B8 – Leaf miners

Introduction

Leaf miners can damage a wide range of herbs but symptoms are most commonly found on labiates such as basil and mints. The adult feeding and egg-laying punctures in the leaves, and the mines caused by the larvae can make fresh cut and potted herbs unmarketable.

 

Recognition and host plants

Adult leaf miners are small, robust flies like miniature house flies, about 2 mm long. They can be seen on host plants (e.g. mints and basil) or on yellow sticky traps placed just above the plants. The larvae are off-white, legless maggots, found inside the leaf mines. The pupae are oval, about 2 mm long and vary in colour from white, yellow, brown or black, depending on species. Some species e.g. Chromatomyia spp. pupate at the end of the mines in the leaf, whereas others e.g. Liriomyza spp. pupate in the ground.

Leaf miner species that could occur on protected herbs include:

Native species:

Phytomyza petoei
This species can attack mints, particularly Mentha spicata (spearmint). The adult fly is dark grey-black. The larva usually drops from the leaf to pupate, but sometimes the oval, grey-brown pupae can be found in the leaf tissue at the end of the mines.

Phytomyza gymnostoma
This species, the allium leaf miner has been found in recent years in the UK on leek and onion. So far its distribution has been limited to the West Midlands and it has been found only in allotments and gardens. However, it has been recorded as a serious pest of chives in mainland Europe. Herb growers should contact PHSI if leaf miner symptoms (feeding punctures or mines) are found on chives. If the pest occurs on a commercial nursery, PHSI may decide that statutory action is required. The adult fly is dark grey-black. The larvae are off-white and larger than those of most other leaf miners; 5-8 mm long. The larvae pupate at the end of the mines towards the base of the leaves. The pupae are orange-brown and 3-4 mm long.

Chromatomyia species: (NB these have have only recently been renamed from Phytomyza and may be referred to as Phytomyza species in text-books etc.)

Two native leaf miner species, Chromatomyia syngenesiae (chrysanthemum leaf miner) and Chromatomyia horticola can infest mint and basil, in addition to many ornamental bedding and pot plants including chrysanthemum. Weed hosts include groundsel and sow thistle. The adult flies of these two species have a grey-black body (Fig. 1) and the larvae are greenish-white or off-white and up to 3 mm long (Fig. 2). The pupae are oval and whitish-grey to yellow-brown, and are found in the leaf tissue at the end of the mines (Fig. 3).

Liriomyza species

Native species:

A native species, Liriomyza bryoniae (tomato leaf miner)might occasionally damage mint, e.g. on nurseries that also grow tomatoes. This is established in the UK as an important pest of tomatoes. However, it is a regulated pest for Ireland and Northern Ireland and is included in plant passporting controls.

Quarantine species:

Three quarantine Liriomyza species are not native to the UK and are notifiable to Fera PHSI whenever found or suspected:

Liriomyza huidobrensis (South American leaf miner) and Liriomyza trifolii (American serpentine leaf miner) can infest many edible and ornamental plants and weeds. Liriomyza sativae (vegetable leaf miner) has not yet been recorded in the UK, but has been intercepted by PHSI on imported basil, so is a potential threat.

There is a risk of importing these quarantine leaf miner species on young plant material from overseas. Any leaf miners suspected to be a Liriomyza species should be treated as a suspected quarantine pest.

If the presence of a quarantine species in suspected, you must immediately inform your local Plant Health and Seeds Inspector (contact details are available on the Fera website at: www.fera.defra.gov.uk then selecting the plants, bees and seeds tab followed by the plant health tab or tel: 01904 465625.

The adult fly of Liriomyza species is black or grey-black with yellow markings, and a characteristic bright yellow spot on its back (Fig. 4). The older larvae are cream with a yellow/orange patch at the front (L. huidobrensis and L. bryoniae) or yellow-orange all over (L. trifolii and L. sativae). The larvae drop out of the mines to pupate in the ground or compost. The pupae are yellow-orange to black in L. huidobrensis and L. bryoniae (Fig. 5) and yellow-orange to pale golden brown in L. trifolii and L. sativae.

Symptoms

Feeding punctures
The first sign of leaf miner damage is the presence of the adult feeding punctures (whitish spots) in the upper leaf surfaces (Fig. 6). Chromatomyia species usually make oval punctures up to 1 mm diameter, whereas Liriomyza species make smaller, round punctures about 0.2 mm diameter. Phytomyza gymnostoma makes small, irregularly round punctures, often arranged in a line along the leaf at the tip end.

