The following is information on scouting and monitoring for fungus gnats in the greenhouse from Rutgers University.
Monitoring for Fungus Gnats
When using yellow sticky traps to capture adult fungus gnats it is most effective to place traps horizontally (flat) near the root medium. Sticky traps placed in this position typically increases catch by 50% over traps set-up in the traditional vertical position at canopy level. Adult fungus gnats are weak flyers and generally will not be found in high numbers around the tops of crop canopies. Yellow traps should also be placed under benches if the floor is not cement.
Potato disks or wedges placed within the medium to attract fungus gnat larvae can determine density counts. The disks are typically 1 to 2 inches in diameter and are pressed ½ inch into the root medium. The wedges (French fry shape) are approximately ½ inch square and 1.5 to 2 inches long. The disks are best used in propagation areas while the wedges are best used with more established, deeper-rooted crops. Place the disks every 100 sq. ft. in propagation areas and the wedges every 1000 sq. ft. in production areas. Count fungus gnat larvae feeding on potato 48 hours after placement in media. It has been shown that after 72 hours the potato pieces may dry-out and lose their drawing capabilities. Or worse yet, the pieces may begin to rot, promoting a breeding ground for the larvae.
Some action thresholds have been determined for fungus gnat larvae when using the potato disks. Within propagation areas as few as 3-5 larvae per disk (after 48 hours) can cause significant damage to the small, shallow root systems. Alternatively, when using the potato wedges in a 6-inch pot, it may require as many as 15-20 larvae per disk (after 48 hours) before any meaningful root damage occurs.
Fungus gnat larvae. Photo by Whitney Cranshaw, Colorado State University, Bugwood.org
Information from "Pest Counts and Action Thresholds in the Greenhouse" by Steven K. Rettke, Ornamental IPM Program Associate, Rutgers University in the August 20, 2009 edition of the Plant & Pest Advisory, Landscape, Nursery & Turf Edition; A Rutgers Cooperative Extension Publication