Some of the most common pests affecting fruit and vegetables are listed here alphabetically.
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Allium leaf miner
What is it? A tiny white maggot which feeds on leeks and onions, causing similar damage as the leek moth caterpillar (see below).
Symptoms: White spots, leaf splits and distorted plants which eventually rot.
Prevention and/or treatment: Clear away any debris at harvest, dig over soil to expose pupae to predators. Do not put infected plants or debris straight into the compost heap. Instead destroy them by soaking them for a couple of weeks then bury the mush. To prevent occurrence, cover susceptible plants with an ultra-fine mesh cover (the fly is tiny). The eggs are laid throughout the year, and can overwinter in the soil, so it is difficult to create total barrier control.
What is it? Red and black ants are most common in the garden. Red ants tend to sting, while black ants are less aggressive. New nests are created in late summer.
Symptoms: Ants 'farm' aphids and scale insects by protecting them from predators and to 'harvest' the sweet residue they exude, thus spreading the diseases these pests can carry. Ant nests can undermine plants in the soil, greenhouse and pots, causing them to wilt or die. And they can irritatingly remove newly-sown seeds to feed the colony. Some species build mounds on lawns, making mowing difficult.
Prevention and/or treatment: It is impossible to eliminate ants from a garden, so to some extent you have to learn to live with them. There are some actions you can take – encourage their natural predators such as slow worms and frogs. Water nematode treatments directly onto the nest. They don’t kill the ants but cause them to relocate. Particularly troublesome nests can be doused with water, and use grease bands on fruit trees; similarly grease greenhouse staging legs, or stand them in a moat of water - as ants can’t cross water.
What is it? Aphids are sap-sucking insects that can be found on a very wide range of plants - and in roots, stems and leaves. Often known as greenfly or blackfly, they are one of the most common pests. They can also carry viruses.
Symptoms: In large numbers clustered around tender young growth they cause young shoots to become weak and distorted, sometimes killing the plant.
Prevention and/or treatment: Avoid using too much nitrogen-rich fertiliser which encourages soft leafy growth which is attractive to aphids. Encourage creatures that feed on aphids, such as birds, insects and their larvae, earwigs and bats. Grow flowers that attract hoverflies, lacewings and ladybirds (see Flowers and herbs to attract beneficial insects). During winter, hang up pieces of fat in fruit trees and above rose bushes to attract blue tits which eat aphid eggs. Inspect plants regularly and squash any aphids that are seen. Pick off heavily infested shoots and leaves and drop into a bucket of soapy water. A strong jet of water can also dislodge them.
Apple codling moth larvae
What is it? Known as apple maggots, the larvae are pinkish-white with mottled brown heads, 18-20 mm long. The adult moth is mottled grey-brown in colour, about 8 mm long.
Symptoms: They burrow into the core leaving a prominent, red-ringed, entry hole blocked by dry ‘frass’ (maggot droppings). A large proportion of the fruit flesh can be eaten away and the cavity becomes filled with brown frass.
Prevention and/or treatment: Pheromone traps can be effective, as well as attaching greasebands around the tree. Encourage earwigs (which eat the moth eggs) and Blue tits (which eat the cocoons). Remove and destroy windfalls, birds (including hens) will clear away pupae, and check that tree ties do not contain cocoons.
Pigeons love brassicas! They can strip a plant in winter when there is little other source of nutrition. Other garden birds love berries. To deter them it is worth thinking of two behavioural traits – neophobia (the fear of the new) and habituation (the ability to become tolerant of stimulus). Therefore using deterrents such as static scarecrow will only have limited effectiveness. Either use a complete barrier, such as netting, making sure there is no entrance or possibility of the bird getting trapped, or a simple system of string and wire threaded across the vegetable patch (CDs are optional!). This appears to make it difficult for birds to judge flight access.
What is it? Small white winged insects on the underside of leaves of brassica plants, which fly up in clouds when disturbed. The young whitefly, known as ‘scales’, remain on the leaves.
Symptoms: The whitefly themselves can cause severe distortion and stunted growth, and the scales make leaves unappetising and covered in sticky honeydew that is exuded by the feeding insects. Sooty or black moulds often grow on the honeydew.
Prevention and/or treatment: Most plants can tolerate quite a high infestation. Limitation measures include creating a healthy soil, which produces strong, resistant plant growth. Fennel, coriander and cow parsley will attract parasitic wasps which lay eggs inside the whitefly scales. Their larvae consume them from the inside out. Remove infected leaves before the immobile young whitefly ‘scales’ turn into adults. Alternatively insecticidal soap or sprays based on vegetable oils can be effective. Biocontrols will only work in a greenhouse/polytunnel.
What is it? Adult flies are about 8 mm long, shiny black with reddish head, orange legs and transparent wings. The larvae are 8-10 mm long, creamy-white in colour.
Symptoms: Young seedlings can die, and mature plants are infested with rusty brown tunnels. There may be no foliage symptoms.
