Sap-sucking insects, probably the most troublesome group of pests in the garden. There are several different species, all of which have evolved to exploit specific plants. Modern agricultural and horticultural methods have tended to increase their numbers and diversification. While the life cycles of different species may vary, they tend to follow a general pattern. Aphids overwinter as eggs deposited on shoots and on the bark of trees and shrubs. The eggs are timed to hatch as the sap rises in the spring. The larvae travel towards the emerging buds and begin to feed. From now onwards their whole system is geared to feeding, growing and increasing in numbers. Rapidly increasing in size, they go through a series of moults, continuing to feed through the stage equivalent to pupation in other insects. They go on to develop wing buds, but are incapable of flying. The flightless females are viviparous – live-bearing. Further generations of entirely live-bearing females are produced, asexually. At this stage the tips of the host plants are often covered in aphids. The necessity to colonise other plants results in a generation of winged females, which have limited flying capability and are carried by wind currents. Many insects are lost at this stage as they have little control over their movements, but the sheer volume means that sufficient aphids will reach suitable species of host plants. The female will continue to produce live-bearing females asexually until, with shortening days, provision must be made for winter. Winged egglaying females and males to fertilise them are produced and eggs are laid on the original host to overwinter and restart the cycle again the following spring.
By sucking the sap, aphids weaken and distort growth which as well as stressing the plant, leads to making it vulnerable to a range of secondary problems. The sap is rich in sugars, but low in protein content, so aphids have to take in much more sugar than they can use in order to get sufficient protein. Excess sugar is excreted as ‘honeydew’. The plant leaves and stems thus become sticky and are soon covered with the black fungus sooty mould – the leaves look as though they have been dusted with a black powder. Aphids, along with all sap-sucking insects, are responsible for the spread of viruses which can greatly reduce a plant’s performance and for which there is no known cure.
Aphids can be controlled by a systemic pesticide such as amidacloprid, or a contact pesticide containing bifenthrin. Organic treatments include spraying with rape seed oil or pyrethrins. For any treatment to be successful it is important that it is applied immediately the aphids are seen.