B5 - Whiteflies

Introduction

Whiteflies can infest many species of protected herbs.  The presence of whiteflies and/or their sticky honeydew and the associated sooty moulds are unacceptable on fresh cut or pot herbs supplied to supermarkets.

  

Recognition and host plants

The most common whitefly species found on protected herbs is the glasshouse whitefly, Trialeurodes vaporariorum.  The adults are small, white, moth-like insects, approximately 1 mm long, which tend to hold their wings flat across the back when at rest (Fig.1).  They are usually found on the underside of the leaves and in growing points, and make short flights from the plants when disturbed.  The yellowish, conical eggs are laid on the undersides of leaves. These turn black just before hatching into the first nymphal stage or ‘crawler', which is active for only a few days. The second, third and fourth nymphal or ‘scale' stages are immobile and flattened, and turn from greenish-white when young to white when fully grown. The final scale stage or ‘pupa' is thicker than the preceding stages and has short waxy hairs fringing the oval body (Fig. 2).  The adult whitefly emerges from the ‘pupa' by cutting a slit in the top of the scale.

The  tobacco whitefly Bemisia tabaci, a quarantine pest, can also infest herbs, and imported material is a potential source of this pest.  Tobacco whitefly adults are slightly smaller than those of the glasshouse whitefly, and tend to hold their wings slightly apart and at an angle to the body when at rest, exposing the yellow body (Fig. 3).  The mature scales of tobacco whitefly are yellow rather than white as in the glasshouse whitefly, and are slightly pointed at the rear end (Fig. 4).  

Any whiteflies suspected to be Bemisia tabaci should be immediately reported to your local Defra Plant Health and Seeds Inspector (PHSI).  Contact details are available from:

PHSI HQ, York, Tel:  01904 455174,

or on the Defra website: www.defra.gov.uk/planth/senior.htm

Glasshouse whitefly has a wide host range and commonly-infested herb species include sage, lemon verbena, mint, marjoram, bergamot, basil, balm and rue.   The pest is also common on many protected vegetable and ornamental crops, e.g. cucumber, tomato, fuchsia and poinsettia.  Weed hosts include sowthistle, nettle, chickweed and dandelion.  Tobacco whitefly has a similar host range to glasshouse whitefly and is most commonly imported on young plants or cuttings. 

Symptoms

Whitefly adults, eggs and scales are visible on the undersides of leaves.  In heavy infestations, the sticky honeydew excreted by the whiteflies allows the growth of sooty moulds on the upper surface of leaves below the infested ones.  Both whitefly species can transmit plant viruses, but to date no viral disease of herbs attributable to whitefly transmission has been recorded in the UK.

  

Sources of infestation and favourable conditions

Glasshouse whitefly can occur all year round on herbs under protection and can also survive outdoors in mild winters on host plants including weeds.  The eggs are the most cold-hardy stage and can survive for a few days at temperatures as low as -6°C.  In HDC project PC 178, the most common source of the pest on protected herbs was thought to be infested stock plants (Bennison, 2001).  The pest can also overwinter on production plants, including those not normally acting as hosts, such as protected parsley and coriander grown for cutting.  In cool temperatures, the adults tend to stay within patches of infested plants until overcrowded or disturbed.  In warm temperatures, the adults are more active and readily fly to other host plants,  within the glasshouse or tunnel and/or from other adjacent protected and outdoor crops and weeds.  Nurseries growing other whitefly-susceptible edible or ornamental crops are at particular risk from the pest, especially when infested plants are disturbed or removed, when adult whiteflies may disperse to find other host plants.  Glasshouse whitefly population growth can occur between 8°C and 35°C with the optimum temperatures for development  in the range 20-25°C.

The source of tobacco whitefly in the UK is imported plants or cuttings.  This species is less well adapted to cool temperatures than the glasshouse whitefly and is not permanently established here.  Bemisia cannot survive below 0°C and thus is unlikely to survive UK winters outdoors, although it could survive under protection.  Bemisia population growth can occur between 16°C and 30°C, whilst the optimum temperature range for development is 25-30°C.

  

Integrated Pest Management (protected herbs)

The IPM strategies summarised here are relevant to both glasshouse whitefly and tobacco whitefly.  However, this information should not be relied on if the presence of tobacco whitefly is suspected.  If the presence of this species is confirmed, PHSI will issue a Notice outlining specific measures to eradicate the pest and prevent it spreading to other nurseries.  This will be designed in consultation with the grower and will include full details of the required cultural, biological and/or chemical control methods, as appropriate for the nursery concerned. 

