B6 – Thrips

Introduction

Thrips are common pests of protected herbs. The damage caused by their feeding can make fresh cut and potted herbs unmarketable.

Recognition and host plants

The most common thrips species damaging protected herbs is the western flower thrips (WFT), Frankliniella occidentalis. Herbs commonly damaged include basil, chives, mint, parsley, tarragon and thyme. WFT is also a widespread pest of other protected crops e.g. cucumber, pepper and many ornamental species, and can also occur on outdoor crops, e.g. ornamentals and strawberry. The onion thrips, Thrips tabaci, also occurs on both protected and outdoor herbs, particularly on chives. Onion thrips are also common pests of other protected and outdoor ornamental and vegetable crops, particularly alliums e.g. leek and onion. Recognition of different thrips species is difficult, and identification to species can only be done using a high power microscope and with specialist knowledge. However, pest species can easily be distinguished from ‘incidental’ species such as cereal thrips, which fly into glasshouses but do not damage herb crops. The presence of thrips larvae on plants indicates that the thrips are breeding on the crop and that damage is likely to be occurring.

WFT adults are small and slender, about 2 mm long, with fringed wings held along the back when at rest, although these can be held apart on sticky traps. The female is usually yellow at the front end and brown at the back (Fig. 1 and 2). The male is yellow and slightly smaller than the female (Fig. 2). The eggs are not visible as the female lays them in plant tissue. There are two larval stages, both of which are wingless. The first stage larva is very small and clear or white. The second stage larva is yellow and about the same length as the adult (Fig. 4). There are two pupal stages, the pre-pupa and the pupa. Usually these are not visible on plants as most of the larvae drop to the ground to pupate in the growing substrate or on the bench or floor. The pre-pupa has short immature wing buds and forward-pointing antennae (Fig. 5). The pupa has longer immature wings and the antennae are folded backwards over the body (Fig. 6).

The onion thrips is very similar in appearance to WFT, but the adults are all female in the UK. The adults are greyish yellow to brown (Fig. 3) and the second stage larvae are greenish. The larvae drop to the ground to pupate as in WFT. Grain or cereal thrips, e.g. Limothrips cerealium, often fly into glasshouses during the summer months, particularly during harvesting of nearby cereal crops. They are commonly known as ‘thunderflies’ or ‘thunderbugs’ as they take flight in large numbers in the warm, humid conditions that are often followed by thunderstorms. Cereal thrips adults are dark brown or black and can be distinguished from either WFT or onion thrips on sticky traps (Fig. 7). Cereal thrips do not damage or breed on herbs.

Both WFT and onion thrips have very wide host ranges, including many weed species. Weed hosts for WFT include chickweed, clover, fleabane, mallow, shepherd’s purse and thistle.

Quarantine thrips species: Thrips species currently listed as quarantine pests (e.g. Thrips palmi and Scirtothrips dorsalis) are not recorded as significant pests of herbs, and there have been no recent interceptions of quarantine thrips species on imported plant material. However, S. dorsalis is now established on other crops e.g. peppers in Israel, and there is a risk that this thrips species could be imported as a ‘hitch hiker’ on young herb plants. S. dorsalis adults are very small, about 1 mm long, with pale yellow bodies and dark wings. The larvae are pale and similar to those of other thrips species. T. palmi adults are small, about 1 mm long with a yellow body. The larvae are creamy yellow to pale orange and up to 1 mm long. Any suspect thrips should be immediately reported to your local Defra Plant Health and Seeds Inspector (PHSI). Contact details are available from FERA PHSI, Tel: 01904 465625, website: www.fera.defra.gov.uk then selecting the plants, bees and seeds tab followed by the plant health tab.

