The following is information on scouting and monitoring for aphids in the greenhouse.
It is probably not possible to use action thresholds to manage aphid populations. If winged aphids are found on sticky cards, then populations are usually already high. As a result, plant inspections are the only reliable way to scout for aphids. To simplify scouting efforts attempt to group aphid-susceptible plant species together. (e.g., chrysanthemum, sunflower, gazania, portulaca, pepper, and others).
The distribution pattern of aphids in the greenhouse is typically spotty, with clumped populations (e.g., Melon aphids). On the other hand, Green Peach aphid species have a greater tendency to sometimes move throughout the crop. This behavior forces scouting to be more widespread. Look for plant symptoms such as distorted, discolored terminal tissue and for various aphid signs such as honeydew, sooty mold, cast skins and the actual aphids themselves.
Aphid damage to pepper plant. Photo from the University of Connecticut.
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