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Thrips

Ravageurs et maladies
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Thrips.
Photo: Jardin botanique de Montréal (Daniel Fortin)

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Description

Summary

Over 5,000 species of thrips have been identified around the globe, hundreds of them in North America. Western flower thrips (Frankliniella occidentalis) and onion thrips (Thrips tabaci) are the greenhouse species that cause the most damage in Quebec. Not all species of thrips are harmful, however. Some eat small insects (including other thrips) and mites. Six-spotted thrips (Scolothrips sexmaculatus), for instance, are predators of spider mites. Many species feed only on spores of fungi, moss, algae and lichen and live in plant litter and dead wood.

Phytophagous thrips attack a wide range of ornamental and food plants. These gregarious insects pierce the cells of leaves, flowers, stems, buds and fruit and suck out their contents. Damaged tissues become spotted, banded or scarred and are often deformed. In addition to weakening plants, thrips may also transmit viruses.

Signs and symptoms

  • Thrips use their modified mandibles to pierce plant tissues, and suck out the juices. The empty cells then fill with air, causing tiny white or silvery spots to appear. When there is considerable damage, these spots form silvery streaks down the length of the leaves. Damaged flowers have pale or dark spots, depending on the colour of the petals (white flowers seem to be more attractive to thrips). Attacked tissues are often deformed and may dry out and drop prematurely.
  • Thrips often feed inside developing buds. Their damage is not apparent until the leaves and flowers open: leaves may be misshapen, dwarfed or stunted and petals scarred and deformed. Buds may also open only partially or not at all.
  • Thrips feces (small, shiny black deposits) are usually visible on damaged tissues.
  • Thrips also feed on the nectar and pollen of plants (such as African violets), causing flowers to age prematurely.
  • Fruit damage from thrips' egg laying and feeding shows up as corky scars, lines or spots and/or deformed fruit.
  • These pests may also damage bulbs in storage. The plants grown from these bulbs will be stunted, with reduced blooms.
  • Some species cause galls to form on their hosts.
  • These insects weaken plants and stunt their growth.
  • Some thrips transmit viruses to plants.

Latin name (genus)

Catinathrips, Frankliniella, Heliothrips, Hercinothrips, Neohydathothrips, Taeniothrips, Thrips, etc.

Host plants

Indoor plants: African violet, angel's trumpet, begonia, Cape primrose, crassula, croton, cyclamen, dieffenbachia, ficus, gerbera, gloxinia, hibiscus, impatiens, lady's eardrops, orchid, rhododendron, yucca, etc.

Annuals, perennials and bulbs: aster, chrysanthemum, dahlia, daylily, geranium, gladiolus, iris, lily, peony, petunia, pink, snapdragon, sweet pea, verbena, zinnia, etc.

Trees and shrubs: birch, hydrangea, linden, maple, privet, rose, willow, etc.

Vegetable and fruit plants: apple, asparagus, bean, blueberry, cabbage, carrot, cherry, corn, cucumber, garlic, grape, onion, pea, pear, potato, raspberry, strawberry, sweet pepper, tomato, etc.

Development cycle

Description and life cycle

Thrips belong to the order Thysanoptera. Their metamorphosis is intermediate between complete and incomplete. They usually go through six stages: egg, larva (two stages), “prepupa” and “pupa” (pseudopupal stages) and adult.

Eggs: Kidney-shaped and relatively large compared with the size of the females.

Larvae: Resemble adults, but smaller and wingless. Often cream coloured.

"Prepupae" and "pupae": Resemble adults. "Prepupae" have atrophied wings. "Pupae" have more developed wings and long antennae that curve backwards. During these two stages, they do not feed and remain largely immobile.

Adults: 1 to 2 mm long, resembling grains of rice. Usually yellow, brown, black or white. Males are generally smaller than females. Although thrips rarely fly, their two pairs of feathery wings allow them to be carried by the wind over long distances. Some species are apterous (wingless). Thrips have sucking mouthparts.