  

Cultural control

• Keep stock plants in separate structures from those used for propagation or production.
• Avoid taking cuttings from infested mother plants.
• Maintain strict weed control in and around glasshouses and tunnels. 
• Dispose of infested plants carefully.
• Clean bench or floor coverings between crops, as whitefly adults can disperse when removing plants, and may survive on plant debris.  
• Yellow sticky traps (in addition to those used above plants for regular monitoring) may be useful in doorways or under vents, or in between batches of plants, to intercept migrating whiteflies.  Long ‘curtain' traps may be used for mass trapping but will also catch large numbers of flying beneficial insects, e.g. parasitic wasps, so should be used and positioned with care.
• Consider using ‘trap' plants that are more attractive to whiteflies than the herb crops. Potted tobacco and aubergine plants have proved successful trap plants for glasshouse whitefly in poinsettia crops in the UK and Canada respectively.  Trap plants need to be managed carefully to avoid them becoming breeding sites for the pest.  Research is being done in Canada to use the aubergine trap plants as ‘banker plants' for whitefly biological control agents and this technique is worth trying in herbs (see below).  

  

Biological control

Several biological control agents are commercially available for the control of whiteflies.   Biological control strategies for whiteflies within an IPM programme should be planned carefully.  If necessary, seek advice from the supplier or a consultant.

1.    Encarsia formosa
This yellow and black parasitic wasp is only about 0.6 mm long (Fig. 5).  Encarsia  is more effective against the glasshouse whitefly than the tobacco whitefly, and kills whiteflies in two ways.  Firstly, it acts like a predator, by ‘stinging' the young whitefly scales and sucking out the body fluids.  This is known as ‘host-feeding'.  Secondly, Encarsia acts as a parasite, where the female wasp lays individual eggs in each whitefly scale (preferring the third or fourth scale stages) and the young parasite then develops inside the body of the whitefly scale.  Parasitised glasshouse whitefly scales turn black (Fig. 2). Those of tobacco whitefly turn yellowish brown, and the yellow and black body of the developing Encarsia adult can be seen inside (Fig. 6).  When fully developed, the adult Encarsia emerges from a round hole cut in the top of the scale.  Host-feeding could play a more important role than parasitism in whitefly control on fresh herbs, on which there is ‘zero tolerance' of pests, as it causes the small scales to shrivel and die before they reach the preferred (and more visible) stage for parasitism.

Encarsia is supplied as black parasitised glasshouse whitefly scales, either on small cards or in tubes.  For successful control of glasshouse whitefly, Encarsia is best introduced weekly, starting as soon as temperatures are suitable and ideally before the first whitefly is seen.  Encarsia can complete its life cycle between 12°C and 35°C but the adult parasites need temperatures above 15°C to be able to fly and thus find whitefly scales efficiently.  Optimum temperatures for Encarsia are in the range 20-25°C, but practical experience, e.g. on early season protected nursery stock, indicates that effective control can be given if temperatures are at least 18°C for a few hours a day.  

Encarsia  gives successful control of glasshouse whitefly on many protected crops, including cucumber, tomato and various ornamentals.  However, use of Encarsia on protected herbs has given variable results.  Possible reasons for this were investigated in HDC project PC 178, and included low early season temperatures preventing Encarsia flight and establishment; inadequate parasite introduction rates; hairy or strongly scented herbs repelling or impeding Encarsia; short herb production periods preventing Encarsia from completing its life cycle on the crop; and lack of effective IPM-compatible pesticides (Bennison, 2001).

The trap / banker plant method is worth trying with Encarsia on herbs.  If whitefly could be lured from herbs to long-season trap plants such as aubergine, on which Encarsia works very well, this could improve biological control and make it more cost-effective.      

2.    Eretmocerus emericus
This parasitic wasp is similar to Encarsia but is yellow in colour. It is more effecive against the tobacco whitefly than Encarsia, and is less susceptible to pesticides.  Eretmocerus will also kill glasshouse whitefly, both by host-feeding and by parasitism, but the parasitised scales are yellow rather than black.  Optimum temperatures for Eretmocerus are above 20°C.  Mixtures of Encarsia and Eretmocerus are available.  If only glasshouse whitefly is present, Eretmocerus should not be needed.            

3. Amblyseius swirskii
This predatory mite feeds on  whitefly eggs and young scales as well as on thrips larvae.  The predator is very similar in appearance to Amblyseius cucumeris, which is widely used for thrips control.  The adult mites are oval, about 0.4 mm long and are pale orange when they have fed on thrips or whitefly (Fig. 7). The predators are supplied either in a carrier in bottles for direct release onto plants, or in paper sachets for placing on or between plants as a controlled release system.  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.  Optimum temperatures for A. swirskii are 25-28°C and the minimum temperature for activity is 15°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 whitefly and thrips. 

4. Lacewings
Lacewing larvae, Chrysoperla carnea are mainly used for aphid control, but they will also feed on other prey including whiteflies.  See the aphid section for further details. 