Mines
The larvae tunnel through the leaf in between the upper and lower surfaces, causing the characteristic whitish leaf mines (Fig. 7). Phytomyza petoei makes an irregular linear mine, sometimes with additional blotch mines (see http://www.leafmines.co.uk/html/Diptera/P.petoei.htm). The mines of both Chromatomyia spp.and Liriomyza huidobrensis follow a loose, meandering pattern, but those of L. huidobrensis are more likely to be close to the mid-vein (Fig. 6). The mines of L. trifolii are usually tightly coiled and almost blotch-like, although they can also be linear or loosely meandering. Phytomyza gymnostoma makes narrow linear mines.

Frass trails
Chromatomyia larvae leave their frass (droppings) in the mines as rows of black dots (Fig. 8), whereas those of Liriomyza appear as a continuous black line with occasional breaks.

Table 1. Key recognition features for leaf miner species that may occur on herbs

Leaf miner species	Adult feeding punctures	Adult body colour	Leaf mines	Pupae
Phytomyza petoei	Irregularly round	Grey-black bodyNo yellow spot on back	Linear but irregular	Grey-brown pupa in soil or compost, or in leaf at end of mine
Allium leaf miner (Phytomyza gymnostoma) **	Irregularly round	Grey-black bodyNo yellow spot on back	Narrow and linear	Orange-brown pupa in leaf at end of mine
Chrysanthemum leaf miner (Chromatomyia syngenesiae)	Oval	Grey-black bodyNo yellow spot on back	White, loosely- meandering, serpentine mines with dark dots of frass at irregular intervals	Whitish-grey to yellow/brown pupa in leaf at end of mine
Chromatomyia horticola	Oval or irregularly round	Grey-black bodyNo yellow spot on back	Loosely meandering	Whitish pupa in leaf at end of mine
South American leaf miner (Liriomyza huidobrensis) *	Round	Black body.Yellow spot on back.	Linear or loosely- meandering mines, usually close to mid-vein, with continuous trail of dark frass.	Yellow-orange to blackish pupa in soil, compost or substrate, or occasionally hanging from underside of leaf or lodged on lower leaf.
American serpentine leaf miner (Liriomyza trifolii) *	Round	Grey-black body with yellow markings.Yellow spot on back.	Meandering to convoluted, tightly coiled mines, with continuous trail of dark frass.	Yellow-orange to pale golden brown pupa in soil, compost or substrate
Tomato leaf miner (Liriomyza bryoniae) * for EU plant passports only	Round	Black body.Yellow spot on back.	Loose, meandering mines with continuous trail of dark frass.	Yellow-orange to dark brown pupa in soil, compost or substrate.

* Quarantine pest. If suspected, contact your local PHSI.

** Not currently classed as quarantine pest, but if this species occurs on a commercial nursery, contact your local PHSI.

 

Sources of infestation and favourable conditions

Sources of leaf miners include:

• Cuttings taken from infested mother plants.
• Bought-in young plants infested with leaf miner adults, eggs, larvae or pupae. Importing plant material increases the risk of quarantine Liriomyza species.
• Chromatomyia species pupae may survive in leaf debris from previous crops, and Liriomyza species pupae may survive in soil, compost, ground or bench coverings, or in pots and trays used for growing previous crops.
• Adult leaf miners can fly from infested crops within the glasshouse or tunnel or from cultivated or weed hosts outdoors between spring and early autumn.

Warm conditions favour leaf miner development. However, it is important to remember that Liriomyza species pupae can overwinter in the ground in unheated structures and be a source of infestation the following year.

 

Integrated Pest Management (protected herbs)

The control strategies for leaf miners summarised here are relevant to both native and quarantine species. However, do not rely on this information if you suspect a quarantine leaf miner species may be present as, if 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

• If a previous crop has been infested with leaf miners, use a thorough clean-up programme to minimise carry-over of the pest to following crops.
• Check stock plants, incoming plant material, crop plants and yellow sticky traps regularly.
• Do not take cuttings from infested mother plants.
• Use new or clean pots, trays and matting for each crop.
• Maintain strict weed control in and around glasshouses and tunnels, including in empty structures in between crops.
• Removal of punctured or mined leaves can be done to clean up plants before sale, and is a requirement under PHSI Notice if a quarantine leaf miner is confirmed on a production nursery.
• Heavily-infested plants should be removed completely. If a quarantine leaf miner is confirmed, destruction of infested plant material is the most effective means of eradication.
• Dispose of unwanted infested plants and any plant debris promptly and carefully.
• Long ‘roller’ traps e.g. ‘Mastertrap’ can be useful for mass trapping in heavy infestations, between infested crops and new batches of plants, or in empty glasshouses. They may be recommended by PHSI if a quarantine species is present. However, they can also catch flying biological control agents so they should be positioned and timed with care.

