Thrips

Some 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

Quick facts

Scientific name: Insect order Thysanoptera
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. A few species can cause damage to garden plants.

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 (nymph) 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 discolouration 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 affects 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 feeds on 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) Found on 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 discolouration 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 feed on various garden shrubs, especially Viburnum tinus.

Control

Check susceptible plants frequently from spring onwards so action can be taken before a damaging population has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.

Non-pesticide control

  • Tolerate some thrips damage, despite extensive silvery mottling established outdoor shrubs will usually survive the presence of glasshouse thrips 
  • Biological controls are available for use in glasshouses. These include predatory mites, Amblyseius species, Hypoaspis species and Macrocheles roibustulus (sold as Mighty Mite). There is also a nematode biological control sold as Fruit and Vegetable Protection’.  These can be purchased from biological control suppliers (downloads pdf document)   
  • Hanging blue sticky traps (widely available from garden suppliers) above or among the plants in glasshouses can trap thrips and help monitor and reduce numbers. This is not recommended out of doors as the traps will capture a large number of non-target invertebrates

Pesticide control

  • Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of thrips. These pesticides have a very short persistence and so may require reapplication to keep thrips in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults. 
  • Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action (e.g. Ecofective Bug Control, RHS Bug and Mildew Control, SB Plant Invigorator and Westland Resolva Natural Power Bug & Mildew). These are not considered organic.
  • More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
  • In glasshouses it is possible to use glasshouse fumigants. Glasshouse should be sealed and instructions on the product label must be followed. An organic fumigant based on garlic is available as Pelsis Pest-Stop Biofume Greenhouse Fumigator and can be used when crop plants are present. Products based on the synthetic pyrethroid permethrin are available as DeadFast Greenhouse Smoke Generator, DeadFast Greenhouse Smoke Fumigator
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number applications, spray interval and harvest interval.
Plants in flower should not be sprayed due to the danger to bees and other pollinating insects.
Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.

Downloads

Pesticides for gardeners (pdf document)
Biological control suppliers (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 nymphs, either in the soil or concealed on the host plant.


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