Western flower thrips feeding causes bloom distortion and poor fruit set. Egg-laying punctures create "pansy spot" on some cultivars. Adults can be sampled using either beating trays or blue sticky cards.
Adult thrips are attracted to blooming plants, and an apple orchard in bloom constitutes a huge number of flowers relative to weeds or native plants. Recent research suggests that thrips prefer to lay their eggs in vegetative tissues, but some percentage will also be laid in the fruit; this makes it difficult to relate the numbers of adults found to the potential damage. In the past, we have recommended sprays before or during bloom; however, newer information indicates that the best time for preventing damage is at about 5 mm (about 1/5 inch) fruitlet size (closer to petal fall than to bloom). This timing has the added benefit of greater safety to pollinators. Note that this newer timing for optimum thrips control is NOT the same timing as optimum Campylomma control.
(Elizabeth Beers, WSU-TFREC)