Is it a pest?

Many insects are useful in the garden. Pollinators such as bees help plants to produce fruit and seeds.

When do insects and other animals become pests? The simple answer is when they damage plants more than you're prepared to put up with.

For gardeners who grow vegetables or flowers for competitions or shows, producing perfect specimens is important. But most gardeners have a more relaxed approach and don't mind the odd blemish or a few chewed leaves. Most plants can tolerate a few insects feeding on them before there is any noticeable drop in yield or quality.

Preventing pest problems

Healthy soil
The soil must be fertile, with soil animals allowed to play their vital role. Feed the soil by returning organic matter to it as compost or mulch. Fallen leaves, grass clippings and rotted manure can all be used as mulches. Soil kept healthy this way will not need commercial fertilisers to boost its fertility.

Healthy plants
A healthy plant can overcome the effects of diseases or pests better than a weak one. If you have a plant with a pest problem, try to work out why it is so vulnerable. Does it need more or less water? Does it need more or less nitrogen or other nutrients? Does it need more sun? Grow plants that do well in your local soil and climate.

Young, tender seedlings are more vulnerable to pest damage than older, more established plants, so make sure the soil is good enough to give the plants or seeds a rapid start in life. Water frequently to encourage steady growth.

Discourage pests
Keep weeds down – a dense cover keeps the soil surface damp which encourages slugs. Aphids can survive the winter on weeds then move on to your new plants in spring. Destroy any heavily infested plants before pests spread to others. The affected leaves or the whole plant should be burnt or buried deeply.

Rotate crops
You can stop pests building up in the soil by growing each type of vegetable in a different patch each year. A regular cycle of rotation should cover 3 to 5 years. Peas and beans make nitrogen available in the soil, so they should be planted the season before crops which need lots of nitrogen – such as cabbages, cauliflowers and broccoli (brassicas). Follow with a root crop such as carrots or onions.

Grow a mixture of plants
A mixture of plants may encourage a greater variety of insects, so there is less chance of one species doing better than others and becoming a pest. It also attracts insects and birds which feed on those you want to control.

Growing "companion" plants together in the same area can also help control pests. They can repel insects you don't want (for example, scented marigolds repel whitefly), or attract pests away from plants you don't want damaged (nasturtiums attract aphids away from other plants). Natural enemies of some pests are attracted by the flowers of carrot, parsnip, parsley and dill plants, for example parasitic wasps which feed on leafroller caterpillars and tomato fruitworms.

Some varieties of vegetables and fruit are more resistant to common pests than others, so if the ones you're growing constantly suffer pest damage try growing other varieties.

If pests get out of hand

Try these methods. They will help to bring pest numbers down if you are willing to put up with a little damage.

Hand picking
Rubbing affected parts of the plant between your thumb and fingers to squash insects, or removing caterpillars, slugs and snails by hand may seem time-consuming. However, these methods can be effective in small gardens or where only a few plants are being damaged.

Repellents
Sprays made from garlic, onion or aromatic herbs such as tansy, mint, rosemary or basil can repel common pests. A garlic spray can deter aphids, mites and white butterflies. Try crushing several cloves of garlic, add 1 litre of boiling water, leave to cool, then strain through a sieve. Add 1 teaspoon of soap or detergents to help the spray stick to the leaves. You will probably need to spray several times, 1-2 days apart.

Other sprays
Sprays of soapy water can be used for soft-bodied insects such as aphids, mealybugs, mites and whiteflies. To make a soap spray, dissolve a quarter cake of soap in a little hot water, then add 4-5 litres of cold water. Higher concentrations may damage plants.

Often a forceful spray of plain water is all that is needed to remove insects. Like soap sprays, this needs to be repeated.

Biological control
Some insects, such as ladybirds, hover flies, lacewings and ground beetles, as well as spiders and mites, eat garden pests. Others live on or in them as parasites. You can attract them to the garden by planting flowers such as Michaelmas daisies, calendulas, Californian bluebells and sea hollies.

Birds may also help control potential pests. The greater the range of plants in the garden, the more different predators and parasites will be attracted.

Controlling common pests

Aphids
Aphids can damage leaves, vegetables, fruit trees, ornamental plants and houseplants.

