Injurious Insects

INJURIOUS INSECTS Injurious insects include members of almost all orders and comprise species which destroy cultivated plants and forest trees, others which injure grain and stored products, manufactured goods and raw materials; there are again species which infest domestic animals as well as those which molest or harm man himself. An idea of the losses occasioned by insect pests may be gained from the following figures. The Mexican cotton-boll weevil is estimated to cost the cotton-growing States of North America from 120,000, 00o to £3o,0OO,o0o annually; the codling moth causes over 12, 400,000 loss to fruit growers in the United States each year ; rice leaf-hoppers in 1914 entailed a loss of nearly £i,oO0,O0o in one division only of the Central Provinces of India. In Great Britain the ox warble fly has been estimated to injure hides to the extent of 1400,00o per annum and the frit fly to destroy eight bushels of oats in each acre grown in an average year. The aggregate losses from insect pests to farm crops alone in the United States were estimated to amount in 1912 to a sum greater than the total cost of education in that country for the same year.

It has to be remembered that the very methods man has adopted in civilization favour the multiplication and spread of insect pests. The cultivation of large areas of country devoted to the production of a single crop provides unlimited means for the rapid multiplication of pests affecting that crop ; at the same time the growth of railroad and maritime communications facilitates the spread of insects from one country into another. From among 73 of the worst insect pests of the United States, 37 have been introduced from foreign countries. Of these latter most of them have entered America unaccompanied by natural enemies that help to keep them in check in the lands whence they originally came. The European corn borer, cotton-boll weevil, San Jose scale, gypsy and brown-tail moths, alfalfa weevil, oriental peach moth and Japanese beetle, along with other pests, have all entered the United States from foreign lands, and have found conditions highly favourable to their multiplication and spread. Similarly the woolly aphis and greenhouse white-fly are believed to have entered the British Isles and other European countries from abroad, the grape-vine Phylloxera, a North American insect threatened in 1870 to destroy vine culture in Europe, and quite recently the Colorado potato beetle has become established in the Gironde area of France. Quarantine regulations, if strict, do much to preclude the entry of foreign pests into a country, but the in nate difficulties attending the total exclusion of such immigrants are too great for legislative methods to be anything more than only partially effective.

Some of the more important insect pests are enumerated below, while further information is given in the separate articles devoted to the various orders to which these pests belong.

Insects Affecting Cultivated Crops.

Among the more im portant pests of cereals, frit fly (Oscinella frit) takes a serious toll of oats and to a lesser degree wheat, barley and rye over most of Europe, and the gout fly (Chlorops taeniopus) attacks barley. Wireworms or injurious larvae of click beetles (family Elateri dae) attack the roots, not only of cereals but of many other crops also, and at times cause great damage. Among other pests, the Hessian fly (Mayetiola destructor) attacks wheat on both sides of the Atlantic, while the chinch lug (Blissus leucopterus) is exceedingly destructive to all kinds of cereals and grasses over eastern North America. Caterpillars of the type known as army worms and cut worms, together with locusts (q.v.) are important enemies of many crops in various parts of the world. The Euro pean corn borer (Pyrausta nubilalis) is an Old World pest of maize and other plants, but since 1917, when it was discovered near Boston (Mass.), it has become one of the worst of all immi grant insects found in the United States and Canada, where it now threatens the very existence of the maize crop and has largely resisted attempts to check its ravages and increasing spread.

Clovers and lucerne are subject to injury from various insects especially root-feeding larvae of weevils (Sitona) ; although lu cerne is relatively free from serious pests in Europe, the alfalfa weevil (Phytonomus posticus) is a major pest in Utah, Colorado and adjacent States, having been imported from Europe. Vege tables suffer many pests and certain of these occur in both Europe and North America, notable examples being the onion fly (Hylervyia antiqua), the cabbage root fly (Pegomyia brassicae), carrot fly (Psila rosae), the asparagus beetle (Crioceris asparagi) and the cabbage white-butterflies (Pieris). In the United States curcubitaceous plants suffer heavily from the striped cucumber beetle (Diabrotica vittata) and the squash bug (Anasa tristis). In Great Britain and most parts of Europe the potato is singularly free from pests of the foliage, but in North America it is consist ently attacked by the Colorado beetle (Leptinotarsa decemline ata). Beans, mangolds and sugar-beet are infested by the bean aphis (Aphis ruifaicis) over much of Europe, while flea beetles (Phyllotreta spp.) riddle the foliage and destroy seedling tur nips, mangolds and other crops.

