homopteran

homopteran, (order Homoptera), any of more than 32,000 species of sucking insects, the members of which exhibit considerable diversity in body size. All of the Homoptera are plant feeders, with mouthparts adapted for sucking plant sap from a wide assortment of trees and wild and cultivated plants. Many homopterans cause injuries or destruction to plants, including fruit trees and grain crops, and can be vectors of plant diseases. A few provide secretions or other products that are beneficial and have commercial value. Most members of the Homoptera fall into one of two large groups; the Auchenorrhyncha, which consists of the cicadas, treehoppers, froghoppers or spittlebugs, leafhoppers, and planthoppers or fulgorids; and the Sternorrhyncha, which includes aphids or plant lice, phylloxerans, coccids, scales, whiteflies, and mealybugs.

General features

Size range

Most homopterans range from 4 to 12 mm (1/6 to 1/2 inch) in length. There are certain species of cicadas in Borneo and Java, however, that are 8 cm (3.1 inches) long with wingspreads of 20 cm (7.9 inches). The large fulgorid or lanternfly can attain this size also. On the other hand, some of the tiniest scale insects are only 0.5 mm (0.02 inch) in length.

Distribution and abundance

Although Homoptera species are distributed throughout the world, the relative numbers of individual species vary in a given locale. Only one cicadid species is known in Great Britain, and fewer than 12 in all of Europe. However, more than 200 cicadid species are known in North America, and about 180 in Australia.

The abundance of any species in a given environment depends upon the biotic potential of the insect, the abundance of the food plant, and other factors favourable for development of large populations. Certain species never reproduce in excessive numbers, while others, considered pests, produce many offspring. Insect species that feed on available crops or other plants present in quantities sufficient to support them normally develop large populations; for example, the oyster shell scale (Lepidosaphes ulmi) on fruit trees and ornamentals; the greenbug (Toxoptera graminum) on wheat; and the potato leafhopper (Empoasca fabae) on potatoes, beans, and alfalfa. Grape leafhoppers (Erythroneura) frequently develop large populations that result in heavy plant losses.

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Importance

Homopterans, because all species feed on sap sucked from plants, often cause injuries or destruction to the plants that nourish them. When such plants are cultivated crops (e.g., grains or fruit trees) or valued ornamentals, the economic loss resulting from infestations is severe. In addition, some homopteran species act as vectors of virus- and bacteria-caused diseases of their plant hosts. The check exerted upon insect pests by other insects is an important mechanism of natural control of populations. Predacious insects feed on small, weak species; parasitic insects live on or in a host and feed at its expense. Aphids, for example, are parasitized principally by members of the Hymenoptera; two important aphid predators are ladybird beetles and lacewings. Pests also may be controlled by chemical and biological methods (e.g., development of resistant plants, as with European grapevines).

The homopterans are responsible for injuring numerous plants of economic importance. Cicadas or dogday harvestflies, sometimes mistakenly called locusts, are well-known pests that have an annual life cycle. They are characterized by their large size and the strident song of the male. Periodical cicadas emerge every 13 or 17 years in large numbers, swarm in trees, mate, and lay eggs in green twigs. Permanent damage to fruit twigs is caused by the egg deposition slits; when the weakened twigs mature into fruit-bearing limbs, they break under the weight of the fruit, and the crop is lost. Failures of this sort can be avoided by not planting young fruit trees in years of cicada emergence.

Leafhoppers cause various types of plant injury by interfering with the normal physiology of the plant. The salivary secretion of the potato leafhopper, for example, causes leaf cell hypertrophy that impairs transport of sugars. The resulting sugar accumulation in the leaves destroys chlorophyll and causes the leaves to turn brown and die. This injury, termed “hopper burn,” can result in complete loss of a potato crop if not controlled. Another type of injury is caused by leafhoppers that feed upon plant mesophyll tissue. In addition to removing excessive amounts of sap, these insects also destroy the plant’s chlorophyll, resulting in yellow spots on the leaves, which eventually turn yellow or brown. Erythroneura, Typhlocyba, and Empoasca species cause this injury to apple trees and grapevines. Grape leafhoppers reduce growth and foliage function and cause formation of grapes that are inferior in size, colour, flavour, and sugar content. Plants also are injured when insects lay eggs in green twigs. The egg punctures of several leafhoppers and treehoppers reduce the flexibility of plant limbs. Plant stunting and severe curling of leaves occur when the leafhopper Empoasca fabae punctures the undersurfaces of leaves and veins of bush beans and inhibits growth. This leafhopper also feeds on alfalfa and causes leaves to turn yellow and drop off. In the same way, aphids and mealybugs cause leaf curling on potatoes and many ornamental plants, and the potato psyllid feeds on potato leaves and causes curling and yellowing known as “potato yellows.”