• Attract natural enemies with flowering plants. Ladybirds and the larvae of hover flies are avid aphid eaters. The adult flies feed on the nectar of French marigolds, nasturtiums and poppies, so plant these among the plants aphids might damage. Put out food to encourage birds.
• Put silver foil on the ground between rows of low plants – it confuses aphids and stops them landing on those plants.
• If the plant isn't fragile, hose it to wash off aphids.
• Spray with garlic or soapy water.
• Grow resistant plant varieties. Topweight is a carrot variety that tolerates aphids.

Carrot rust fly larvae
Carrot rust fly larvae feed on roots of carrot, celery and parsnip plants and parsley.

• Put lawn clippings around the plants – this confuses the flies' sense of smell and stops them landing to lay eggs.
• Put a polythene "fence" (800mm high) around the crop. This stops flies from finding egg-laying sites near the ground.
• Push earth up around plants to make it harder for larvae to get to the roots.
• Thin when plants are small, so you don't damage the roots left in the ground. Flies use their sense of smell to find targets, so disturbing the plants will attract them.
• The Egmont Gold variety of carrot tolerates carrot rust fly better than others.

Codling moth caterpillars
Codling moth caterpillars feed inside fruit, especially apples.

• Remove flaking bark from tree trunks to stop the caterpillars making cocoons there over winter. A band of corrugated cardboard around the trunk will stop codling moth caterpillars as they migrate down (late summer-autumn). The band makes a good site for them to make their cocoons. It can be removed and burnt after the fruit is harvested.

Cutworms
Cutworms chew any young seedlings.

• Push tin or cardboard cylinders 25mm into the ground around young plants to protect stems – toilet rolls are ideal.
• Hoe lightly around plants to expose cutworms, then remove them.

Leafroller caterpillars
Leafroller caterpillars feed on leaves and fruit.

• Plant carrots, parsnips, parsley or dill. If left to flower they will attract parasitic ichneumon wasps.
• Mow under trees to remove other refuges.
• Pick damaged leaves and burn them.

Mealybugs
Mealybugs can damage leaves, fruit trees, houseplants and vegetables.

• Attract natural enemies – lacewings – with flowers or clover.
• Spray bugs with soapy water or hose underside of leaves.
• Kill bugs on houseplants by touching them with a cotton swab dipped in methylated spirits.

Scale insects
Scale insects can affect leaves, fruit and branches of ornamentals and houseplants.

• Many natural enemies such as ladybirds and parasitic wasps are active on unsprayed trees, and often control scale adequately for the home gardener.
• Remove and burn infested branches and stems.
• Kill bugs on houseplants by touching them with a cotton swab dipped in methylated spirits, or simply wipe them off the leaves.

Slugs and snails
Slugs and snails feed on leaves of many vegetables and green house crops.

• Lure with containers partly filled with raw potato slices, lettuce leaves, bran or stale beer. Empty traps regularly. Check under old wood planks, loose bricks and plant pots for slug and snail refuges.
• Hoe frequently to expose slugs and eggs to birds and cold.

Spider mites
Spider mites suck green pigment from the leaves of greenhouse crops and fruit trees.

• Mist the greenhouse with a fine water spray during hot weather. Mites prefer dry conditions. Higher humidity may encourage diseases though, so watch for signs of mildew.
• Remove unwanted plants from greenhouse in autumn.

Thrips
Thrips damage ornamentals, houseplants, fruit and vegetables.

• Attract natural enemies – lacewings – with flowers or clover.
• Spray insects with soapy or plain water.
• Remove infested flowers and buds, and burn.
• Control flowering weeds which may provide food.

White butterfly and diamond back moth caterpillars
White butterfly and diamond back moth caterpillars particularly affect cabbages, broccoli, and cauliflowers (brassicas).

• Attract natural enemies – spiders and parasitic wasps – by planting carrots, parsnips, parsley, or dill, and allowing them to flower.
• Plant tomatoes or wormwood (a herb) among brassicas. These help to repel white butterflies.
• Plant cabbages with cauliflowers and broccoli – butterflies will prefer the cabbages.
• Repel butterflies with garlic spray.

Whitefly
Whitefly often damage greenhouse crops.

• Put fine mesh screens over vents and doors to stop insects getting in.
• Spray with soapy water, but don't spray too strongly or in bright light as this may damage the leaves.