The following is information on monitoring for western flower thrips in the greenhouse.
Western Flower Thrips
There are no universally accepted thresholds for the western flower thrips (WFT) because of numerous variables that cause the threshold number to change. A general guideline to start with might be 15 thrips per yellow sticky card per week per 1000 sq. ft. This arbitrary number is only a suggested starting point and it may often be necessary to refine your own action thresholds with experience.
If releasing predatory mites for biological controls it may be necessary to begin when as few as 2 thrips/ysc/ wk/1000sqft are observed. Plants that are sensitive to thrips damage such as African violet and streptocarpus crops may have a threshold of less than 10 adult thrips captured on sticky traps per week per 1000 sq. ft. Alternatively, moderately sensitive plants such as impatiens, rose, gerbera, mum, and gloxinia crops may have action thresholds ranging as high as between 18 to 30 thrips/ trap/wk/1000sqft. (If tospoviruses (INSV or TSSV) are present within a crop, then thrips thresholds are essentially one (1)). A poinsettia crop has a low sensitivity to thrips damage and can have an action threshold of 40 or more adults captured per trap/wk/1000sqft.
When these various threshold guidelines are reached it should be a signal to begin examining individual crop plants more closely, especially those plants closest to the sticky traps. However, the distribution pattern of the Western Flower Thrips in the greenhouse is random. Therefore, the thrips could potentially be found anywhere throughout the greenhouse. Some methods to scout for thrips on plants include the following: 1- Tapping the plant (especially flowers) over a piece of white paper to dislodge the thrips. 2- Exhaling carbon dioxide on the flowers in order to agitate the thrips and coerce them to leave their cryptic hiding places. 3- Pullingback and closely examining the nectar producing flower organs with a hand lens to detect thrips presence.
Western flower thrips adult and nymph. Photo by Jack T. Reed, Mississippi 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