The froghoppers, often called spittlebugs because immature stages live in spittlelike masses, feed on a variety of plants. One important species, the meadow spittlebug (Philaenus spumarius), feeds extensively upon clover and alfalfa and causes severe stunting that can result in loss of up to 50 percent of a crop. Scale insects, unless parasitized, produce enormous populations on green twigs, young limbs, leaves, and fruit; when tree bark or shrubs become encrusted with one or more layers of scales, the entire plant often dies. Damage is caused to apples by the rosy apple aphid. Females of the third seasonal generation remain on the apple leaves until after small apples have formed. Many aphids crawl onto these tiny apples and puncture them causing dimpling of the fruit and normal incision of tiny apples. The cluster of apples, known as aphid apples, are small and gnarled.

More than 100 species of leafhoppers are known organisms causing plant diseases. Some important plant disease viruses transmitted by leafhoppers are aster yellows (transmitted by Macrosteles fascifrons); potato yellow dwarf (several species of Aceratagallia and Agallopsis); and phony peach disease and Pierce’s disease of grape (species of Cuerna, Homalodisca, and Oncometopia). Corn stunt is transmitted by species of Dalbulus; curly top of sugar beet by Circulifer tenellus; eastern and western x-disease by species of Colladonus; and elm phloem necrosis by Scaphoideus luteolus. One species of spittlebug is a vector for a yellow virus of peaches. Aphids are vectors for several virus mosaic disease organisms. A membracid species transmits the virus that causes pseudocurly top of tomato and tobacco, and two species of fulgorids are vectors of virus disease organisms of rice. The whitefly Bemisia tabaci transmits the virus that causes tobacco leaf curl, and species of mealybugs are vectors of the virus that causes pineapple wilt. The bacteria that cause fire blight disease on pear, apple, and quince trees are transmitted by several types of insects including leafhoppers. The bacterial pathogen, Neofabraea perennans, that causes perennial canker of apple is transmitted by the woolly apple aphid.

Because homopterans suck more sap from plants than they need, the surplus is excreted from the tip of the abdomen as sweet droplets known as honeydew. If the insect is feeding on apple foliage and honeydew falls on apples, a sooty fungus (sooty mold) grows in each droplet. The apples become black spotted and are no longer marketable. Many other homopterans also produce honeydew, with sooty mold growing on whatever the honeydew lands on.

Of great economic importance are insects that secrete lac on twigs in tropical and subtropical regions. The lac is refined and used in preparing shellac and varnishes. More than 4 million pounds of lac are refined annually. Other waxes secreted by aphids and scale insects are used in candlemaking, medicines, and candies.

Although few homopterans produce food for man, the tamarisk manna scale, Trabutina mannipara, is thought to have produced the biblical manna for the children of Israel. The females produce large quantities of honeydew that solidify in thick layers on plant leaves in arid regions. This sugarlike material, still collected by natives of Arabia and Iraq, is considered a great delicacy. The term manna often refers to plant products also. Certain species of scale insects produce a gum that was used as chewing gum by tribes of North American Indians. Female root-inhabiting scale insects (species of Margarodes) enclose their bodies in gold and bronze coloured wax cysts that are used in strings of beads. Certain colour patterns and designs of the forewings of tropical species of leafhoppers and planthoppers have been used in artwork by various peoples. For many generations the Mexican Indians have used a black, white, and red colour design in their art. The design is that of the forewings of a brilliantly coloured Agrosoma leafhopper, found on bushes along streams.

The scales of several species of scale insects, including the Old World kermes and New World cochineal, have been used to produce red dyes for clothing, foods, and medicines and in emulsions to colour film.