 

Biological control

Biological control agents are available for control of both native and quarantine leaf miners. Choice will depend on time of year and leaf miner density.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. Parasitic wasps
Dacnusa sibirica and Diglyphus isaea are small parasitic wasps, about 2-3 mm long. Dacnusa is black with long antennae (Fig. 9) and Diglyphus is metallic green with short antennae (Fig. 10). Dacnusa and Diglyphus can be supplied alone or together in a mixture.

Dacnusa is an internal parasitoid; the females lay their eggs directly into the living leaf miner larvae within the leaf mines and the parasitoids develop inside the host larvae. The parasitised leaf miner larvae continue to cause damage and develop to the pupal stage, but then the adult parasitoid emerges from the pupa instead of the adult leaf miner. Dacnusa is best used during the winter and spring, at low leaf miner densities. It will work at temperatures as low as 10°C but is less effective in hot sunny weather.

Diglyphus is an external parasitoid. Females lay their eggs onto the surface of the leaf miner larvae living within the leaf mines. The Diglyphus larvae lie in the mine alongside the leaf miner larvae and feed externally on their body contents. Diglyphus larvae are colourless when young and are pale green when older. They pupate inside the mine and the pupae are pale turquoise, turning black just before emergence from the leaf as adults. Diglyphus larvae and pupae can be seen inside the mines with the dead leaf miner larvae if the leaf mines are split open. Diglyphus can be used at any time of year at temperatures as low as 10°C, but establishes better at higher leaf miner densities, as the adults need to ‘host-feed’ on the young leaf miner larvae in order to produce eggs. Thus both Diglyphus adults and larvae kill leaf miner larvae and stop further mining. An indicator of good Diglyphus establishment is the presence of very small, narrow ‘stopped’ leaf mines.

2. Entomopathogenic nematodes
Steinernema feltiae has been shown to give some control of leaf miner larvae when used as a foliar spray against thrips. The nematodes enter the leaf mines through the leaf miner feeding punctures or exit holes, so they are more effective in higher leaf miner densities where the availabilty of such entries is higher. Once inside the mine the nematodes swim to find a leaf miner larva, enter the body and release symbiotic bacteria that kill it. The nematodes need to be applied carefully, following all label recommendations, e.g. the suspension of nematodes should be applied to just before run-off, with a recommended wetter, in the late afternoon or early evening when relative humidity is high.

Monitoring within IPM

• Monitor plants regularly for signs of adult leaf punctures or mines made by larvae.
• Check yellow sticky traps for adult leaf miners. Contact PHSI immediately if a quarantine species is suspected.
• If using leaf miner parasitic wasps, an Entomologist can check for percentage parasitism of leaf miner larvae. Parasitism by Dacnusa can only be checked by dissection of leaf miner larvae, or allowing pupae to emerge into either leaf miner or Dacnusa adults. Successful parasitism by Diglyphus can be recognised by the presence of very small, narrow ‘stopped’ leaf mines, or by the presence of Diglyphus larvae or pupae inside larger mines.

 

Chemical control (protected herbs)

Leaf miners are difficult to control with pesticides. Larvae are protected within the leaves, and pupae are either in the leaf tissue or in the growing medium or substrate, depending on species. L. huidobrensis can be resistant to certain pesticides (see Table 2 on the homepage), thus any failures in chemical control could indicate the presence of a quarantine leaf miner species. Contact PHSI if a quarantine species is suspected.

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).

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 leaf miner adults. Both act by contact only, so good plant coverage is necessary:

• Eradicoat or Majestik (maltodextrin) acts by physical means. Approved for use on protected edible and ono-edible crops.

‘Moderately harmful’ in IPM

• Calypso or Agrovista Reggae (thiacloprid) has a EAMU for use on protected herbs. It is a neonicotinoid insecticide with contact, translaminar and systemic action, and will kill leaf miner larvae within the leaf tissue. Specific Resistance Management Guidelines are given on the EAMU.
• Dynamec, Acaramik and Clayton Abba (abamectin) have Extension of Authorisation for Minor Use (EAMU) for use on protected herbs. Abamectin has both contact and translaminar action and will kill leaf miner larvae within the leaf tissue. There is no known resistance to abamectin in native leaf miners, but Liriomyza huidobrensis can be resistant. Abamectin is harmful to many biological control agents. However, its persistence is short, thus it can be used within IPM if timed carefully. Repeated applications will be more harmful to biological control agents than single applications.

‘Harmful’ in IPM
The pyrethroid products (cypermethrin and deltamethrin) have EAMUs for use on protected leafy herbs and will control native leaf miner adults (L. huidobrensis adults may be resistant). Pyrethroid insecticides are harmful to all biological control agents except for entomopathogenic nematodes, for up to three months after application, thus they are usually 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 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 15/07: ‘Leaf miners of bedding and pot plants’.