Symptoms

Thrips adults and larvae feed by piercing plant cells and sucking out their contents, producing small white or silvery flecks or patches on the leaves. This symptom is common on chives, basil and mint (Figs 8 and 10). Small black faecal specks can usually be seen within the bleached areas (Fig. 9). Thrips damage can be confused with leafhopper damage. Thrips feeding marks are more irregular than the circular spots made by leafhoppers, and the thrips flecking often has a silvery appearance (Fig. 10). Thrips feeding on newly developing leaves can also cause leaf crinkling and distortion when the leaf grows, e.g. on parsley and tarragon. WFT can transmit tospoviruses, including Tomato spotted wilt virus (TSWV) and Impatiens necrotic spot virus (INSV). These viruses have a wide host range including basil and mint and have occurred on other protected crops in the UK, particularly ornamentals, but so far have not been a problem on herbs. The onion thrips can transmit a similar tospovirus, Iris yellow spot virus (IYSV). The first and only case of IYSV in the UK was confirmed in 2008 on cut flower lisianthus. Chive is a known host of IYSV in other countries but not yet in the UK. This virus is notifiable to PHSI in the UK. See Section B.10 for a description of damage symptoms and further details.

Sources of infestation and favourable conditions

On protected herbs, both WFT and onion thrips can be found all year round. Bought-in plant material is a possible source of thrips. However, the main source of the pests is often a preceding infested crop or an infested adjacent herb crop within the glasshouse or tunnel. Adult thrips can fly from crop to crop and can also emerge from pupae in the soil, compost or bench or floor covering. Other sources of the pests are crops in adjacent structures, outdoor host crops, or weeds in or around the glasshouse or tunnel. Both WFT and onion thrips can survive mild winters outdoors. Thrips development and activity are favoured by warmer temperatures. WFT can develop between 10°C and 35°C, the development rate varying with temperature and host plant. For example, on chrysanthemum, the time from egg to adult is about 39 days at 15°C but only 13 days at 25°C. The onion thrips has a similar development rate at 25°C on cucumber. Thus the pests can breed very quickly during the summer months and can go through many generations per year, particularly in heated structures.

Integrated Pest Management (protected herbs)

Cultural control

• Check all incoming plant material for thrips or damage.
• Maintain strict weed control in and around glasshouses and tunnels.
• Dispose of unwanted infested plants promptly and carefully.
• Clean bench or floor coverings between crops, as thrips adults can take flight when plants are being removed and may survive on plant debris.
• In addition to the blue or yellow sticky traps used for monitoring thrips, extra traps may be useful in doorways or under vents, to catch thrips adults flying from batches of infested plants, or migrating in from outdoor hosts. Recent research in Horticulture LINK project HL01107 (SF 120) has shown that use of long ‘roller’ traps in protected strawberry can trap large numbers of thrips and reduce fruit damage (Cross et al., 2013). The ‘roller’ trap may also catch flying beneficial insects, e.g. parasitic wasps, so they should only be used if necessary and should be positioned with care.

Biological control

Several biological control agents are commercially available for the control of thrips. Timings and rates of release within an Integrated Pest Management (IPM) programme should be planned carefully. If necessary, seek advice from the supplier or a consultant.

1. Neoseiulus (Amblyseius) cucumeris
This predatory mite gives very good control of both WFT and onion thrips on protected herbs. Usually, this is the only biological control agent that needs to be used for thrips control on herbs. The adult mites are oval, straw-coloured and about 0.4 mm long (Fig. 11). Although they are very active, they are difficult to find on herb plants as they are usually in the same hidden places as the thrips larvae on which they feed. As N. cucumeris feeds only on young thrips larvae and not on the adults or pupae, successful control depends on a preventive introduction programme. On herbs, controlled-release sachets are usually used from crop emergence (Fig. 12) and these give effective control of thrips throughout the life of a 5-6 week herb crop, as the predators emerge from the sachets over several weeks. N. cucumeris can also be supplied in tubs with bran or vermiculite as a carrier, but if using this method, the predators need to be sprinkled over the plants weekly. N. cucumeris can develop at temperatures between 8°C and 35°C, but needs a relative humidity of at least 65% in the leaf microclimate for the eggs to survive.

2. Amblyseius swirskii
This predatory mite feeds on thrips larvae and also on whitefly eggs and young scales, and is very similar in appearance to N. cucumeris (Fig. 13). A. swirskii has only been available in the UK since 2006. The predator is not native to the UK and only has a licence for release in fully protected glasshouses or tunnels. Amblyseius swirskii needs warmer temperatures than N. cucumeris; the minimum temperature for activity is 15°C and optimum temperaturesare 25-28°C. Most of the research with the predator has been done in the Netherlands on protected salad crops e.g. cucumber and pepper. There is very little current experience on using A. swirskii on protected herbs, although UK growers are beginning to experiment with it. Initial results have been promising on mint, which is susceptible to both thrips and whitefly.

