Aphids are the most common of all plant pests and almost every plant from the smallest annual to the tallest oak tree can be infested at one time or another by one or more species.
There are numerous species of aphids – some are specific to one type or group of plants (such as the black bean aphid), whereas others will attack a wide range of plants.
Different species have different lifecycles, but most spend the main part of the year as wingless females, which give birth to live young. This means that even 1 aphid can quickly produce a large colony. Winged, flying forms develop when overcrowding or seasonal changes dictate moves to other plants.
Most aphid species overwinter as eggs on the host plant, but some remain as active feeding aphids.
These little bugs feed by sucking the sap of plants, weakening them and reducing their growth. They can also transmit viruses to the plant. Affected plant leaves can become curled or distorted in shape.
Aphid colonies usually gather on the undersides of the leaves – or on flowers or growth points – and as they suck the sap, they excrete a sugary honeydew onto the leaves below, which develop shiny glaze. Particularly in damp conditions, sooty moulds will grow on the honeydew. This is not only unsightly, but it will gradually deprive the leaf of light and eventually the plant could die.
Aphids also shed their skins as they increase in size. Cast off skins also gather on the leaf surfaces below forming a whitish dust, which is often the first tell-tale sign some people see.
Small colonies and individual aphids can be controlled by simply wiping them off the plant with a damp cloth or sponge, or by squashing them.
Where this isn't feasible, or practical, then you can control aphids by spraying affected plants with an insecticide.
A contact, natural-based insecticide, such as pyrethrin (a natural plant extract) will control most aphids and other plant pests. As their name suggests, a contact insecticide only kills the insect pests it comes into contact with.
Systemic insecticides also kill on contact, but are then also absorbed inside the plant, and move all around the plant, providing protection against further attack for up to 3 or more weeks.
Some aphids, such as woolly aphid, have a protective covering that contact insecticides cannot penetrate. This means they need to be sprayed with a systemic insecticide.
Always check that the insecticide is approved for use on edible crops, before treating aphids on fruit and vegetables.