Prevention and/or treatment: Flies are attracted by the smell of bruised foliage, so pull carrots and weed around them on a dry evening with no wind – or on a very windy day. Similarly sow sparsely to avoid the need for thinning. There is some evidence that growing carrots with onions (four rows of onions to one of carrots) can help minimise damage, again because the onion masks the smell. Fleece gives excellent protection as does a vertical fence-like barrier around three or four rows of carrots. This needs only be 70 cm–1 m high, as carrot flies are weak fliers. Growing carrots in a container on a table top also helps, as it lifts them above ground level.
What is it? The common earwig can be a nuisance in the garden, damaging the petals of flowers such as dahlias, clematis, delphiniums, pansies and chrysanthemums. However, earwigs do also have a beneficial role in the garden, feeding on aphids and other small insect pests, including the apple codling moth.
Symptoms: Large ragged holes in flower petals. Although often found in cavities inside fruits (peaches, apples, pears) earwigs are not usually responsible for the initial damage; they tend to take over and extend a wound caused by other fruit pests.
Prevention and/or treatment: Create strategically placed nests (using upturned flowerpots with straw and newspaper) which can be emptied of earwig inhabitants by shaking them over a bowl of soapy water.
What is it? Sawfly larvae are green with black spots and a shiny black head. When fully grown they reach approximately 30 mm. The eggs are about 1 mm long and pale greenish-white in colour. They feed on the leaf edges and are difficult to spot.
Symptoms: The larvae strip the plant of its leaves, leaving a weakened and defoliated plant that often produces a poor crop the following year.
Prevention and/or treatment: The adult sawfly lays its eggs from April onwards, so inspect bushes from then. New generations will be hatching low down in the bush. Larvae should be picked off by hand or sprayed – either with direct jets of water or with a nematode control (available online). Remove mulches from around the plant in late autumn, to allow birds (hens particularly!) to clear up the cocoons in the soil.
What is it? Adult whiteflies are small moth-like insects, 1-2 mm long, white/creamy yellow in colour, with white wings.
Symptoms: Covers the underside of leaves which turn yellow from the sticky honeydew excreted, and can develop sooty black moulds.
Prevention and/or treatment: Prevention, as always, is healthy soil that creates strong growth and resistant plants. Yellow sticky tapes can help, especially if you tap the plant directly underneath. Spray badly infected plants with insecticidal soap. During winter, scrub the greenhouse down using warm soapy water, to remove the eggs of any overwintering pests. Ensure any overwintering plant stock, such as fuchsias, are clear of whitefly infestations. Throw out badly infected plants. Bio controls are available.
What is it? The adult is a tiny, inconspicuous, brown moth. The caterpillars are up to 13 mm long, yellow-green in colour with grey-brown patches and a yellowish brown head.
Symptoms: The caterpillars tunnel into the plant, creating brown and white patches on the leaves and eating the stem and bulb.
Prevention and/or treatment: Destroy infected plants, clear all debris and dig over the ground after harvest. Pick off caterpillars by hand. Use horticultural fleece to protect plants from egg laying moths. Encourage predators such as birds, frogs, bats etc who will eat the larvae.
The mole can live anywhere where there is sufficient depth of well-drained topsoil, but is most common in grassland and deciduous woodland – and organic gardens where there are high populations of earthworms! Mole hills are made of very fine-textured, friable soil and were traditionally used to make potting composts. Mole hills can be unsightly in a lawn, and make mowing difficult. There are ways of trapping moles humanely (probably best left to professionals) but you can also deter them from creating their tunnels by flooding, noise, vibration, the smell of human urine, barriers and even digging them out. Spurge (Euphorbia) can also repel moles.
New Zealand Flatworm
What is it? This invader came to Britain in the 60s. It is a threat to our native earthworm as it destroys them. They are easily recognised, with their flat bodies (5-15cms long), dark markings on the top above a pale underside, pointed at both ends and covered in sticky mucus.
Symptoms: The flatworm encircles the earthworm and covers it in mucus, which causes the worm to disintegrate and become digestible. Flatworms can travel - but they are usually found in cool, damp conditions. Hence their presence in the northwest of the UK, N Ireland and Scotland. Warmer temperatures in the south can kill them. Their eggs resemble hard, flattened black currants.
Prevention: It is difficult to eradicate the flatworm. Best practice is to create refuges ie stones, logs, weighted down polythene sheets in cool shady areas. Then, once the worms have gathered, destroy them – either by putting in hot water (>30c), sprinkling with salt, or squashing them. This has to be done regularly, as part of a sustained campaign. It is recommended that you do not touch them, the mucus can cause skin irritation. Wear gloves. Any sightings should be logged onto the OPAL survey web page.
Pea and bean weevil
What is it? Adult weevils are small, brown/grey in colour and short-snouted, between 5- 6 mm in length. The larvae are legless, white with a brown head and are found in the soil around leguminous crops.
