B12 - Flea Beetle

Introduction

Flea beetles derive their common name from their ability to jump many times their own height, like fleas.  The adult beetles and their larvae feed on plant material and can cause crop losses on both outdoor and protected cruciferous herbs and sorrel.

  

Recognition and host plants

The common features of all flea beetle adults are their massively developed hind legs, which provide the muscular power for the insect to propel themselves into the air.  The adult beetles are  oval, shiny insects with domed backs.  Some species are up to 6 mm long, but those of most interest to herb growers are smaller than this, typically between 1.5 and 3 mm long.  Generally, the insects are black, sometimes with a metallic green, blue or bronze tinge or, more obviously, with a large yellow  longitudinal stripes (Fig. 1).  There are also some brown or yellowish species.  Flea beetles tend to specialise in feeding on plants of certain families.  Herb crops commonly damaged are sorrel (Polygonaceae) and Cruciferae e.g. rocket and mustards.  Eight species of Phyllotreta, (including P. undulata, P. nemorum and P. diademata), occur on Cruciferae and Chaetocnema concinna occurs on sorrel and spinach (Chenopodiaceae).

The larvae of flea beetles are off-white with a brown head, three short pairs of legs at the front, and often dark plates on the body.  They feed within plant tissues.  Most species, e.g. the turnip flea beetle Phyllotreta cruciferae, burrow into roots, but one, the large striped flea beetle Phyllotreta nemorum, mines within leaves, petioles and stems.

Symptoms

Adult flea beetles feed on the cotyledons and foliage, producing small pits or ‘shot-holes' that make leafy herbs unmarketable.  Seedlings and rootlets can also be attacked underground, leading to plant losses or reduced plant vigour.  The leaf mines produced by the larva of Phyllotreta nemorum disfigure leaves and/or cotyledons.

  

Sources of infestation and favourable conditions

Flea beetle adults are known to overwinter in plant debris in hedgerows, field margins etc. and move into outdoor and protected tunnel-grown host crops in spring, when temperatures reach 14°C or more.  However, it is also possible that adults could overwinter in tunnels, particularly if any plant material is available for shelter.  The adults will fly readily in warm still, dry, conditions but are less active if it is cold or wet.  Peak adult activity typically occurs between the second week of April and the third week of May, but severe outbreaks can occur through June and July, particularly in prolonged dry spells.  There is only one generation a year but the adults of the new generation can cause damage between the completion of their development in late June, July or early August and the onset of hibernation in September.

  

Integrated Pest Management (protected herbs)

Cultural control

• Ensure all plant debris is cleared up in tunnels after harvesting susceptible crops in the autumn, to reduce the possibility of flea beetle adults overwintering in the tunnel substrate.
• As a known source of the pest is from outdoors, using fine-meshed crop covers, or screens on tunnel doors, can be very effective in minimising invasion of crops, thereby reducing damage.  However, covers or screens need to be in place before activity starts, and susceptible crops should not follow previous host crops if the possibility of beetles emerging under the covers is to be avoided.
• The mobility of flea beetles allows them to find food crops more than a kilometre from the overwintering site.  As a result, siting any susceptible outdoor crops far enough from the previous year's flea beetle-infested crops to avoid infestation, whilst a useful idea, is frequently impractical.  In extreme cases, if outdoor susceptible crops are also grown on site, it may be necessary to take a break from growing favoured crops during peak beetle activity.
• Trap cropping in brassica crops was investigated in HDC project FV 222, using summer turnip, Chinese cabbage and Chinese mustard (Brassica juncea var crispifolia) sown in strips in between the crop plants.  The first two trap crops were more attractive to flea beetles than either broccoli or swede, but the Chinese mustard was ineffective.  Damage was worst in the crop immediately adjacent to the trap-crop strips and the project concluded that such strips needed to be at least 20 metres apart for optimum effect.   Work in the USA resulted in a recommendation for trap crop strips to be 50m apart.  Trap crops sown about two weeks before the main crop were most effective, and a mixed trap crop of summer turnip and Chinese cabbage attracted a wider range of flea beetle species than either trap crop alone.   A difficulty with trap cropping is that where the main crop (e.g. rocket, Eruca sativa) is at least as attractive to flea beetles as the trap crop, the method is unlikely to be effective.
• In the USA and Israel, and to a much lesser extent in the UK, suction machines have been developed for pest removal from crops.   This method was investigated in 2008, in HDC project FV 330, for control of leafhoppers on outdoor herbs, and the method may also remove flea beetle adults as it could exploit their jumping behaviour. Frequent treatment would probably be necessary.

  

Biological control

There are no biological control agents specifically available for flea beetles.  Some nematode products used for control of other pests, e.g. Capsanem (Steinernema carpocapsae) may give some control of flea beetle larvae (see Section B.7, caterpillars), but have not been tested against flea beetles.

  

Chemical control

Controlling flea beetles with pesticides can be difficult, as the adults often jump off the plants when disturbed and the larvae are well protected in the roots, leaves etc.  Even successful treatments may not have a lasting effect because successive waves of insects tend to arrive in suitable host crops, requiring frequent re-treatment.  Seed dressings of some insecticides, such as the neonicotinoid imidacloprid, have given 60-70% control in trials on crops other than herbs, but none are approved for use on herb seed.

If a pesticide is needed, it 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).

For full, 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:       

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

• Calypso (thiacloprid), has contact, translaminar and systemic action, and has a Specific Off-label Approval (SOLA) for use against aphids on protected herbs. The product may give some control of flea beetles.
• Spruzit (pyrethrins plus oil) is approved for use on any protected edible crop and will give some control of flea beetles. The product is harmful to many biological control agents but has only limited persistence, thus it can be used with care within IPM programmes. N.B. Spruzit has been known to cause leaf damage to curly parsley. The product should be tested on a few plants first for any herb species, before using on a large scale, particularly if more than one application is made. See product label for full details.

‘Harmful' in IPM
Various pyrethroid products (cypermethrin and deltamethrin) have SOLAs for use on protected leafy herbs.  They will give some control of flea beetles.  Frequent re-treatments may be necessary to control immigrating individuals.  Pyrethroid insecticides are harmful to biological control agents for up to three months after application, thus they are incompatible with IPM.

• 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 Project Report FV 222.  Brassicas: biology and control of brassica flea beetles by integrating trap crops with insecticide use.

HDC Project Report FV 330 (2009).  Field-grown herbs:  Evaluation of a mechanical method for the cultural control of leafhoppers.