Thrips

Several species of thrips can cause a mottling on foliage and flowers of a wide range of garden and glasshouse plants.

Western flower thrips ( Frankliniella occidentalis ) on Primula obconica

Western flower thrips (Frankliniella occidentalis) on Primula obconica

Quick facts

Scientific name: Insect order Thysanura
Plants affected: A wide range of plants can be affected
Main symptom: Mottled foliage and flowers
Most active: April-September

What are thrips?

Thrips (also known as thunder flies) are an order of small insects, many of which feed by sucking sap from leaves and flowers.

Thrips vary in colour but otherwise show little obvious diversity in their appearance. The adult insects are narrow bodied and up to 2mm in length. They have two pairs of strap-like wings, which are fringed with hairs, and these are folded back over the dorsal surface of the thrips when it is at rest. The immature nymphal stages are wingless, generally creamy yellow and paler than the yellowish-brown or blackish-brown adults. There are several species of thrips that can cause damage in glasshouses and gardens.

They can be hard to detect, shaking foliage over a white sheet of paper or similar can reveal the thrips which can then be examined, ideally with a magnifying lens.

Symptoms

Leaves damaged by thrips often become dull green and later develop a silvery-white discoloration on the upper surface. The discoloured areas are usually marked by many tiny black excrement spots.

When thrips feed on developing tissues at the shoot tip or in flower buds they are likely to cause distorted growth. Flower petals are marked by a white flecking where the pigments have been lost and heavy attacks may prevent flower buds from opening.

Some thrips, such as onion thrips and western flower thrips, can transmit plant viruses.

The problem

There are several species of thrips that can cause damage in glasshouses and gardens.

Gladiolus thrips (Thrips simplex) Mainly attacks gladiolus during July to September, but also on freesia, causing white flecks on foliage and flowers. Heavy attacks cause the petals to turn brown and buds fail to open. To prevent overwintering thrips from feeding store corms in a cool frost-free place.

Pea thrips (Kakothrips pisivorus) Found on garden peas during June to August; causes stunted growth with a silvery brown discolouration on the foliage and pods; the latter may remain flat with just a few peas developing at the stalk end.

Privet thrips (Dendrothrips ornatus) The adults are brownish-black with a banded appearance of white and dark markings on their wings. They feed on the leaves of privet and lilac during May to October, resulting in the gradual development of silvery-brown foliage by late summer, and may cause some premature defoliation.

Banded palm thrips (Parthenothrips dracaenae) Occurs all year round on various glasshouse and houseplants, especially those with relatively tough leaves such as Ficus, Dracaena, Citrus, Monstera, Schefflera and palms. A blackish-brown thrips with banded brown and white wings that causes extensive silvering of the leaves.

Western flower thrips (Frankliniella occidentalis) A North American species present in Britain since 1986.  Attacks the foliage and/or flowers of many glasshouse plants, especially tomato, cucumber, streptocarpus, African violet, fuchsia, gloxinia, achimenes, pelargonium, cyclamen, chrysanthemum, verbena, Primula obconica and Impatiens. Feeding causes silvering of the leaves, stunted growth, flecking and premature senescence of flowers.  When buying houseplants, check the flowers carefully and avoid any showing signs of thrips or pale flecking on the petals.

Onion thrips (Thrips tabaci) Causes a whitish mottling on the leaves of onion and leeks. It also attacks a wide range of other plants, including carnation, chrysanthemum, begonia, cyclamen, dahlia, tomato and cucumber. It causes silvering of the foliage and on some plants, such as dahlia, feeding at the shoot tips results in severely stunted growth and distorted leaves.

Honeysuckle thrips (Thrips flavus) Attacks many garden plants but is most frequently found on honeysuckle during May to October. The foliage becomes extensively silvery-brown, especially on honeysuckles growing in warm sheltered situations, such as against a wall.

Glasshouse thrips  (Heliothrips haemorrhoidalis) Occurs in heated glasshouses throughout the year, causing a silvery discoloration of the upper leaf surface of many plants. The foliage is also marked with small brownish spots caused by the thrips' excrement. The adults are blackish-brown with yellowish-brown nymphs. This thrips can also survive out of doors all year round and attack various garden shrubs in sheltered situations, especially Viburnum tinus.

Control

Non-chemical control

  • Biological  control in the form of a predatory mites, Amblyseius species, Hypoaspis species and Macrocheles robustulus (sold as Mighty Mite), are sometimes available to control thrips in greenhouses. A predatory bug Orius laevigatus is also available.  
  • Sticky traps, widely available from plant centres, can be used to monitor thrips in glasshouses. Where available blue traps can be more efficient at trapping thrips than yellow


Chemical control

  • Thrips can be controlled by spraying infested plants thoroughly with insecticides
  • On food plants pesticides are more restricted and manufacturer’s instructions must be followed for each food crop including maximum dosage and harvest interval, organic products can be used on most edible plants. For pea thrips, some formulations of deltamethrin, lambda-cyhalothrin, pyrethrum or plant oils and extracts can be used; onion thrips can show resistance to many of the products but gardeners who wish to try spraying can use deltamethrin, pyrethrum or plant oils and extracts; for onions only pyrethrum or plant oils and extracts can be used
  • Two or three applications may be required as eggs and soil-dwelling stages will not be affected and Western flower thrips can be particularly difficult to control as it has a high degree of tolerance to many insecticides
  • Leaves damaged by thrips feeding will not regain their green colour but new growth will develop normally once control has been achieved
  • Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Defenders Bug Killer, Ecofective Bug Killer (also contains fatty acids)), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic Pest & Disease Control, Origins Bug Control) can give good control of thrips. These pesticides have a very short persistence and so may require reapplication to keep thrips numbers in check
  • More persistent insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Ultimate Fruit & Vegetable Pest Killer) and cypermethrin (e.g. Py Bug Killer)
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) can also be used
  • Plants in flower should not be sprayed due the danger to pollinating insects
  • Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Downloads

Pesticides for gardeners ( Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biological controls and their suppliers  (Adobe Acrobat pdf document)

Biology

A typical thrips will lay up to 100 eggs at a rate of one or two per day. These are often deposited on the younger leaves or in flower buds of host plants. Eggs hatch into nymphs which, like the adult insects, feed by sucking sap.  There are two feeding nymphal stages before they go into pre-pupal and pupal stages. These non-feeding stages take place in the soil and/or in sheltered places on the host plant. The feeding nymphs are entirely wingless; wing buds are present on the pre-pupal and pupal stages, although wings are not fully formed until the adult thrips emerge.

The length of the life cycle varies and is affected by temperature. Under ideal conditions the life cycle is completed in 24-35 days and thrips in glasshouses may continue breeding throughout the year. Thrips on garden plants usually have two or three generations a year but may have more during hot summers. Outdoor thrips overwinter as adults or larvae, either in the soil or concealed on the host plant.


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