Your guide to using insecticides in the garden

No garden is pest-free, but when undesirable insects begin to take control of an area of your garden you may want to consider using insecticides. Which insecticide should you use when pests threaten your garden?

• Use pesticides only if an infestation is causing damage and cannot be otherwise controlled.

Start with the least toxic substance possible and gradually move to stronger measures as needed.

Insecticides are derived from a range of organic and synthetic sources and are available in liquid or dry form.

• Many gardeners prefer organic types, which are made from bacteria, viruses, fungi, fatty acids, minerals, oils and plants; they decompose quickly into nontoxic substances that won't harm the environment.

All-purpose synthetic pesticides that contain carbaryl, permethrin or malathion remain active for a few days and kill both harmful and helpful insects.

• Insecticides are generally sprayed or dusted on plants or soil, where they suffocate, paralyze or poison insects on contact or by ingestion.
• You must completely cover the affected plant or soil surface, including leaf undersides.
• Many organic pesticides must be ingested by insects, but soaps and oils kill through direct contact, as do most synthetic insecticides.

• Systemic insecticides are absorbed by the plant and circulated by its sap; they can't be washed away by rain. Pests are killed when they feed on any part of the plant.
• Systemic insecticides should be used only on ornamental plants, never on edible ones.

• Be sure to identify the pest correctly and choose the appropriate pesticide.

Find out about the pest's life cycle and habits so you can treat it effectively.

Many synthetic pesticides harm bees and other desirable insects.

• Try to use the most "specific" substance possible — one that targets the pest you want to destroy — and don't apply a bee-killing pesticide during bloom time, when bees are most active.

Horticultural oils, which include dormant oil and summer oil, are used to smother eggs and developing insects on trees and ornamentals.

• Use the heavier dormant oil in late winter or early spring, once temperatures are over 4°C but before plants leaf out.
• Use the lighter summer oils any time the temperature is below 29°C.

• Insecticidal soap is one of the best cures for soft-bodied pests, such as aphids, mites and leaf miners.

It's safe on most plants and nontoxic to beneficial insects and animals. But don't spray it in direct sun, in extreme heat or during drought, because it can injure the leaves of some plants.

Brew your own insecticidal soap by mixing 10 millilitres (two teaspoons) of dishwashing liquid with a few drops of vegetable oil and four litres (one gallon) of water.

• Use a plastic spray bottle to apply it; but wash the bottle thoroughly if it previously held household cleansers.

Some pesticides — including natural ones — can scorch, discolour or damage foliage.

• Always read the label and test the substance first on a small, inconspicuous part of the plant if you're not sure of its safety.