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UC Pest Management Guidelines


Adult western flower thrips.

Lettuce

Western Flower Thrips

Scientific Name: Frankliniella occidentalis

(Reviewed 10/09, updated 10/09)

In this Guideline:


DESCRIPTION OF THE PEST

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. Western flower thrips vary greatly in regards to the color of the adults; 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 spring, populations build up on weeds and other vegetation and move into lettuce when these plants begin to dry out. On lettuce plants, adults reproduce and rapidly colonize into large populations.

DAMAGE

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 life. 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.

MANAGEMENT

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.

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

Cultural Controls
Western flower thrips feed on virus-infected weeds, ornamentals, or other vegetation surrounding the field; then they fly into the lettuce field and transmit the viruses. Therefore, managing thrips includes removing weeds and other possible hosts of tospoviruses from around lettuce fields, plowing down and destroying old, harvested lettuce fields to reduce spread of thrips to younger fields. Avoid planting downwind from crops that harbor thrips, such as small grain crops. Using clean culture by quickly removing plant residues from harvested crops before thrips migrate to later plantings helps reduce populations. In addition, sprinkler irrigation can help suppress thrips, because it washes them off plants.

Monitoring and Treatment Decisions
Monitor fields regularly. This can be done when monitoring for aphids and other pests. In the low desert, thrips tend to be a problem late in the season (March & April); in coastal areas population levels are highest from May through October.

Thrips can generally be found throughout the plant, feeding on the undersides of leaves but often hide in difficult-to-find-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 3-5 thrips are found on a small plant, there is probably 3 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. Treat 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 2-3 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 it is more difficult to obtain with larger plants.

Common name Amount/Acre** R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider information relating to the impact on natural enemies and honey bees and environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. SPINETORAM
  (Radiant SC) 6–10 fl oz 4 1
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: Often used only for thrips control and not for caterpillars to reduce the potential for the development of insecticide resistance.
 
B. SPINOSAD
  (Entrust)# 1.25–2.5 oz 4 1
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: Often used only for thrips control and not for caterpillars to reduce the potential for the development of insecticide resistance.
 
C. METHOMYL*
  (Lannate SP) 0.25–1 lb 48 0.25–0.5 lb: 7
  over 0.5 lb: 10
  . . . or . . .
  (Lannate LV) 0.75–3 pt 48 0.75 to 1.5 pt: 7
  over 1.5 pt: 10
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: Will also control loopers. Do not use if leafminers are present. Caused leaf area reductions of nearly 38% in seedlings of the Mesa variety.
 
D. ZETA-CYPERMETHRIN*
  (Mustang Max) 4 oz 12 5
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: For use on head lettuce only. Do not use if leafminers are present.
 
E. LAMBDA-CYHALOTHRIN*
  (Warrior) 2.56–3.84 fl oz 24 1
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: Do not exceed 0.3 lb a.i./acre/season. Do not use if leafminers are present.
 
** Mix with enough water to provide complete coverage.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
* Permit required from county agricultural commissioner for purchase or use.
1 Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.
# Acceptable for organically grown produce.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
Insects and Other Arthropods
E. T. Natwick, UC Cooperative Extension, Imperial County
Acknowledgment for contributions to Insects and Mites:
W. E. Chaney, UC Cooperative Extension, Monterey County
N. C. Toscano, Entomology, UC Riverside

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