The agricultural crops play a very essential role in the life of human beings. They provide us food, fiber as well as many important products of our daily use and brighten our standard of living. The agricultural crops are although been affected by a number of organisms that are called pests which are responsible for the decrease in the food as well as the market value of the agricultural products. One such dangerous harm causing organisms are the aphids.
Aphids belong to the largest animal phylum Arthropoda and class Insecta. The common name of aphids is plant lice and they are tiny insects belonging to the order Hemiptera and family Aphididae. It has been recorded 4,400 species of aphids are present all over the world out of which 250 species are responsible for causing serious threat to our agricultural crops as well as ornamental plants. Their body size varies from 1-10 mm. Some animals act as predators of aphids for example ladybird beetles, parasitic wasps, hoverfly larvae, lacewings, crab spiders, aphid midge larvae and the entomopathogenic fungi. Aphids are although cosmopolitan but are widely distributed in the temperate zones. Aphids are easily transported from one place to the other by wind for example the currant lettuce aphids has migrated from New Zealand to Tasmania through wind itself. Human beings have also been noticed to be responsible for the spread of aphids by transporting the infected plant materials from one place to other.
The aphids bear soft bodies which may be green, yellow, black, brown or pink in colour. The antennae of aphids are six segmented and their mouth parts are very well adapted for piercing and sucking. They suck the sap of the plants with the help of stylets which are enclosed in a rostrum. The stylets are the modifications of the mandibles and the maxilla the common structures present in the mouth of the insects. The legs of aphids are long, very thin; two jointed and have two clawed tarsi. Most species of aphids possess a pair of abdominal tubes called the siphunculi or cornicle which is responsible for the secretion of a triacylglycerol containing, quick hardening defensive droplet designated as cornicle wax. Different types of defensive fluids are also secreted by different species of aphids. Aphids bear a tail like structure called the cauda just above their rectal apertures and have a pair of compound eyes that bears an opercular tubercle consisting of three lenses called triommatidia. When the nutrition from the host plant decreases and the availability of suitable food declines the aphids produce winged offsprings called alates which fly to search suitable food sources in different directions. Mouth parts or even the eyes have been found to be absent in some forms and some species.
Majority of aphid species are monophagous which means they prefer to suck the sap from only one host species but the green peach aphid Myzus persicae tends to feed on a number of plant species of different plant families. They feed on the sap of the phloem vessels of the plants like the scale insects and the cicadas. As the phloem vessel is punctured the sap which was under high pressure directly enters the food canal of the aphid. They are also responsible for transmission of a number of plant viruses especially concerned with the potatoes, cereals, sugar beets and the citrus plants. These plant viruses are sometimes responsible for the death of the plants also.
Large numbers of animal species show a mutualistic relationship with aphids. Many species of ants feed on the honey dew secreted by the aphids. Honey dew is secreted by the terminal part of the alimentary canal of the aphids. Some species of the farming ants protect the aphid eggs in their nests and also provide protection to their young ones by bringing them back to the nests. A very fantastic example of the mutual relationship has been found between aphids and ants for the Lycaenid butterflies. These butterflies lay their eggs in the vicinity of aphids and the caterpillars that hatch out from the eggs feed on aphids. Here in this case the ants do not defend the aphids but they carry the caterpillars to their nests and in this way provide protection to the aphids. After bringing the caterpillars to their nests the ants feed the caterpillars and the caterpillars produce honey dew for the ants. When the caterpillar is ready to undergo pupation it pupates in the cocoon at the entrance gate of the nest and after two weeks the adult butterfly emerges out. Some bees of the coniferous forests also make use of honey dew secreted by the aphids for making the forest honey. Many aphids species are also responsible for housing the endosymbiotic bacteria inside their bodies in special cells called bacteriocytes and these bacteria synthesize certain amino acids which the aphids are unable to get from the sap of the plants.
Reproduction in aphids may be very simple or very complex depending upon the environmental conditions. The life cycle of aphids includes both asexual and the sexual reproduction and production of both eggs as well as live young and they switch between both woody and herbaceous plants throughout the year. Many aphid species show cyclical parthenogenesis. In the spring and the summer seasons only females are present in a population. The overwintered eggs hatch in the spring in the form of females and these females are called as fundatrices. These females reproduce parthenogenetically and produce young ones. The females undergo special meiosis and the eggs have same genetic constitution like that of the mother. The embryos develop inside the mother’s ovarioles and the mother directly gives birth to the first instar nymphs which are also females. These females are identical to their mothers except the body size and they are called as viginoparae. This life cycle continues throughout summer and multiple generations are produced whose life span is of 20-40 days only.
In autumn aphids undergo sexual reproduction and produce eggs. Fluctuations in temperature and photoperiod forces the female aphids to reproduce patheneogentically and it produce sexual males and females. The males resemble their mothers except that they are haploid. Sexual aphids lack mouthparts and wings. The sexual males and females copulate and produce eggs that develop outside the body of the mother. The eggs hatch in the spring in the form of females. Some aphids also have telescoping generations which means that the mother who is having daughter inside her is also housing a parthenogentic developed daughter insider her body. The availability of food also affects the mode of reproduction.
Aphids are believed to have evolved in the Permian period about 280 million years ago. The population of aphids can easily decline under extreme conditions of weather. As we all know that aphids are the soft bodied insects they may be affected by bacterial, fungal and viral infections and other predators also. The aphids defend themselves by feigning themselves as dead and they also drop from the plants when they feel the presence of any predator. Aphids are also responsible for decreasing the growth rate of plants which may include yellowing of leaves, mottling, stunting, browning, twisting etc.
Now these days scientists are working on the biocontrol agents that directly help in regulating the aphid population. Ladybird beetles are one such biocontrol agents. Work is under progress. Many syrphid fly larvae also feed on aphids. Aphids although are dangerous for our agricultural crops but survive dynamically in the environment.