The cotton crop in many parts of the world has a formidable enemy in the pink bollworm (Pectinophora gossypiella), while bugs of the genus Dysdercus known as cotton stainers also cause great damage, especially in the West Indies. The Mexican cotton boll weevil (Anthonomus grandis) is one of the most severe of all insects affecting this crop, but is confined to Central America and the United States. In the latter country vast sums of money are annually spent in combating it. Sugar cane is especially at tacked by moth borers, notably the widely spread species Dia traea saccharalis. Other moth borers include Chilo simplex in India, where it is a most important enemy of the crop, and the grey borer (Laspeyresia schistaceana) of Java. The borer weevil (Rhabdocnemis obscura) attacks not only sugar cane, but also banana and various palms all over the tropics, while in Trinidad, the frog-hopper (Tomaspis saccharin) causes immense damage and is exceedingly difficult to control. At one time the leaf-hopper (Perkinsiella saccharicida) threatened the existence of the sugar cane industry in the Hawaiian Islands until it became effectually suppressed by biological methods of control.

In North America tobacco suffers from the flea beetle (Epitrix parvula) and caterpillars of hawk moths among other pests. In Ceylon tea is affected by the shot-hole borer beetle (Xyleborus fornicatus), while in Assam the Capsid bug Helopeltis theivora is a major pest locally known as tea blight. Coffee has numer ous pests including moth borers, beetle borers, leaf-miners and numerous scale insects.

The insect enemies of fruit and fruit trees would require sev eral columns for their enumeration alone. Scale insects are of great importance in many lands. The San Jose scale (Aspidiotus perniciosus) in warm climates attacks all fruit and many other kinds of tree and multiplies with great rapidity. In North Amer ica it is a permanent enemy, extending all over the United States and into southern Canada. The fluted scale (Icerya purchasi) of Australia has spread throughout the world wherever citrus fruits are grown, but is effectively controlled by biological measures. Citrus fruits suffer from many other scale insects and from mealy bugs. In California several species of these pests are being re pressed by biological measures, but spraying and fumigating under tents are resorted to on a large scale also. Among aphides the woolly aphis (Eriosoma lanigera) of the apple is prevalent in all parts of the world wherever that fruit is extensively cultivated, and mention must also be made of the Phylloxera of the vine (q.v.). Among beetle pests the plum curculio (Conotraejielus nenuphar) is well known in North America, where the recently in troduced Japanese beetle (Popillia japonica) skeletonizes the leaves and destroys the fruit of many trees and shrubs; its larvae, it may be added, are destructive below ground to lawns and golf greens. In Europe the apple blossom weevil (Anthonomus pom orum) destroys the opening flower buds, and among moth pests, caterpillars of the winter moth (Cheimatobia brumata) and other Lepidoptera destroy the foliage of various fruit trees; in North America those known as canker worms and tent caterpillars are exceedingly destructive. Since 1916 the oriental peach moth (Laspeyresia molesta) has become an immigrant pest attacking plum, cherry, peach, quince and apple with great severity in the eastern United States. The stem borer leopard moth (Zeuzera pyrina) attacks both fruit and shade trees in several parts of the world, while the codling moth (Cydia pomonella) has spread throughout the world where the apple is cultivated and causes vast damage by its caterpillars, boring into the fruit. Among fly pests the Mediterranean fruit fly (Ceratitis capitata) and its allies attack an enormous variety of succulent fruits in most warm countries with the exception of California. Other pests include the pear fruit midge (Contarinia pyrivora) and the pear leaf midge (Perrisia pyri). Hymenopterous pests include various sawfly caterpillars attacking apple, pear, cherry, gooseberry and currants, and most species occur in both Europe and North America.

Insects Affecting Forest and Shade Trees.

In Europe the oak and other forest trees suffer defoliation by various caterpil lars, notably those of the pea green moth (Tortrix viridana), nun moth (Lymantria monacha) and of numerous Geometers such as the pine moth (Bupalis piniaria), winter moth (Cheimatobia bru mata), march moth (Anisopteryx aescularia) and mottled umber moth (Hibernia defoliaria). In America the gypsy moth (Por thetria dispar) and the brown-tail moth (Euproctis chrysorrhoea) have both been accidentally introduced from Europe and defoliate a great variety of trees. The first-mentioned species especially has entailed and is still entailing enormous expense in efforts to control its spread; already some $25,000,000 have been devoted to destroy ing these two pests by the legislatures of the eastern States, apart from very large allocations of Federal money for the same pur pose. The European elm-leaf beetle (Galerucella luteola) is another serious defoliator, and in parts of the United States thousands of fine elms have been killed by this insect. Sawfly caterpillars of various species are very destructive defoliators of larch and other coniferas on both sides of the Atlantic, and these same trees also suffer heavily from shoot-destroying moth larvae, notably, the pine-shoot moth (Evetria buoliana) in Europe and restricted regions in the United States, and the spruce and balsam bud-worm (Harmolegia f umi f erana), in the United States and Canada. Great injury to conifers is also caused by aphides of the genus Chermes and its allies. Perhaps the most severe of all forest pests are the bark and ambrosia beetles (Scolytidae), which attack conifers and to an extent some broad-leafed trees. The species Dendroctonus ponderosae is stated to have destroyed in 1919 more than a billion cubic feet of timber in the Black Hills forest reserve in the United States. Other species are almost equally de structive both in the United States and Canada and probably few coniferous forests in any part of the world are exempt from the depredations of these beetles.

