Texas Agricultural Extension Service
UC-017

WESTERN FLOWER THRIPS
ON ORNAMENTAL PLANTS

Bastiaan M. Drees
and
Charles L. Cole
Extension Entomologists

Thrips are bladder-footed insects from the order Thysanoptera, meaning fringed wings. There are 6,000 thrips species in the United States. Most important are the onion, citrus, greenhouse, gladiolus, Cuban laurel and western flower thrips (Thrips tabaci, Scirtothrips citri, Heliothrips haemorrhoidalis, Gynaikothrips ficorum and Frankliniella occidentalis, respectively). They are characterized by having a single (left) mandible, thought to have evolved from their pollen-feeding behavior. This sword-like mandible is extruded when the mouth cone is compressed on plant tissue. Saliva lubricates the mouthparts and causes some external digestion to occur. The extruded mandible slashes open epidermal cells. The contents of the opened cells are then sucked in through the cone. Distribution of the western flower thrips in Texas, 1989

Thrips are important pests of ornamental plants because of the physical damage caused by their feeding activities to tissues of developing and mature flowers and leaves. Thrips also feed on pollen and some species are predaceous. In addition, thrips can vector disease. The tomato spotted wilt virus (SPWV) can be transmitted by six species of thrips, three species in the United states: onion, tobacco (F. fusca) and western flower thrips (WFT). The WFT is found throughout Texas (Fig. 1).

Thrips adults and larvae are thigmotactic, preferring to hide in complex plant parts, flower buds and other folded tissue where they are difficult to detect and to reach with insecticides. Some plants are more attractive to thrips (WFT) than others (chrysanthemum, gloxinia, impatiens, tomato, vegetables and grasses).

Western flower thrips life cycle

Life cycle

Adult female lays eggs inside of plant tissue (Fig. 2). One female can lay 150 to 300 eggs. Just before hatching the egg "oozes" out of the tissue and hatches. Within hours the larvae molt to the second instar. They molt into a second larvae instar and then into a pre-pupal, non-feeding, stage which crawls to the soil (although sometimes it stays in flowers) where it pupates. If the plants have tomato spotted wilt (TSWV), the adult emerging from the pupa will be infected. At 68oF, the WFT takes about 19 days to develop from egg to adult, and the adult can live for about 57 days. At higher temperatures, development can occur within 8 days. Adult thrips fly when temperatures exceed 63-65oF when light is over 100 ft candles (This may be the best time to apply insecticides).


Disease transmission

Thrips feed on epidermal, and subepidermal mesophyll cells. Thrips acquires TSWV from infected plant tissue. There is latent period (biological interaction) before transmission. Larval thrips acquire the virus (perhaps adults do also, but fail to transmit it due to a long latent period) after feeding for 15-30 minutes. During the latent period the virus travels to the midgut where it enters the hemolymph and moves to the salivary glands. Multiplication of the virus within the thrips may occur. The latent period is 3-12 days. The virus is retained by the infected thrips for life. Transmission is via the salivary fluid. If damaged cells survive long enough, the virus can enter adjacent healthy cells to complete transmission. Adult and larval thrips can transmit the virus. The virus is not transmitted to subsequent generations through egg production. Research has suggested that spraying with milk (proteins) may interfere with transmission (tissue recognition). Stylet oils do not trap virus particles, but rather are selectively toxic to newly infected cells on the plant.


Natural enemies

Insidious plant bug, Orius tristicolor; predaceous thrips, Aeolothrips duvali; and a parasitic wasp, Thripoctenus americensis.

Non-chemical methods of suppression:

  • control weeds to avoid virus reservoirs and thrips harborage areas
  • avoid continuous cropping
  • produce non-susceptible crops in alternation with the production with susceptible ones
  • start with noninfested stock
  • use exclusion screen (over vents and doors, and even within the greenhouse) using appropriate materials (such as Visqueen®, Visqueen Film Products, Customer Service, P.O. Box 2448, Richmond, VA 23218, (804/788-6222) assuring no reduction of air flow by employing appropriate design principles available from the manufacturer
  • use fresh media or apply a soil treatment (steam sterilization) between crops (copper sulfate)
  • isolating stock plants, rooting areas from production areas
  • isolate crop cohorts
  • group sensitive plants together (those supporting high populations and those showing early symptoms)
  • avoid putting sensitive plants close to vents or doors
  • worker education: transplanting, pinching, watering, destroy prepupal stages
  • dispose of plant residues quickly
  • rogue infected plants
  • thrips populations are sometimes controlled or suppressed by rain, overhead irrigation or misting
  • monitor adult populations

    Chemical control:

  • different responses in different greenhouses
  • eggs (in leaf tissue), larval (in flower parts, rolled leaves, buds) and pupal (in soil) stages of thrips are often protected form exposure to insecticide applications
  • thrips adults are highly mobile and actively migrate to and within the greenhouse
  • determine whether the population is a migratory population of adults or a breeding population within the greenhouse
  • WFT is tolerant/resistant to many insecticides (organophos phates, carbamates, synthetic pyrethroids) and populations often explode during or after insecticide treatment programs applied for other pests
  • dip plants entering the green house
  • adult thrips fly when temperatures exceed 63-65oF when light is over 100 ft candles (This may be the best time to apply insecticides)
  • thrips adults and larvae are thigmotactic, preferring to hide in complex plant parts, flower buds and other folded tissue where they are difficult to detect and to reach with insecticides
  • droplet sizes of less than 100 microns are best for thrips control since this droplet size penetrates the foliage best
  • treatment schedule: 2 to 3 applications 5 days apart. Assure good penetration of foliage
  • rotate insecticides by class (every 2 weeks, based on the thrips generation time)
  • see alternate method of application to increase coverage and retard resistance development
  • avoid tank mixtures
  • sugar has improved the efficacy of some insecticides when added to the spray by acting as a feeding stimulant for thrips, but it is not effective when used with contact insecticides

    Source: Proceedings of the Fourth Conference on Insect and Disease Management on Ornamentals, 1988 (Ali, Hall and Parella, ed.), Society of American Florists


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    1990

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    Last modified: March 18, 1997 by Edgar Cross