Aphids are insects in the order Homoptera, which are also known as plant lice. Some 3,800 species of aphids have been identified worldwide with 1,300 species occurring in North America, which includes some 80 species that are pests of crops and ornamental plants. Aphids have a distinctive pear-shaped body, and most are soft and green in color. The wings are transparent and are held in a tent-like position over the abdomen, which has a short tail, called a cauda. The legs are long and thin, and the antennae are thin and have six segments. Two tube-like structures, called cornicles, project from the fifth or sixth abdominal segments of aphids. The cornicles excrete a defensive chemical when the aphid is threatened.
Aphids have a complicated life cycle and reproduction
habits that make them extremely adaptable to their
host plants and environments. When aphid eggs that have
overwintered on their host plants hatch in the spring, they
produce females without wings. These females and are
capable of reproducing asexually, a process called
parthenogenesis. Several asexual generations of aphids
may be produced during a growing season.
When it becomes necessary to move to another plant, females with wings are produced and move to another host plant. As winter approaches, both males and females are produced and their fertilized eggs again overwinter until the next spring. Sometimes winged females that produce asexually also migrate to new hosts.
The lack of wings among generations of aphids that have no need to migrate is seen as an adaptive advantage, since it helps keep them from being blown away in windy weather.
An intimate, symbiotic relationship exists between
ants and aphids. They are often compared to cattle, with
the ants acting as protectors and ranchers. What aphids
have that ants want is something called honeydew, a sweet
substance that is excreted by aphids through their anus
and contains surplus sugar from the aphid’s diet. Ants protect
aphid eggs during the winter, and carry the newly
hatched aphids to new host plants, where the aphids feed
on the leaves and the ants get a supply of honeydew.
Because of their ability to reproduce rapidly and grow large colonies, their feeding on plants causes yellowing, stunting, mottling, browning, and curling of leaves, as well as inhibiting the ability of the host plant to produce crops. Infestations by aphids can cause plants to die, and the insects can carry other diseases, such as plant viruses, from one plant to another. Their saliva is also toxic to plant tissues. Among the biological controls of aphid infestations in agriculture and horticulture are lacewings, sometimes called “aphid lions,” lady beetles or ladybird beetles (ladybugs), and syrphid flies. Pesticides, including diazinon, disyston, malathion, nicotine sulfate, and others, are also used to control aphids. On a smaller scale, some gardeners control aphids by simply washing them off with a spray of soapy water.
Exoskeleton—A hard, shell-like structure that
serves both to protect the vital organs of animals
without an internal skeleton and to support their
Honeydew—A sweet substance excreted by aphids that ants need.
Parthenogenesis—Asexual reproduction without the fertilization of eggs.
Spiracles—Openings that lead to a system of tubes that supply air to insects.
Symbiosis—A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.
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