Determining Pest Counts and Action Thresholds in the Greenhouse

Posted by Brian Sparks|April 26, 2022

In a recent post on the Rutgers Cooperative Extension Plant and Pest Advisory page, Steve Rettke of the Agricultural and Natural Resources Program outlines how there are essentially three options available when scouting your greenhouse crops for insect/mite pests.

No scouting performed, with pesticides being applied on a calendar timetable.
Simply scouting for pest existence, with pesticides applied when presence is observed.
Scouting crop and making pesticide application decisions based on pest counts and action thresholds.

The third option is part of an integrated pest management (IPM) approach that has been promoted throughout the green industry the past few decades.

“Greenhouse pest populations are measured by trapping or direct plant inspection, and both involve determining pest numbers,” Rettke says. “Counting pests and using action thresholds requires time and knowledge, but results in less pesticide use, reduced potential for insect resistance, and can improve plant quality. It is important to remember that trapping (e.g., yellow, or blue sticky cards) improves the efficiency when scouting your greenhouse but does not replace the actual inspection of individual crop plants. This is particularly the case when scouting for aphids and mites.”

In the post, Rettke outlines sticky card counts and action thresholds for several primary greenhouse pests: Western flower thrips, aphids, fungus gnats, whiteflies, and two-spotted spider mites.

“The scouting and counting of insects/mites help to detect when they are first present. Therefore, treatments are made before large populations build up, but not before it becomes necessary,” Rettke says. “Tracking pest numbers over time allows for the use of action thresholds, or when pest density levels threaten crop salability and economic loss. When pest densities and damage are low, it is not efficient to spend 95% of your time controlling the last 5% of the pest.”