Western Flower Thrips

Adult western flower thrips are minute, slender-bodied insects possessing two pairs of long, narrow wings, the margins of which are fringed with long hairs. The bodies of adult thrips can be yellow, orange, brown, or black. Color of adults western flower thrips varies greatly; there are light, dark, and intermediate "morphs." When resting on foliage, the flower thrips abdomen extends beyond the wing tips, and thick, bristlelike hairs can be seen at the tip of the abdomen. Nymphs are white or yellow with small dark eyes. The larvae are white, yellow, or orange. In the spring, numbers increase on weeds and other vegetation and move into lettuce when these plants begin to senesce (dry out). On lettuce plants, adults reproduce and rapidly colonize into large numbers.

Western flower thrips feed on lettuce and vector plant viruses. Thrips cause injury to lettuce by puncturing leaves and sucking the plant sap. Punctured leaves take on a silvery appearance that eventually turns to brown scarring and can be confused with windburn or blown sand damage. Look for the presence of small, black fecal specks in the damaged area to confirm thrips damage.

Western flower thrips is the most important vector of Tomato spotted wilt virus and the only known thrips species to vector Impatiens necrotic spot virus. Only the larval stage can acquire these tospoviruses. They ingest the virus when they feed on infected plants and remain infective throughout their lives. As juveniles grow into adults and develop wings, they fly to other plants and spread the pathogen. For more information on symptoms of these viruses see TOSPOVIRUSES.

Managing vegetation in and around lettuce, biological control, and cultural practices are important in reducing the potential for damage from western flower thrips. Adjacent crops such as grapes and citrus can also harbor thrips. When thrips are present on the lettuce crop, insecticides are often the only viable control alternative.

Natural enemies, including predaceous mites, minute pirate bugs, and lacewings, are often found feeding on thrips. These natural enemies are very susceptible to insecticide sprays, however, and may not be present in fields where insecticides have been used.

Western flower thrips feed on weeds, ornamentals, or other vegetation surrounding the field that might be infected with virus; then they fly into the lettuce field and transmit the viruses.

• Remove weeds and other possible hosts of tospoviruses from around lettuce fields.
• Quickly remove or plow down plant residues from harvested lettuce fields to reduce spread of thrips to younger fields.
• Avoid planting downwind from crops that harbor thrips, such as small grain crops.
• Sprinkler irrigation can help suppress thrips, because it washes them off plants.

Monitor fields regularly. This can be done when monitoring for aphids and other pests.

• In the low desert, western flower thrips are a problem in January–March.
• In coastal areas thrips numbers increase in April and continue through the year.

Thrips can generally be found throughout the plant, feeding on the undersides of leaves and especially in difficult-to-check-and-treat places such as leaf folds. Several methods are available to monitor thrips:

• Carefully examine plant parts for the presence of thrips and feeding scars, including folds in leaf tissue near the base of the plant. If three to five thrips are found on a small plant, there is probably three times as many hidden within folds in the leaves or that had dispersed from the plant.
• Place blue or yellow sticky traps along the field edges when temperatures exceed 63 to 65°F during the day, to indicate when adult thrips begin to immigrate into field from adjacent vegetation.
• Beat lettuce plants from different parts of the field over a sheet, tray, or sticky surface where they can be counted and identified. Morning is the ideal time for beating, as adults are less active then.

Treatment guidelines have not been developed; treatment is usually made at the first sign of injury. Initiate treatments when thrips numbers are low and scarring on young leaves is first observed, particularly when temperatures are increasing. Apply selected insecticide in the afternoon when adults are most active. Several products are registered; spreading surfactants help insecticides reach areas where larvae are hidden.

Following treatment, sample at 2 to 3 day intervals. Application frequency will depend on product residual activity and immigration of adults from surrounding vegetation. Plant size is an important factor contributing to insecticide efficacy. Good coverage underneath the leaf and near the base of the plant is more difficult to obtain with larger plants.