Symptoms: Semi-circular notches eaten out of the edges of the leaves of peas, broad beans, clovers and vetches. Severe infestations can cause seedling losses, especially in cold wet conditions. Older plants are little affected, although they can get into the pods and beans themselves.
Prevention and/or treatment: It is generally recognised that there is little you can do to protect your plants from this pest, except, as always, encourage strong, fast growth by providing plants with the best possible growing conditions.
What is it? The yellowish-brown larvae are 6-8 mm long, with brown markings and brown heads. They mature into brown adult beetles 3-4 mm long, covered with fine hair, which lay tiny cream eggs in fruit blossoms
Symptoms: Only occasionally causes severe damage. Burrows into the fruit, causing it to become hard or distorted.
Prevention and/or treatment: Fork the soil around the canes at the end of the season to bring the beetles and pupae to the surface, to be eaten by birds. Repeat this several times through the autumn/winter. This technique effectively disrupts the life cycle of the beetle
Slugs and snails
What is it? Slugs and snails are soft bodied, gastropod molluscs that move along on a single muscular foot and secrete slime. They scrape their food up with a spiky, rasping tongue. Snails tend to hibernate in the winter and are unable to move through the soil, whereas slugs can be active all year round both above and below ground. Snails are able to climb higher as they retreat into their shells to prevent drying out. Both slugs and snails mostly feed by night.
Symptoms: The tell-tale trail of slime, the seedlings completely eaten, the large holes in leaves, and even the hollowing out of potato tubers are all depressing indicators of slug presence.
Prevention and/or treatment: Protection of vulnerable plants is the key - and it's important not to rely on only one method. Always renew barriers after rain, and accept that some damage is inevitable. The following may help: dig to disrupt both slug and its eggs; encourage natural controls such a beetles, frogs, birds and hedgehogs; frequently inspect your plants and hand pick off (particularly in damp weather and at night); create barriers of dry material which slugs find hard to traverse, such as grit, sheep wool – and renew when wet after rain.
You can also put a thick layer of dry oats or bran around small vulnerable plants for slugs to gorge on and dehydrate- making easy pickings for the birds. Again, renew when wet after rain. If you use traps (a can or saucer with dregs of beer) empty them frequently. To avoid killing ground beetles which eat the slugs, it would be better to put your beer into a saucer with raised edges. Use of nemotodes (microscopic organisms, available to buy online) can have some success, but they only work once in a season, and the conditions are very specific for the nemotodes to function.
If you have to use slug pellets (and yes, we all lose our patience at some stage!) make sure they are approved for organic growing, use SPARINGLY, and store safely. Most contain ferric phosphate, which will break down in the soil. However, they also contain other chemicals which can affect earthworms. Try the Organic Gardening Catalogue. Using non-organic slug pellets is to be avoided at all costs. Not only do they kill the slugs, but they also can affect the hedgehogs, thrushes, frogs and other wildlife that eat the slugs. See also Slugs and snails.
What is it? Vine weevil larvae are up to 1 cm in length with a plump, creamy white body and a brown head. When found, they are usually curled into a ‘C’. The adults, also 1 cm long, are dull matt black with ridges running down their back and a pronounced ‘snout’.
Symptoms: Serious pests of a wide variety of plants, particularly Fuchsia, Primula, Cyclamen and Begonia. The larvae are the most damaging, usually found in the compost of container grown plants, but they can also attack the roots of plants in open ground. Adult weevils make holes in leaves, which usually don’t harm the plant. Plants wilt as though short of water, and when taken out of the pot the root system will have virtually vanished and the larvae will be clearly visible in the compost.
Prevention and/or treatment: Be vigilant, if caught in time, plants can be saved either by re-potting in a new pot with fresh soil, or by re-planting in a different place. Larvae should be destroyed. Put a strip of wide PVC tape, such as brown plastic parcel tape, around individual pots and tubs, and smear this liberally with insect barrier glue that the weevils cannot cross. You can often trap the adults on a warm August night. Use of nemotodes (microscopic organisms, available to buy online) can have some success, but the conditions are very specific for the nemotodes to function.
What is it? Larvae are tough skinned, cylindrical, golden yellow to orange brown in colour and reach up to 25 mm in length when mature. Three pairs of thin small legs are located behind the head. Wireworms are the larvae of various species of click beetle
Symptoms: They attack the underground parts of plants, damaging roots, tubers, corms and stems. Favourites are potato, beetroot and carrot as well anemone, dahlia, and gladioli. Small holes 2-3 mm across is seen on the outside of the tuber or root. A network of tunnels is often invaded and enlarged by other pests such as slugs or woodlice. Further bacterial and fungal rots may develop making them unsuitable for storage.
Prevention and/or treatment: It is not easy to prevent or control the larvae – thorough digging before planting and harvesting will expose them to predators such as birds. Harvesting a potato and other root crops early will limit the damage