The following is information on scouting and monitoring for whiteflies in the greenhouse.
Although the use of yellow sticky traps can improve scouting efficiency, when scouting for whitefly it is especially important to also inspect crop foliage. It is critical to start scouting early so whitefly populations are not allowed to build-up. High populations of whiteflies are one of the more difficult pests to suppress in the greenhouse.
Typically on infested plant foliage a consistent top to bottom distribution of whitefly growth stages can be observed. For example, adults will usually be found on the undersides of the upper canopy leaves. When inspecting for eggs, concentrate on the undersides of lower adjacent leaves just below the upper canopy. Smaller scales (1st /2nd instar nymphs) are then found on the undersides of foliage below the leaves containing eggs. Larger scales (3rd/4th instar nymphs) are found on the undersides of the next level of lower/older foliage. Finally, whitefly adults will be emerging from pupae found on the lowest/ oldest leaves closest to the soil media.
Similar to aphids, whiteflies often produce sticky honeydew with the corresponding growth of the black sooty mold fungus. If this becomes readily visible, then it is certain that high whitefly infestations are already present within the crop.
When using biological controls (e.g., Encarsia formosa (parasitic wasps)) it is necessary to estimate counts of whitefly scales (nymphs) within a pest management unit in order to determine how many beneficials to release. It has been determined a release ratio of 30:1 (scale to wasp) will prevent a population build-up of whiteflies. An even smaller release ratio of 150:1 (scale to wasp) will only be required if most of the scale nymph counts are early 1st/2nd instars. When using any kind of biological control tactic, it is crucial to start releases early before high pest levels are reached.
Greenhouse whitefly nymphs. Photo by David Riley, University of Georgia, 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