Larger cards tend to catch a greater diversity of insects than smaller cards, and trap catches tend to increase with size. Thrips Light blue cards are more effective in trapping WFT and may be used over thrips-sensitive plants such as roses or African violets. Yellow cards are more effective for general pest monitoring, and adult thrips may be easier to see on a yellow card compared to a blue card. Use a handlens to distinguish adult thrips from grains of peatmoss and other debris. WFT adults are narrow, 0.04 to 0.08 inches long, with 2 pairs of fringed wings that are held parallel over their body.
Females tend to be dark brown and slightly larger than the small, tan or yellow males.
Dark brown to black onion thrips may occasionally be caught on cards as they enter the greenhouse from outside. Placing cards at bench level tends to catch more thrips compared to card placement at hanging basket or floor levels. Card counts may be higher at the ends of the greenhouse, where the passively carried thrips tend to drop out of air circulation patterns. Under mist propagation, fewer thrips will be caught on sticky cards, as adults are less able to fly, due to the water on their fringed wings. In February, it may be helpful to place cards just above the floor level, to detect any early emerging thrips that have overwintered in the greenhouse.