B4 - Aphids

Introduction

Several species of aphids can infest herbs grown under protection and/or outdoors.  The presence of aphids or their cast skins on the plants, the sticky honeydew excreted by aphids and the associated sooty moulds, and the direct damage caused by aphid feeding can all make potted and cut fresh herbs unmarketable. 

  

Recognition and host plants

It is important to know which aphid species is present, as different species can vary in their susceptibility to both biological control agents and to pesticides.  Aphids can be winged or wingless. The larger wingless aphids are easiest to recognise, using the diagnostic features of body colour, and the shape and colour of the two siphunculi (‘exhaust pipes') at the rear end.  The main aphid species infesting protected herbs are:

Polyphagous aphid species
Some aphid species infesting protected herbs are common pests of many plant species.  These include:

• 1. The peach-potato aphid, Myzus persicae, which is particularly common on basil and can also occur on parsley. The body is green, pink or red and the siphunculi are slightly swollen and have dusky tips (Fig. 1).
• 2. The melon and cotton aphid, Aphis gossypii can infest protected herbs e.g. basil, coriander, lemon verbena and parsley. This small aphid varies from yellowish-green to olive green, dark mottled green or black, sometimes with patches of white powdery wax on the body. The siphunculi are short and black (Fig. 2). This aphid can form dense colonies on growing points and stems.
• 3. The glasshouse-potato aphid, Aulacorthum solani, is a common pest of ornamentals and occasionally infests herbs including sage. The body is shiny green with darker green patches around the bases of the siphunculi (Fig. 3). The siphunculi are green with distinct black tips.
• 4. The shallot aphid, Myzus ascalonicus, can infest many plant families, including the Alliaceae e.g. chives, where it forms dense colonies at the stem bases. The body is pale green or dirty yellow and the siphunculi are slightly swollen (Fig. 4).

Aphid species infesting certain plant families or species
These include:

• 1. The willow-carrot aphid, Cavariella aegopodii, which is common on umbelliferous herbs e.g. parsley, coriander, dill and chervil. The body is yellowish-green and the siphunculi are slightly swollen at the tips (Fig. 5). There is a protuberance at the rear end of the body above the cauda (‘tail'), giving the appearance of a double ‘tail'. This aphid can be a serious pest of protected parsley.
• 2. The hawthorn-parsley aphid, Dysaphis apiifolia, can also be a serious pest of protected parsley. This species can form dense colonies at the stem bases and is yellowish-green, pinkish-green or greyish-green, with a slight powdery coating (Fig. 6). The siphunculi are black, with rusty-orange patches around the bases.
• 3. The mint aphid, Ovatus crataegarius, is mottled green, commonly found on mint stems and leaves (Fig. 7). This aphid is often mistaken for the peach-potato aphid. It can be recognised by its body colour, the two prominent bumps on the head between the antennae, and the tapered green siphunculi without dusky tips.

Many weed species in the same plant families as the cultivated plant hosts can also be hosts for the aphid species described above.

Symptoms

Damage symptoms depend on the aphid species but can include leaf distortion and curling, e.g. peach-potato aphid on basil (Fig. 8).  Aphids and/or white cast aphid skins are visible on the leaves or stems, often with sticky honeydew excreted by the aphids, which allows the growth of sooty moulds.  Some aphid species can also transmit viruses that may infect herbs, e.g. the willow-carrot aphid can transmit parsnip yellow fleck virus, which can occur on carrot, parsnip and umbelliferous herbs e.g. parsley, chervil and coriander.  The virus symptoms include stunting, leaf yellowing or reddening and even the death of growing points in subjects such as chervil, if the ‘helper' virus anthriscus yellows (that is also transmitted by willow-carrot aphid) is also present. However, these viruses are thought to be uncommon on protected herbs, as no virus symptoms were detected on parsley, chervil or coriander infested with willow-carrot aphid on nurseries visited in HDC project PC 178 (Bennison, 2001). 

