aphid

Any of the family of small insects, Aphididae, in the order Hemiptera, suborder Homoptera, that live by sucking sap from plants. There are many species, often adapted to particular plants; some are agricultural pests.

In some stages of their life cycle, wingless females rapidly produce large numbers of live young by parthenogenesis, leading to enormous infestations, and numbers can approach 2 billion per hectare/1 billion per acre. They can also cause damage by transmitting viral diseases. An aphid that damages cypress and cedar trees appeared in Malawi in 1985 and by 1991 was attacking millions of trees in central and East Africa. Some research suggests, however, that aphids may help promote fertility in the soil through the waste they secrete, which is termed honeydew. Aphids are also known as plant lice, greenflies, or blackflies.

Features The characteristic features of an aphid are either two pairs of membranous wings or no wings (wings may be present or absent in different members of the same species); piercing and sucking mouth parts; and a pair of cornicles or tubes which extend from the posterior end of the oval-shaped abdomen and which secrete a waxy fluid as a protection against predacious enemies. This is distinct from the ‘honeydew’ secreted through the anus, which is often removed by attending ants.

Natural enemies Aphids have a number of natural enemies which may broadly be grouped as follows. (1) Aphid parasites, for example, the wasp Aphidius testaceipes. The female wasp deposits eggs within the aphid's body, which hatch into larvae that feed on the internal organs of the host and kill it. (2) Flies belonging to the family Syrphidae. (3) Beetles belonging to the family Coccinellidae, for example, the ladybird. However, these natural enemies usually fail to keep the population of aphids down effectively, and the farmer anxious to protect crops has to resort to chemical and cultural methods as well as the biological method of encouraging natural enemies.

Chemical and cultural control Chemical control includes the use of chemicals such as parathion and methyl parathion. There are three main cultural methods of control. One is to grow vigorous varieties of plants that can withstand the attack of aphids. Such healthy plants are obtained by preparing friable, firm seedbeds, planting good seed, providing adequate fertilizer and moisture, and rotating crops systematically. Aphids are known to injure barley, oats, and wheat sown in the autumn more severely when they follow grain sorghum in rotation, than when they follow soybeans, corn, or wheat. The second method is to grow resistant varieties of grain. The third method is to destroy volunteer (left over from previous crop) grains, which provide food and shelter for the aphids, by either ploughing or discing.