Thrips (also known as thunder flies) are an order of insects, the Thysanoptera. Around 150 species are found in Britain. Whilst many feed by sucking sap from leaves and flowers many do not cause noticeable damage to host plants. A few species can however cause mottling to some garden and indoor 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.

Thrips 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.

Leaves fed upon 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 can cause distorted growth. Flower petals are marked by a white flecking where the pigments have been lost and heavy feeding damage may prevent flower buds from opening.

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

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.  Feeds on 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.

Check susceptible plants frequently from spring onwards so action can be taken before a damaging population has developed. When choosing management options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section and avoiding pesticides. Within pesticides the shorter persistence products (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. Pesticide treatments are likely to kill natural enemies and are only likely to be successful if the entire plant can be reached. 

Non-pesticide

• 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 a predatory bug (Orius laevigatus) and 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. The nematodes can potentially affect other insects and the predators can feed on invertebrates other than thrips. These can be purchased from biological control suppliers (downloads pdf document)   
• Encourage natural enemies in gardens, for example the predatory bug Orius laevigatus can occur naturally  
• 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

Pesticides

• If non-pesticide options do not manage the thrips populations, control may be achieved by spraying with pesticides. Spraying at dusk is likely to be more effective
• Organic sprays, such as natural pyrethrum (e.g. Bug Clear Ultra 2, Neudorff Bug Free Bug and Larvae Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, BugClear Fruit & Veg, Rose Clear 3 in 1 Action, Vitax Rose Guard) can give good control of thrips. These pesticides have a very short persistence and so may require reapplication to keep thrips numbers in check. Thrips hidden in distorted leaves may be unaffected by these products. Plant oil and fatty acid products are less likely to affect larger insects
• Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action against thrips (e.g. Ecofective Bug Control, Growing Success Bug Stop and SB Plant Invigorator). These products contain some synthetic ingredients and so are not considered organic
• More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer). These products have long lasting action against insects including those that are beneficial
• Pesticides, with both systemic (absorbed and transported through plant tissues) and contact action, are available. These include Flupyradifurone (Provanto Smart Bug Killer) for use on ornamentals and selected edibles and the neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra). These pesticides are widely considered to be the most environmentally damaging, remain active for a long time and will kill beneficial invertebrates
• 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 2, Vitax Greenhouse Smoke Fumigator

Downloads

Pesticides for gardeners (pdf document)
Biological control suppliers (pdf document)
 

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.