While thrips may reproduce sexually, they usually do so by parthenogenesis (without fertilization by a male).

Females may lay 150 to 300 eggs in their short lifetime (30 to 45 days). In most species, a female uses her ovipositor (egg-laying organ) to insert her eggs in plant tissues (leaves, flowers, leafstalks, tender stems, buds and fruit). Species without an ovipositor lay their eggs on plant surfaces. Incubation takes 2 to 8 days. The young larvae (1st instar larvae) feed for a short time before undergoing a first moult. At the end of the second larval stage, they stop feeding, drop to the ground and bury themselves. They then turn into “prepupae” and “pupae” before becoming adults. Some species spend their entire life cycle on their host plants.

Depending on the species, some thrips (often only the females) may overwinter as adults or immature specimens in plant litter, under the bark on trees and shrubs, in the soil or between the scales of bulbs in storage.

The length of the thrips life cycle varies with species and temperature, with hotter temperatures shortening the life cycle. In general, these insects complete their life cycle in 2 or 3 weeks, with several generations a year.

Prevention and control

Favourable conditions

  • While thrips prefer hot, dry conditions, they can tolerate high humidity levels. A film of water on leaves and flowers deters them, however.
  • High-nitrogen fertilizers and drastic pruning promote the rapid growth of tender shoots that attract thrips.
  • Drought-stressed plants are more vulnerable to attacks by these tiny insect pests.
  • Wind is a major factor in their spread. Thrips may also fly, jump, cling to gardeners' clothing and hair or be spread via infested plants and potting soil.

Identification

  • Inspect plant leaves and flowers regularly. Use a magnifying glass if necessary. Since adult thrips are more active early and late in the day, these are the best times to look for them. They jump, take flight or hide when disturbed.
  • Shake or tap flowers or buds over a sheet of white paper to dislodge any insects that may be hiding on them.
  • Indoor: install yellow, white or blue sticky traps (blue traps seems to be most effective) near vulnerable plants to attract adults. Such traps are available from garden centres, but you can make your own by brushing strips of yellow, white or blue cardboard with a sticky substance like petroleum jelly. Remember to clean off or replace the traps periodically.

Prevention

  • Avoid buying any plants that seem to have been attacked by thrips. Inspect all new plants and isolate infested ones.
  • Inspect bulbs before storing them.
  • Weed your garden regularly, because many weeds serve as host plants.
  • Avoid drastic pruning and heavy feeding with high-nitrogen fertilizers.
  • Keep plants healthy by watering them during dry spells and fertilizing them properly.
  • Do not reuse potting soil from plants that have been attacked by thrips.
  • Regularly disinfect pruning tools with a 70% isopropyl (rubbing) alcohol solution.
  • Inspect all susceptible plants regularly in order to act promptly.

Physical control

  • Cut off and destroy infested parts. Do not compost them.
  • Dislodge the insects with a strong, steady stream of water.
  • Repot infested indoor plants, using new potting soil.
  • Regular deadheading helps to reduce populations of certain thrips, by eliminating immature stages before they become adults.
  • In fall, rake up and dispose of all plant litter, to reduce the overwintering population.

Biological control

  • Encourage the presence of natural predators: ladybird beetles, lacewings, spiders, predacious and parasitic wasps, predacious mites, etc., by growing a wide variety of plants and avoiding the use of pesticides.
  • In greenhouses, introducing natural predators and parasites (Hypoaspis spp., Amblyseius cucumeris, Orius insidiosus, etc.) gives good results. This control method is difficult to apply at home, however.

Chemical control

  • Thrips quickly develop resistance to pesticides. In addition, their life cycle and behaviour may make them difficult to control.
  • As a last resort, use a low-impact pesticide with insecticidal soap. Make sure to thoroughly coat the undersides of leaves, young shoots and buds. The best time to do so is early morning or evening, when thrips are most active and thus most apt to come into contact with the soap.

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