3. Hypoaspis miles and Hypoaspis aculeifer
These ground-dwelling predatory mites are primarily used for the control of sciarid flies. However, they will also feed on other prey in the compost, soil or on floor or bench coverings, e.g. thrips pupae and springtails. They should not be relied upon as the sole method of thrips control, but could supplement control when used in combination with other biological agents e.g. N. cucumeris. The adult mites are up to 0.8 mm long, off-white with a pale brown shield covering most of the upper surface of the body (Fig. 14). Both H. miles and H. aculeifer need moist compost, soil or substrate and temperatures above 15°C. They are very active and can be seen running on the substrate surface or under pots and trays.

4. Macrocheles robustulus
This predatory mite can give some control of thrips by feeding on the ground-dwelling pupal stages. See section B3 (sciarid flies) for further details.

5. Steinernema feltiae
The entomopathogenic nematodes Steinernema feltiae were first developed as compost drenches for the control of sciarid fly. More recently, they have also been used successfully on chrysanthemum crops, as foliar applications against WFT. Defra-funded research (Bennison, 2006) has shown that although the nematodes can kill WFT adults and larvae on the plants, their key role in controlling WFT is by killing the ground-dwelling life stages, i.e. larvae that drop from the plants to pupate, and the pupal stages in the ground (Fig. 15). Weekly sprays of the nematodes are needed for WFT control. Single drench treatments applied for the control of sciarid flies may give some incidental benefit against thrips but should not be relied upon for thrips control.

6. Orius laevigatus and Orius majusculus
These predatory ‘flower bugs’ feed on both adults and larvae of thrips. They also feed on pollen, and can establish well on flowering crops in advance of thrips infestation. Orius are used successfully on pepper crops in the UK, but have not been used on protected herbs to date, as most herb crops grown for culinary use do not flower, and N. cucumeris is usually effective against thrips.

7. Lacewings
Lacewing larvae, Chrysoperla carnea, are mainly used for aphid control, but they will also feed on other prey including thrips. See Section B.4 for further details.

8. Verticillium lecanii
The ‘whitefly’ strain of this insect-pathogenic fungus (‘Mycotal’) can also give some control of thrips (Fig. 16). See Section B.5 for further details.

9. Beauveria bassiana
This insect – pathogenic fungus (‘Naturalis-L’) can give some reduction in thrips. See section B5 (whiteflies) for further details.

10. Metarhizium anisopliae
Another fungus, (Met52) is commercially available and has an Extension of Approval for Minor Use (EAMU) for control of thrips pupae when used as a mulch or a pre-planting soil treatment in protected and outdoor herbs.

11. Atheta coriaria
This ground-dwelling predatory ‘rove beetle’, which is sold mainly for control of sciarid and shore flies, will also eat thrips larvae and pupae in the ground (after the larvae have dropped to the ground to pupate). See section B.2 and B.3 for further details of this predator.

Monitoring within IPM

• Adult thrips should be monitored using sticky traps (see Section A – Principles of IPM). Blue traps are more attractive to WFT than yellow ones, but thrips of all species are easier to see on yellow traps.
• Both thrips adults and larvae should be monitored on foliage.
• Amblyseius and Neoseiulus spp. are difficult to find on herb plants and efficacy is best assessed by monitoring thrips numbers and damage.
• Hypoaspis spp. can be seen running over the substrate or under pots and trays, but efficacy is best assessed by monitoring thrips numbers and damage.

Chemical control (protected herbs)

Chemical control of thrips is difficult. WFT is resistant to most pesticides, and UK populations of onion thrips collected from salad onion and leek crops have recently been shown to be resistant to the pyrethroid insecticides. In addition, both thrips species are difficult spray targets, as the adults and larvae tend to feed within young unfurled leaves, and the pupal stages in the ground are protected from foliar sprays.

