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Thrips are small, slender insects that are less than 2 mm long. Thrips have two pair of wings fringed with fine hairs. Their colour varies from pale yellow to light or dark brown.
Adult males are usually smaller and paler in colour than females. Colour can vary depending on temperature, being darker when development occurs at cooler temperatures. Western flower thrips have red-coloured pigment in their eyes, while onion thrips eyes are grey.
Eggs are ovoid, translucent and are laid in soft plant tissue.
Larval forms resemble the shape of adults but are smaller, paler in colour and lack wings.
The prepupal and pupal stages are also similar to adults, and can be recognised by presence of wing buds.
A 10x hand lens is needed to distinguish life cycle stages. A microscope and expert assistance is needed to identify the species.
Distribution and host range
Onion thrips and western flower thrips are found widely throughout New Zealand on a broad range of crops.
Onion thrips is a key pest of onions, but will also attack the young growth of capsicum, cucumbers and the flowers of some flower crops.
Western flower thrips attacks a wide range of vegetable and ornamental crops and weeds. A greenhouse strain of this pest is prevalent on greenhouse crops of capsicum, cucumber, egg plant and flower crops including chrysanthemum, carnation, gerbera and rose. It is found on many weed species, including black nightshade, cape gooseberry, and Amaranthus.
Signs and symptoms
Thrips have piercing sucking mouth parts and feed by puncturing the surface of flowers and young leaves and sucking up the plant juices. Both adults and nymphs cause damage. Thrips are often concealed inside flowers making them difficult to see.
Signs and symptoms of thrips include:
Onion thrips and western flower thrips are vectors of tomato spotted wilt virus which causes disease in capsicum, lettuce and causes bronze top on tomato. Some ornamental plants are also affected by this virus. Western flower thrips also transmits impatiens necrotic virus.
Life history and habits
The life cycles of thrips species are essentially similar, and under greenhouse conditions the life cycle is continuous and all stages can be found at any time.
Eggs are inserted into young leaf, leaf stem or flower tissue singly, and the tip of one end of the egg remains visible at the surface. Females can produce up to several hundred eggs in their life time, but this depends on the host plant, availability of pollen and environmental conditions.
After hatching, the small larva moves to a protected position to begin feeding. A second stage larva emerges following a moult and continues feeding.
The prepupa and pupa, are non-feeding stages which progressively develop wing pads. Most prepupae drop to the soil where they complete pupation. Some pupa may remain on the plant in protected positions.
The duration of thrips life cycles is determined by temperature. At 15 °C, a life cycle from egg to adult may take 35-45 days, whereas at 30 °C a life cycle can be completed in 12-15 days.
Thrips adults are capable of flight, and are likely assisted for longer range dispersal by air currents. Onion thrips flights are more common in spring and autumn. Flying thrips are readily attracted to sticking insect traps, and certain attractant chemicals improve trap catches.
High populations of thrips can cause serious economic loss if left uncontrolled. Damage resulting from direct feeding and transmission of virus disease can both occur. Thrips damage to young plants can be most severe.
Thrips can also become a contaminant of flower crops, and tolerance of the presence of thrips on flower crops for export markets is very low.
Monitoring should be used to detect the invasion of thrips into a crop and to indicate the need for control measures. Once established, thrips populations can be difficult to control. The greenhouse strain of western flower thrips is resistant to a number of insecticides, as is onion thrips, and thus control options may be limited in some crops.
Reducing the invasion of thrips into crops and early detection are the key to successful thrips management. An integrated approach to pest management is needed to effectively apply insecticide resistance management strategies.
Adult thrips are easily detected on sticky insect traps and can provide an indication of the presence of thrips. True economic thresholds for thrips have not been established for most crops, and the relationship between the numbers of thrips caught on traps and on a crop is highly variable.
White, yellow and even hot-pink traps will attract thrips, but blue traps are generally recommended as they trap fewer other insects and make detection of thrips easier.
Hang traps near openings just above the top of the crop. It is important to move traps up as the crop grows. Examine the traps weekly.
In some countries, indicator plants, e.g., petunia have been recommended to assist with the monitoring of thrips and virus problems. Sticky traps indicate only flight activity and relative population size, while indicator plants can be used to monitor feeding activity and virus problems.
When biological control agents are being used for thrips control, they should be released when traps first indicate the presence of thrips.
A number of practices are recommended to prevent or minimise the establishment of thrips on greenhouse crops. These include:
A variety of natural enemies (predators, parasites, disease) has been researched for the management of thrips. The predatory mite, Amblyseius cucumeris, is most commonly used against thrips in greenhouse crops.
The pirate bug, Orius vicinus shows great potential as a thrips and generalist predator (including psyllids) on greenhouse crops, and its relatives have been used with huge success in the northern hemisphere.
The predatory mite, ( Hypoaspis aculeifer ), which is normally released for fungus gnat control, may also provide some control of thrips that are pupating in soil or growing media.
Amblyseius cucumeris is available from BioForce Ltd, who sells the product Mite-A ™.
These predators are most effective when used in an Integrated Pest Management programme, when the use of harmful pesticides are avoided and effective crop management practices are adopted.
Biological control agents might not be suitable export flower crops because the tolerance of insects and mites on crops is very low. Consult with BioForce Ltd before applying predators to any export flower crop.
A range of insecticides is claimed to be effective against thrips, although few have specific registration claims for use on greenhouse crops. The western flower thrips strain that invades protected crops (i.e glass, plastic and tunnel houses, as well as indoors) is known to be resistant to many of the thrips insecticides on the market.
The larvae and adults of thrips are more susceptible to insecticides than eggs, pre-pupa and pupa. Insecticide applications should therefore be targeted at larvae and adults.
Resistance to insecticides by thrips is known to occur in New Zealand and elsewhere. Care must be taken to not apply more than the recommended number of applications of insecticide from any one chemical in a year.
Insecticide applications should be made as cluster of sprays, rather than at regular intervals throughout the crop cycle, e.g., 3 sprays, 3-5 days apart depending on the chemical used. A different chemical group should be used at each cluster of sprays.
Apply sprays to crops in a manner to ensure good coverage of the undersides of leaves. Crops recently de-leafed allow better spray penetration. Spot spray ‘hot spot’ areas to minimise the spread and build of greenhouse whitefly in a crop.
Care should be taken to consider the effect of insecticides on beneficial insects or mites that may be being used in integrated pest management programmes. Consult with BioForce Ltd before applying any insecticide when beneficial insects or mites are being used.
If you are uncertain about the identity of any pest in your crop, or need advice on the management of pests contact BioForce Ltd, email@example.com .