This article explains how to keep pest problems to a minimum and keep your garden flourishing without using insecticides. If pests do get out of hand, there are ways of keeping their populations down which don't rely on insecticides. These methods have the added advantage of protecting the wide range of insects and other animals which your garden needs. You can limit pest damage by making sure your garden is a healthy balanced system.
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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.
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.
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.
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.
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.
Try these methods. They will help to bring pest numbers down if you are willing to put up with a little damage.
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.
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.
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.
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.
Aphids can damage leaves, vegetables, fruit trees, ornamental plants and houseplants.
Carrot rust fly larvae
Carrot rust fly larvae feed on roots of carrot, celery and parsnip plants and parsley.
Codling moth caterpillars
Codling moth caterpillars feed inside fruit, especially apples.
Cutworms chew any young seedlings.
Leafroller caterpillars feed on leaves and fruit.
Mealybugs can damage leaves, fruit trees, houseplants and vegetables.
Scale insects can affect leaves, fruit and branches of ornamentals and houseplants.
Slugs and snails
Slugs and snails feed on leaves of many vegetables and green house crops.
Spider mites suck green pigment from the leaves of greenhouse crops and fruit trees.
Thrips damage ornamentals, houseplants, fruit and vegetables.
White butterfly and diamond back moth caterpillars
White butterfly and diamond back moth caterpillars particularly affect cabbages, broccoli, and cauliflowers (brassicas).
Whitefly often damage greenhouse crops.
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