A pesticide should only be used if necessary and should be selected carefully, taking into account pesticide compatibility within IPM (see Section A – Principles of IPM) and harvest interval (see Table 2 on the homepage). Due to existing and potential problems with thrips resistance to pesticides, it is very important to follow Resistance Management Guidelines when using a pesticide (see Section A – Principles of IPM and Table 2 on the homepage).

For specific information on safety of pesticides to individual biological control agents, consult your biological control supplier. General guidelines on the selection and compatibility of pesticides within IPM are given below:

‘Safe in IPM’
The following products are safe to biological agents once spray deposits are dry, and may give some control of thrips. Both act by contact only, so good coverage of the undersides of the leaves is necessary:

• Eradicoat or Majestik (maltodextrin). Acts by physical means. Approved for use on any protected edible or non-edible crop.

‘Moderately harmful’
The following products are harmful to some biological control agents:

• Tracer (spinosad) has a EAMU for use on protected herbs and may be effective against both WFT and onion thrips. The product is safe to many biological control agents but is harmful to parasitic wasps e.g. those used for leaf miner or whitefly control, for up to 2 weeks after application. Specific Resistance Management Guidelines are given on the EAMU and it is very important that these are followed carefully. Over-use can lead to WFT resistance to spinosad, and this has already occurred e.g. in ornamentals and strawberry in the UK.
• Calypso or Agrovista Reggae (thiacloprid) is a neonicotinoid insecticide with an EAMU for use on protected herbs. The product may be effective against both WFT and onion thrips*, but is more harmful to certain biological control agents than nicotine.

* Specific Resistance Management Guidelines are given on the EAMUs for neonicotinoid insecticides, see Section A – Principles of IPM and Table 2 on the homepage).

‘Harmful’ in IPM
The pyrethroid pesticides (cypermethrin and deltamethrin) have EAMUs for use on protected leafy herbs, and pyrethrins (Spruzit and Pyrethrum 5 EC) have approval for use on both outdoor and protected edible crops. However, both WFT and onion thrips are resistant to pyrethroids in the UK, so these pesticides should not be used against thrips. Pyrethroid insecticides are harmful to biological control agents for up to three months after application, thus they are incompatible with IPM. Pyrethrins are harmful to biological control agents for only a few days after application, so could be used with care within IPM programmes, but thrips resistance to pyrethrins is likely in the UK.

• Full details for the use of biological control agents and compatibility of pesticides are available from biological control suppliers or consultants. Also see Section A – Principles of IPM.
• Pesticide approval information in this guideline is current at 29 April 2013.
• Regular changes occur in the approval status of pesticides arising from changes in pesticide legislation or from other reasons. For the most up to date information, please check with a professional supplier or the CRD website http://www.pesticides.gov.uk/. General enquiries on pesticides and detergents are now being handled by the Defra Helpline (as of April 2013): Tel: 08459 33 55 77.
• Always follow label recommendations or statutory conditions for use on Extension of Authorisation for Minor Use (EAMU) notices of approval.
• Always follow instructions for Pesticide Resistance Management guidelines given on the label or EAMU.
• Growers must hold a paper or electronic copy of the current EAMU before using any product under the EAMU arrangements. Any use of a pesticide with a EAMU is at grower’s own risk. Relevant EAMUs are sent to HDC members by HDC, or are available from CRD (see above) or from consultants.
• Use pesticides safely.

Further information

HDC Factsheet 14/09. Thrips on bedding and pot plants.

HDC Factsheet 19/08. Iris yellow spot virus: A potential threat to the onion industry.

HDC Factsheet 23/10. Tomato spotted wilt virus protected edible crops.

Bennison, J.A. (2006). Expoiting knowledge of western flower thrips behaviour to improve efficacy of biological control measures Final Defra report www.defra.gov.uk/science/project_data/DocumentLibrary/HH3102TPC_3988_FRP.doc

Bennison, J.A. (2006). Protected bedding and pot plants: Evaluation of WFT control by Atheta coriaria using an on-nursery rearing system. Final report to HDC on project PC 261.

Cross, J, et al. (2013). Biological, semiochemical and selective chemical management methods for insecticide-resistant western flow thrips on protected strawberry. Second Annual report to Horticulture LINK and HDC for project HL0110 and SF 120.