Because they are tiny (only about 1/25" long) and difficult to see, thrips are most obvious through what they leave behind, for example, dark, fecal pellets and whitened, desiccated plant tissue resulting from mass feedings. With magnification, you can identify the adults as elongated, slim insects which range in color from yellow to black and have double, fringed wings; nymphs are even smaller, wingless, and range in color from yellow to white. Eggs are inserted into leaves, fruit, or stems. Nymphs and adults puncture flower buds, leaves and stems with their single large mandible then slurp the plant juices that seep from the wound.
Symptoms of thrip infestations will be most often apparent on flowers, especially white, yellow, and other light-colored blooms. Thrips puncture petals, causing considerable discoloration and disfiguration. Leaves and onion foliage become bleached or silver, then wither; flowers and fruit become scarred. Heavy infestations will result in scorched-looking leaves and destroyed blossoms. Flowering plants favored by thrips include annual aster, dahlia, daylily, iris, gladiolus, hollyhock, lily, peony, rose, tuberous begonia, and wax begonia. Favored vegetable plants include beans (lima and squash), cabbage, carrots, corn, cucumbers, onions, peas, potatoes, squash (summer and winter) and tomatoes. Among fruits, pears are susceptible to thrip damage.
Because thrips burrow deeply into petal and leaf tissue, controls are most effective when applied as soon as thrips or their damage is spotted. Knock thrips from leaves with a forceful spray of cold water, preferably early in the morning. Use a nozzle that produces a fine spray of water, and thoroughly wash off the undersides of affected leaves. This control is most effective when repeated at least 3 times, either daily or every other day. Alternately, spray infested plants with insecticidal soap every 3 days for 2 weeks. Treat more serious infestations with 2 applications of pyrethrum applied 3 to 4 days apart, making sure you get both upper and lower leaf surfaces. Prune away affected flowers and dispose of them; do not compost them. Since thrips prefer a dry environment, you can discourage their presence by keeping plants adequately misted and watered. Keep in mind that Spanish variety onions are resistant to thrip damage as are a number of cabbage varieties.
Specific recommendations for flowers. Iris thrips produce stunted growth and rusty or sooty areas on leaves. The tops of infested plants eventually turn brown and die out. Serious infestations can kill nearly all of the roots and otherwise leave the plants vulnerable to fungal infection. To control iris thrips, spray insecticidal soap at the base of each plant every 5 to 7 days until the infestation has been controlled. To prevent thrips problems, soak iris divisions and gladiolus corms for 30 minutes in a hot water bath (110-125 degrees F). Dig up gladiolus bulbs early in the fall, let them cure for a few weeks before soaking them, and store at 40 to 45 degrees through the winter before planting again in early spring.