Aphids

How do I know if I have a problem?

Although aphid damage is easiest to spot on ornamental, fruit, and shade trees, they can also infest flower and vegetable gardens. You will find aphid colonies on the underside of leaves, the tips of branches or anywhere there is new plant growth.

Leaves attacked by aphids have spotty yellow discolourations, usually on the undersides. The leaves may later dry out and wilt or curl. Some types of aphids cause plants to form galls (swellings of plant tissues that are globe or spindle-shaped). The galls, which often turn brown, contain many aphids in all stages of development. When aphids moult, they cast off their skins, which look like small white flakes.

Winged adults are produced only when the colony must migrate, like when the colony is overcrowded or the climate is unfavourable.

How can I get rid of aphids?

Physical control

• Repair screens and weather-stripping around doors and windows to keep winged adults away from houseplants.
• Inspect new houseplants before bringing them into your home. If needed, isolate and treat them with one of the methods listed below.
• As a preventative measure, avoid feeding garden and house plants with high-nitrogen fertilizers.
• Monitor house and garden plants often during the growing season. As the flight of winged colonizers cannot be predicted, examine plants weekly to see if you need to take action against a new infestation. Examine the bud area and underside of new leaves for clusters or colonies of small aphids. Once colonies are found on a plant, this means that aphid numbers can increase quickly if no action is taken.
• On small plants, crush the colony by hand or prune the plant to remove the colony. In some cases, this may provide enough control. Pruning and destroying infested leaves or plant sections may also help prevent plant diseases from spreading and reduce the development of new colonies.
• Spray dormant oil on fruit trees in the spring to kill hibernating eggs. Dormant oils are thick oils used mainly on fruit trees to control hibernating mites, scales and other insects. You can buy dormant oils at garden supply stores.
• To control infestations in your home garden, spray the underside of plants with strong jets of water to reduce aphid populations. Monitor plants and repeat as needed (this can be every few days).
• Since aphids like the colour yellow, place pans of water and yellow food colouring close to observed infestations to attract and drown them. This method is useful in vegetable gardens.
• In outdoor gardens, place yellow double-sided tape around the rims of pots and along edges of raised beds to trap the aphids.
• Lay heavy aluminium foil on the ground along rows of plants to repel the aphids.
• Certain plants coriander, basil, catnip, chives, and dill may protect other susceptible plants from infestation. They excrete an odour or oil that is unattractive to aphids and may repel them from the area.

Predators

Try to attract the natural enemies of aphids by planting a variety of flowering plants. Small-flowered plants like sweet alyssum, yarrow, and herbs in the carrot family are often visited by insects that prey on aphids, like the lady beetle, the lacewing, and syrphid flies. Parasitic wasps are also attracted to these flowers.

Some natural predators or parasites can be bought from specialized suppliers and certain garden and greenhouse supply stores. Follow instructions carefully if buying these biological control products.

Chemical products

• Aphid infestations may be treated with insecticides. There are some active ingredients registered for use by consumers on aphids.
• Insecticidal soap sprays, which must be in direct contact with the insects, may also be effective and have no residual effect.

Tall, heavily infested trees may be hard to treat without special equipment. For infestations bad enough to threaten the health of a tree, you may want to hire a licensed pest control operator.