed in crops.

Traps may not be a good tool for deciding treatment need or timing. Immature stages in crops commonly cause most damage, and traps typically capture only airborne adults. Adult trapping sometimes is not a reliable indicator of pest presence or abundance on the crop, and traps must be used in combination with visual inspection of plants.

Unless other guidelines are recommended, use at least one sticky trap per 10,000 square feet (900 sq m) of growing area. When monitoring whiteflies, use about one trap per 1,000 square feet (90 sq m) of growing area. Although actual trap density will be dictated by the growing area and the time and effort devoted to trapping, each pest management unit should have at least one trap. In addition, put one trap by vents and by doors to detect populations migrating in. Also, put a trap in very susceptible crops and do not locate these crops near doors. Use bright yellow traps, each 3 by 5 inches or larger. If western flower thrips is the primary species of concern, consider using blue sticky traps.

Orienting traps horizontally (facing the soil) is sometimes recommended when monitoring pests such as fungus gnats emerging from media. However, in most programs, to catch the most insects, orient the longest part of the trap vertically (up and down). Place each trap so that the bottom of the trap is even with the top of the plant canopy. For rapidly growing crops, locate the trap's bottom a few inches above the canopy so that the plants don't overgrow the traps. As plants grow, move each trap up so that its bottom remains about even with the top of the canopy. Use one or two clothespins to attach each trap to a bamboo post or wood dowel embedded in the growing media or stand. Alternatively, hang traps from rafters or wires strung between posts.

Number each trap and map its location in your growing area. Inspect each trap at least once or twice weekly. It is easiest to replace traps each time you inspect them. Traps can then be wrapped in clear plastic and taken to a more comfortable location for counting. Alternatively, replace traps when they become too fouled to count insects quickly. If traps are reused, note this because catches become cumulative and you must subtract the number of insects present last time the traps were checked.

Because many insects in TRAPS may be harmless or beneficial, carefully identify insects before taking action. High-quality color photographs and line drawings of commonly trapped insects are available in Sticky Trap Monitoring of Insect Pests, ANR Publication 21572. You can also wrap used traps in clear plastic (such as Saran Wrap) to preserve them and take the traps containing unknown pests to a Cooperative Extension or county agricultural department expert for help in identification.

Count the number of each type of pest caught and keep a record of these data. It is not necessary to count all insects on the entire trap; counting the insects in a vertical, 1-inch-wide (2.5-cm-wide) column on both sides of the trap will give results that are representative of the entire trap. Do not reduce traps to 1-inch vertical strips, as smaller traps may be less attractive to insects.

Interpreting Trap Information. Regularly summarize trap data to facilitate comparison, for example, by graphing the average of all traps from each sample date. Interpreting trap information requires knowledge, skill, and practice and may be more art than science. Traps catch both migrating insects as well as adults that emerged from the crop. Canopy density, plant foliage quality, and temperature influence adults' tendency to fly. Wind and ventilation fans can discourage flight, reducing trap catches. The number of adults trapped may temporarily drop after a pesticide application, even if there has been relatively little change in immature populations on foliage. Conversely, adult numbers of some species may temporarily increase in traps after applying an adulticide, so the numbers caught for several days after an application should not be used when comparing adult densities among sample dates. Foliage disturbances, such as sprinkling with water or shaking plants to promote pollination or monitor adults, increase trap catches. Even large numbers of pest species in traps do not necessarily indicate that control action is needed. For more information, see ESTABLISHING TREATMENT THRESHOLDS. Always use traps in combination with foliage inspection to confirm presence of an economically damaging population.



[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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