The solid wood of all types of tree is infested with timber boring larvae of Buprestid and Longicorn beetles, particularly in warm countries where these two families attain their maximum de velopment. It is estimated that 1,200,000 cu.ft. of Sal timber is damaged annually in the Indian forests by the Longicorn, Hopol cerambyx spinicornis. In many parts of the world timber is in jured by wood-wasps (Siricidae) and wood-boring moth larvae, notable examples of the latter belong to the family Cossidae, which includes the leopard moth, goat moth and the bee-hole borer of teak. Mention must also be made of the pine weevil (Hylobius abietis), which is exceedingly destructive to young conifers in Great Britain.

Insects Affecting Stored Products and Manufactured Goods.—Most of the insects which come under this category are cosmopolitan pests that are readily transferred from one country to another through commerce. Among the many pests of stored grain, flour and meal, the granary weevils (Calandra granaria and oryzae) are among the best known, while the mealworms (Tene brio) and the saw-toothed grain beetle (Silvanus surinamensis) also occur with great frequency. Among moths, the caterpillars of the Mediterranean flour moth (Ephestia kuehniella) and the Indian meal moth (Plodia interpunctella) render flour and meal unfit for consumption, while the caterpillars of the Angoumois grain moth (Sitotroga cerealella) attack grain in the field as well as in storage.

Among pests which attack clothing and carpets, the clothes moths (Tinea pellionella and Tineola biselliella) are well known and their larvae also attack wool, hair, feathers and dried skins. Beetles of the genera Anobiurn and Xestobium bore into furniture and the rafters of old buildings, and the larder beetle (Dermestes lardarius) and its allies infest skins, wool, meat, ham and cheese. Stored and manufactured tobacco is attacked by the cigarette beetle (Lasioderma serricorne) and dried museum specimens of natural objects by the beetle Anthrenus scrophulariae, which also attacks carpets and other materials. Drugs such as aconite and opium, together with biscuits, chocolate, pepper, ginger and many other substances, are infested by Sitodrepa paniceum, which is often known as the drugstore beetle.

Insects Affecting Domestic Animals.

Among the most im portant of these are the ox warble flies (Hypoderma), which are prevalent throughout Europe, North America and other parts of the world. They occasion great losses to cattle raisers by the presence of the grubs beneath the skin which cause irritation and deterioration of the flesh, reduction of milk and perforation of the hides; anaphylactic shock may also result when the grubs under the skin are crushed, and illness or even death of the host may supervene. The sheep bot-fly (Oestrus ovis) is of worldwide dis tribution and its larvae infest the sinuses of the head; sheep also suffer from the ked or "tick" (Melophagus ovinus) which causes irritation and loss of flesh, and is especially harmful to lambs. The sheep-maggot flies of Australia cause immense losses through their larvae infesting the wool and skin and causing septic wounds. Horses and mules are very prone to attack from bot flies of the genus Gastrophilus and their larvae infest the throat, stomach and intestines, attaching themselves to the lining walls of the parts concerned. In Russia the flesh fly (Sarcophaga magnifica [W ol farti]) causes great suffering to domestic animals, owing to even the smallest wound becoming infested with its larvae. The screw worm fly (Chrysomyia macellaria) is a very similar type of pest in the south-west United States and its larvae often cause fatal in juries. Lice or Siphunculata are irritating blood-sucking ecto parasites which cause wasting of the flesh when very numerous: horses, cattle and swine are all liable to infestation. Biting lice or Mallophaga are particularly irritating pests of poultry and some species also infest domestic animals ; lastly, fleas of various kinds occur on domestic animals and birds.

Insects Affecting Man.—Many kinds of flies and certain other insects harm or molest man himself in various ways: some of these are merely irritating or annoying, while others act as agents, con veying the pathogenic organisms of some of the most virulent dis eases from one person to another. For a discussion of this subject the reader is referred to MEDICAL ENTOMOLOGY and PARASITOL OGY.

Insect Vectors of Plant Diseases.

During recent years it has become increasingly evident that certain insects disseminate dis eases among plants. Several bacterial diseases, such as fire blight of apples and pears and wilt diseases in other plants, are known to be spread from plant to plant by insects. The recently recognized group of virus diseases known under the names of mosaic, leaf curl and leaf roll, have been shown in some cases to be conveyed from diseased to healthy plants by the punctures of sap-sucking Hemip tera. Thus, curly top in beet is transmitted by the leaf hopper, Eutettix tenella, sugar-cane mosaic by the aphid, Aphis maidis, and aster yellows disease by the leafhopper, Cicadula sexnotata. Recent research points to the growing importance of virus diseases, and their transmission by insects, in plant pathology.