  

Sources of infestation and favourable conditions

Winged aphids can migrate into glasshouses and tunnels through vents and doors from outdoor crops or weeds, or can fly from infested plants in the glasshouse or tunnel.  For example, the source of an infestation of willow-carrot aphid on protected parsley, on a nursery visited in HDC project PC 178, was thought to have been an adjacent outdoor crop of carrots (Bennison, 2001).  Aphids produce winged forms for migration, e.g. to move from winter to summer plant hosts, or to find fresh plant material when either colonies get overcrowded or when the host plant is senescing.  Otherwise, aphids produce wingless offspring, which usually stay on the original plant, but can also walk to adjacent plants.  Aphids can be found all year round under protection, but are more of a problem in heated structures and between spring and autumn in unheated glasshouses or tunnels.  Aphids give birth to live young and large colonies can build up very quickly in warm weather. 

  

Integrated Pest management (protected herbs)

Cultural control

• Maintain strict weed control in and around glasshouses and tunnels. 
• Dispose of infested plants carefully.
• Clean bench or floor coverings between crops, as aphids can be dislodged when removing plants, and may survive on plant debris.  
• Yellow sticky traps may be useful in doorways or under vents, to catch winged aphids migrating in from outdoor hosts or from batches of infested plants.  However, these will also catch flying beneficial insects, e.g. parasitic wasps, so they should be used and positioned with care.

  

Biological control

Several biological control agents are commercially available for the control of aphids.  They should be chosen carefully as some are selective and will not control all aphid species.  Times 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.    Aphidius colemani
Susceptible aphids:  
peach-potato aphid (Myzus persicae)
melon and cotton aphid (Aphis gossypii)

A. colemani is a small (about 2 mm long) brown and black parasitic wasp. The adult female lays a single egg into each aphid (Fig. 9).  The parasite develops inside the aphid body, turning the aphid into a pale brown ‘mummy' (Fig. 12).  When fully developed, the adult parasitic wasp cuts a round hole in the top of the mummy and emerges.  Where IPM is being used aphid parasites can occur naturally, but usually these are not sufficient to provide full control.  A. colemani is supplied as parasitised mummies in bottles.  The adults emerge from the mummies in the opened bottle in the glasshouse or tunnel over the following few days.  Aphidius are best released weekly on a preventive basis, as the adults are very mobile and efficient in finding their hosts, and will often find the first aphids before they are detected by crop monitoring.  Waiting until the first aphids are seen is often too late for starting parasite releases.  Once aphids are seen, parasite release rates should be increased.  A ‘banker plant' system was developed with HDC and MAFF funding (Bennison, 1992) for more cost-effective use of Aphidius in cucumbers.  This system is now commercially available and used in many protected crops, including herbs, both in the UK and overseas.  The banker plants (Fig. 10) are barley plants infested with the bird-cherry aphid, Rhopalosiphum padi (which only infests cereals and grasses).  The banker plants allow large numbers of A. colemani to breed on them in the glasshouse, even before aphids are seen on the herb crop.  The adult Aphidius then fly from the banker plants to the crop plants to find the pest aphids.     

2.  Aphidius ervi
Susceptible aphids : 
glasshouse-potato aphid (Aulacorthum solani)
potato aphid (Macrosiphum euphorbiae) - not usually a pest of herbs.

This black parasitic wasp is very similar to A. colemani, but it is about twice the size (Fig. 11). If a mixture of herbs is grown that is susceptible to a range of aphid species, it may be necessary to release both species of parasitic wasp.  One supplier offers a mixture of aphid parasites in the same tube, i.e. A. colemani, A. ervi and Aphelinus abdominalis.  The latter species attacks the same aphid species as A. ervi but the parasitised aphid mummies are black rather than brown.  Banker plants are also available for A. ervi, (wheat plants infested with the grain aphid, Sitobion avenae - different to those used for A. colemani).     