5.  Verticillium lecanii
This insect-pathogenic fungus (‘Mycotal' Ò) is a microbiological control agent and is thus subject to the PSD approval system (unlike invertebrate predators, parasites and nematode biological control agents).  Mycotal has a SOLA for use on protected herbs, and is supplied as a wettable powder which is applied as a high volume spray.  To be effective, the fungus needs a temperature of at least 18°C and a relative humidity of more than 75% for 10-12 hours after application. Thus it is most suitable for use during warm, humid weather, or in herb propagation houses, or if using crop covers that maintain high humidity.  Once the fungal spores have germinated and the mycelium has colonised the body of the whitefly adult or scale, the fungus grows through the body wall and can be seen as white fluff on the body surface (Fig. 8).  Not all fungicides are compatible with Mycotal, so any used need to be chosen with care and their application timed to cause least harm; if in doubt, check with the Mycotal supplier.

5.  Steinernema feltiae
Foliar sprays of this entomopathogenic nematode have been shown to kill tobacco whitefly scales on various host plants e.g. tomato, cucumber and ornamental plants, in Defra-funded research at Central Science Laboratory, York.  Further research would be needed to determine the potential of the nematodes against whiteflies in commercial conditions.

  

Monitoring within IPM

• Numbers of whitefly adults can be monitored using yellow sticky traps (see Section A - Principles of IPM).
• Check leaf undersides for whitefly adults and scales.
• If using Encarsia, check leaf undersides for percentage parasitised (black) scales.
• Amblyseius swirskii may be difficult to find on herb plants, but might be present on the undersides of young leaves infested with whitefly eggs.

  

Chemical control (protected herbs)

Very few pesticides are approved or effective against whiteflies on herbs.  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).  Whiteflies are commonly resistant to many pesticides and 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:

• Chess WG (pymetrozine) has a Specific Off-label Approval (SOLA) for use on protected herbs. It is an aphid antifeedant but may give some control of whiteflies.

The following products are safe to biological agents once spray deposits are dry, and should give some control of whiteflies.  All act by contact only, so good coverage of the undersides of the leaves is necessary:

• Eradicoat (vegetable oil extracts/glucose polymer), acts by physical means. May currently be used on any crop.
• Majestik (natural plant extracts), acts by physical means. May currently be used on any crop.
• Savona (fatty acids), has a SOLA for use on protected leafy herbs.
• SB Plant Invigorator (surfactants and nutrients). Acts by physical means. May currently be used on any crop.

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

• Nicotine 40% shreds (smoke) is approved on protected herbs, and Stalwart and XL All Nicotine 95% (nicotine sprays) have SOLAs for use on protected herbs. Nicotine should give some control of whiteflies. Its harmful effects on biological control agents only persist for 2-3 days after application, so nicotine can usually be used with care within IPM programmes.
• Calypso (thiacloprid) is a neonicotinoid insecticide with a SOLA for use on protected herbs. The SOLA is for aphid control but whiteflies should also be controlled*.
• Gazelle (acetamiprid) is a neonicotinoid insecticide with a SOLA for use on protected herbs. The SOLA is for aphid control but whiteflies should also be controlled*.

* specific Resistance Management Guidelines are given on the SOLAs for neonicotinoid insecticides, see Section A - Principles of IPM and Table 2 on the homepage). NB. resistance in glasshouse whitefly to imidacloprid, another neonicotinoid insecticide, has recently been recorded in the UK.

‘Harmful' in IPM
Various pyrethroid products (cypermethrin, deltamethrin and lambda-cyhalothrin/pirimicarb mix) have SOLAs for use on protected leafy herbs, and pyrethrins (Spruzit) has approval for use on both outdoor and protected edible crops.    Whiteflies are widely resistant to pyrethroid pesticides and these products are unlikely to give any control of the pest. Pyrethroid insecticides are harmful to biological control agents for up to three months after application, thus they are incompatible with IPM.  Spruzit is harmful to biological control agents for only a few days after application, so could be used with care within IPM programmes, but whitefly resistance to pyrethrins is likely. 

• 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 31 March 2009. 
• 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 with the Information Office at the Pesticides Safety Directorate (PSD) Tel: 01904 640500; or on their website (http://www.pesticides.gov.uk/).
• Always follow label recommendations or statutory conditions for use on Specific Off-Label (SOLA) notices of approval.
• Always follow instructions for Pesticide Resistance Management guidelines given on the label or SOLA.
• Growers must hold a paper or electronic copy of the current SOLA before using any product under the SOLA arrangements.  Any use of a pesticide with a SOLA is at grower's own risk.  Relevant SOLAs are sent to HDC members by HDC, or are available from PSD (see above) or from consultants.
• Use pesticides safely.

  

Further information

HDC Factsheet  14/05.  Control of whiteflies on protected ornamental crops.

Bennison, J. (2001).  Protected herbs:  control of glasshouse whitefly and leafhoppers within IPM programmes.  Final report for HDC project PC 178.

European Whitefly Studies Network website:  http://www.whitefly.org/