What do they look like?
Aphids are tiny, soft-bodied, pear-shaped insects, 2-4 millimetres long. They are slow-moving. There are many different species ranging in colour from shades of green to dark brown, black and powdery grey. Under a magnifying glass, you can see what looks like a pair of tiny tailpipes pointing backward from the tail end. These tubes, called cornicles, are unique to aphids. Most aphids are wingless. A few have wings and look like small dark flies among the wingless aphids. Most species live in crowded colonies, clustered on the undersides of leaves and on new tips and shoots.
What does aphid damage look like?
Aphids suck plant sap with their needle-like mouthparts. This distorts leaves, tips of shoots and flowers. Aphids feeding on the undersides of leaves cause the leaves to curl under, cupping the aphid colony inside. Common aphid species attack beans, cabbage family plants, potatoes, nasturtiums, roses, viburnums, fruit trees or maple trees. With some exceptions, each aphid species feeds only on a particular type of plant or groups of plants—so don’t worry that aphids on beans or maple trees will attack your roses or potatoes (or visa versa). Aphids also produce a sugary liquid called honeydew which drops onto leaves and other objects below. This does not harm the leaves, but it is a serious nuisance on car windshields. Sooty moulds often grow on the honeydew, leaving unsightly black streaks and blotches on leaves and tree trunks.
Aphid Life Cycles
Most aphids multiply quickly because they give birth continuously all summer, without mating or laying eggs. In the fall, ‘true’ male and female aphids are born. They mate and the females lay over-wintering eggs on plants. The females that hatch from the eggs in the spring give birth continuously without mating. Aphids reproduce most quickly in cool spring weather, before the numbers of their natural enemies have built up after winter.
Are aphids really a problem?
The many native insects that prey on aphids include lady beetles, lacewings, aphid midges, hover flies and tiny parasitic wasps (see the Beneficial Insects Info Sheet in this series). They are common and often control aphids before gardeners even notice that aphids are present. On ornamental shrubs and trees, aphid infestations can be unsightly, but the effect is usually temporary and causes little real damage. Large numbers of aphids on vegetable seedlings and bedding plants distort and delay growth. On roses, aphid infestations can ruin flower buds. Aphids on boulevard trees don’t really damage the trees, but they can be a serious nuisance where they drip honeydew onto sidewalks and parked cars. The honeydew also attracts yellowjacket wasps in late summer.
How can I prevent damage?
Lure the beneficial insects that eat aphids into your yard by planting flowers that attract them: sweet alyssum, yarrows, candytuft, dill, cilantro, parsley and many other herbs. (For more information, see the Beneficial Insects Info Sheet in this series.) Aphids thrive on slightly drought-stressed plants, so mulch around plants to keep in soil moisture. Dig compost into the soil to improve the moisture holding capacity. Avoid over fertilizing trees and shrubs with nitrogen, which promotes soft lush growth that can be attractive to aphids.
What can I do to control aphids?
Before taking action, look carefully among the aphids for beneficial insects. If you see predators, you probably won’t need to take action because they often control aphids in a week or two. If predators are not acting fast enough, or small plants are being seriously damaged, spray aphids off of plants with a strong stream of water. It is important to spray water again in 3-4 days to catch any survivors. This is surprisingly effective and does the least harm to aphid predators. As a last resort, spraying insecticidal soap or pyrethrins will knock back aphid numbers (but at the cost of killing beneficial insects among the aphids). Some garden centres sell aphid predators. Beneficial insects are expensive, but may be worth using to control aphids in special situations such as boulevard trees, in rose gardens or greenhouses. The most effective of the commercial aphid predators is the aphid midge, which is a tiny fly (Aphidoletes). Before you buy, talk to garden centre staff or other experts about how to use these insects.
Tips For A Healthy Garden
- Enrich the soil once or twice a year with compost or other organic fertilizers.
- Choose plants adapted to the conditions of sun or shade, moisture and soil acidity. If necessary, correct the drainage and acidity to suit the plants.
- Plant native plants, which are adapted to the local climate. Most are easy to care for and have few pest problems.
- Before buying plants, make sure they are healthy and free of diseases and insect pests.
- Water deeply, but infrequently, to encourage deep rooting.
- Cover the soil between plants and under shrubs with organic mulches. This insulates the soil, keeps in moisture and suppresses weeds.
- Protect and attract native beneficial insect, birds and other animals.