3. Aphidoletes aphidimyza
Susceptible aphids: 
all aphid species attacking herbs.

This is a midge whose larvae are voracious predators of most species of aphid.  It is used successfully on herbs, not only to supplement aphid control by parasites if necessary, but also to control those aphids that are not attacked by commercially-available parasites (i.e. willow-carrot aphid, hawthorn-parsley aphid, mint aphid and shallot aphid).  The predator is supplied as pupae in a carrier in tubes, or in release tubs.  The adults emerge from the pupae and the female midges fly to find aphid colonies amongst which they lay eggs.  The small orange-red eggs hatch in a few days and the yellow/orange maggot-like larvae (up to 2.5 mm long) kill and feed on most aphid species (Fig.13).  The larvae drop from the plants to pupate in the growing medium or on the bench or floor, although pupation is more successful in growing media than on bench or floor coverings.  Aphidoletes needs a minimum of 15 hours daylength to complete its life cycle, so releases between May and September will allow establishment in the crop.

4. Lacewings, hoverflies and ladybirds
Susceptible aphids: 
all aphid species attacking herbs.

Other commercially-available aphid predators are the lacewing larva Chrysoperla carnea (Fig. 14), the hoverfly Episyrphus balteatus and the 2-spot ladybird, Adalia bipunctata.  Like Aphidoletes, these predators will eat most aphid species, so can be useful for controlling those aphids not attacked by parasites.  Lacewing larvae have proved particularly successful in herb crops.  However, unlike the parasites and Aphidoletes, these three predators tend not to breed continuously in the crop.  Thus they are best used as ‘biological pesticides' e.g. as an alternative to a pesticide in aphid ‘hotspots'.  Lacewings are supplied as larvae, hoverflies as pupae on cards and ladybirds as either larvae or adults.  The predators can also occur naturally in crops where IPM is used.  

5.  Verticillium lecanii
Susceptible aphids: 
all aphid species attacking herbs.

This insect-pathogenic fungus (‘Vertalec' Ò) is a microbiological control agent and is thus subject to the PSD approval system (unlike invertebrate predators, parasites and nematode biological control agents).  Vertalec 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 aphid's body, the fungus grows through the body wall and can be seen as white fluff on the surface of the aphid (Fig 15).  Not all fungicides are compatible with Vertalec, so any used need to be chosen with care and their application timed to cause least harm; if in doubt, check with the Vertalec supplier.    

Monitoring within IPM

• Sticky traps are not useful for monitoring aphids as they catch only winged aphids, including non-pest species that may fly into the glasshouse or tunnel.
• Check leaves and stems for live aphids, cast skins and malformed or sticky leaves.
• If using Aphidius, check percentage parasitised aphids (‘mummies') every week.
• If using Aphidoletes or lacewings, check for predator larvae amongst aphid colonies.  

  

Chemical control (protected 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).  Some strains of Myzus persicae and Aphis gossypii are resistant to certain pesticide groups.  Due to existing and potential problems with aphid 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 aphicides within IPM are given below:

‘Safe' in IPM
The following products are safe to biological agents:

• Chess WG (pymetrozine) is an aphid antifeedant and should be effective against all aphid species. It has a Specific Off-label Approval (SOLA) for use on protected herbs.

The following products are safe to biological agents once spray deposits are dry, and should give some control of aphids.  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 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:

• Aphox, Dovetail and Phantom (pirimicarb) have SOLAs for use on protected herbs. They have ony a short-term harmful effect on some biological control agents, so can usually be used with care within IPM programmes. Aphis gossypii and some strains of M. persicae are resistant to pirimicarb.
• 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 be effective against all aphid species. 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 and Agrovista Reggae (thiacloprid) are neonicotinoid insecticides with a SOLA for use on protected herbs. These products should be effective against all aphid species*.
• Gazelle (acetamiprid) is a neonicotinoid insecticide with a SOLA for use on protected herbs. The product should be effective against all aphid species*.

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

‘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.    Some strains of Aphis gossypii and Myzus persicae are resistant to pyrethroids and pyrethrins.  Both groups of pesticides are effective against other aphid species infesting herbs, but are purely contact in action, so good coverage of the undersides of the leaves is necessary.  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. 

• 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

Bennison, J.  (1992).  Cucumbers:  aphids, Integrated Control.  Final report for HDC project PC 69.

Bennison, J. (2001).  Protected herbs:  control of glasshouse whitefly and leafhoppers within IPM programmes.  Final report for HDC project PC 178.  (This report also includes information on incidence and control of the willow-carrot aphid on protected herbs).