Biology: Insects: Aphids

Aphids are representative of a group of insects which obtain their food by piercing plant or animal tissues with sharp mouthparts and sucking up the body fluids. To gardeners and farmers, aphids can be serious pests. The gardener will be familiar with ‘greenfly’ on roses and ‘blackfly’ on broad bean plants.

Life cycle. In the summer months, all the aphids seen will be females and most of those on plants will be wingless. They reproduce at this time by a process called ‘parthenogenesis’ in which no sexual process occurs. The females do not lay eggs but deliver their young, called nymphs, alive.

The nymphs are like miniature adults and soon reach full size after a series of ‘moults’. From time to time, some of these nymphs develop wings and fly off to infest new plants.

In Autumn, some of the winged forms are males which fly off to a nearby tree or shrub. They are joined by winged females which produce wingless daughters. These daughters then mate with the males and lay eggs on the branches of the tree. The eggs have thick shells and can withstand low temperatures throughout the winter. In Spring, the eggs hatch to wingless females which feed on the young tree leaves, producing daughters parthenogenetically. Some of these daughters will be winged and fly away to reach the plants on which they feed during the Summer.

Feeding. Aphids have elongated mouth parts which fit together to form a piercing and sucking tube, a proboscis. On a leaf, they insert this proboscis through the leaf tissue until it reaches the food-conducting cells (phloem cells). They inject a little saliva which starts to digest the cell contents and then suck up the fluid into their gut.

The plant sap contains more sugars than amino acids (protein precursors) and the aphids excrete a solution of the excess sugar through their anus. This liquid is popularly called ‘honey dew’ and when it falls on the leaf, it encourages the growth of mould. However, ants have a great liking for this ‘honey dew’ and clamber over the feeding aphids to collect it.

The feeding habits of aphids damage the leaves, making them curl up, dry out and fall off. This effect coupled with the fact that the plant is deprived of some of its nutrients can cause a loss in yield from crop plants and vegetables. Aphids also spread plant viruses by their feeding habits.

Control The aphids are preyed upon by ladybirds and their larvae and the larvae of lacewings. These predators control the aphid population to some extent but gardeners and farmers often have to resort to pesticides. Some of these simply kill the aphids on contact and may be as innocuous as soap solution. Since the aphid penetrates the plant leaf in order to feed, spreading insecticide on the leaf surface is ineffective and recourse is made to systemic insecticides. The plant absorbs these insecticides into its tissues so that the aphid is poisoned when it takes up the cellular fluids. The insecticides break down after a short time so they are not present in the harvested crop.

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