Insect injurious to staple crops

Preface

The sources of information concerning the insects affecting our staple crops are widely scattered throughout tlie bulletins of the State Agricultural Experiment Stations and of the United States Department of Agriculture, a few books on Economic Entomology, and various other publications. Few men but the entomologist have the desire or ability to glean an account of any given insect from the first-mentioned publications, if they know of the existence of others than those published by their own State station. The few books which have been written upon American Economic Entomology usually give but a brief and summary account of any given insect, too brief — it seems to the writer — to give a very clear understanding of the matter.

In preparing the following pages the author has been more and more impressed by the fact that for the control of most of the worst insect 2:)ests of our staple crops, the farmer must depend very largely upon general methods of farm practice. This being tlie case, it is essential that he have a correct knowledge of the pest to be combated ; such a knowledge of its life-history as will nuike plain the reason for the effect of any given procedure against it. Thus the better class of farmers may find a work in which each

Yl PREFACE.

insect is treated somewhat comjireliensively as to life-history, habits, and remedies, yet without being exhaustive or technical, to be of considerable service to them. To furnish such a guide to the more intelligent class of practical farmers has been the aim of the writer, who trusts that the following pages will be read as such and not as in any way a contribution to science.

The author wishes most uiu'eservedly to disclaim any originality for the contents of the work, and to state that unless otherwise noted all the facts are merely comjoilcd from the writings of others. Free use has been made of the writings of all the most prominent American entomologists. Where the treatment of a group of insects has been largely drawn from one or two sources, they will often be indicated by quotations in tlie text.

Many of the following chapters or parts of them have previously appeared in various agricultural journals during the past three or four years, to the editors of which the author desires to express his thanks for their courtesy in allowing him to here republish them; namely. The Country Gentleman, The Farmers' Review, Farm and Fireside, Farm News, The National Eural, Texas Farm and Ranch, The American Agriculturist, The National Stockman and Farmer, and The Practical Farmer.

The author is particularly indebted to Prof. M. V. Slingerland, of Cornell University, for kindly reading portions of the manuscript and for several suggestions of value; to Dr. L. 0. Howard for assistance in part of Chapter X; and to his wife, Anna Cecilia Sanderson, for a large amount of clerical assistance.

E. DwiuHT Sandkhson".

Newark, Dkl., February, 1901.

SOURCES OF ILLUSTRATIONS.

Electrotypes of the following figures were purchased, being secured through the kindness of the parties named : Fig. 9, from Houghton, Mifflin & Co. ; Fig. 102, from the Iowa Ag. Exp. Sta., through Prof. H. E. Summers; Figs. 7, 136, and 137, from Dr. J. B. Smith; Figs. 51, 52, 60, 84, and 85, from Cornell Univ. Ag. Exp. Sta., through Prof. M. V. Slingerland; Figs. 25 and 28, from The Farmers' Eeview, Chicago; Figs. 74, 75, 76, 77, and 78, from Dr. S. A. Forbes; Figs. 71 and 72, from The Country Gentleman, for which they were originally redrawn by the author; Fig. 53, from the Xebr. Agr. Exp. Sta., through Prof. L. Bruner; and Figs. 11, 24, 31, 50, 59, 61, 62, 64, 66, 67, 68, 79, 80, 81, 82, 83, 88, 89, 90, 91, 92, 93, 94, 97, 104, 105, 106, 107, 108, 109, 111, 112, 113, 114, 116, 117, 118, 119, 120, 121, 122, 123, 124, 126, 127, 128, 129, 131, 134, 135, 136, 143, 146, 154, 155, 156, 157, 158, 159, 160, 161, 162, from the United States Department of Agriculture, through the kindness of Dr. L. 0. Howard and Mr. F. H. Chittenden, of the Division of Entomology and the Division of Publications, to which gentlemen we are under especial obligation.

The following figures were kindly loanerl by the parties named: Figs. 29, 30, 32, 33, 34, 35, 36, 37, 38, 39, 40,

vii

Vlll SOURCES OF ILLUSTRATIONS,

41, 42, 44, 45, 46, 47, 48, 110, and 150, from the Md. Agr. Exp. Sta., through Prof. A. L. Quaiiitance, these having been first used by the author iu The Farmers' Eeview; Figs. 65, 132, and 133, from the Ky. Agr. Exp. Sta., through Prof. H. Garman; Fig. 125, from the Fhi. Agr. Exp. Sta., through Prof. H. A. Gossard; Figs. 54, 55, 56, 57, 58, 63, 70, 96, 98, 101, and 103, from the Ohio Agr. Exp. Sta., through Prof. F. M. Webster; Fig. 49, from the Minn. Agr. Exp. Sta., through Director W. M. Liggett; Figs. 12 and 95, from the Del. Coll. Agr. Exp. Sta.; Fig. 151, from the Peninsula Horticultural Society, through Prof. Wesley Webb.

Figs. 19, 20, 21, 66, 138, 139, 140, 141, 144, and 147 were loaned by the author. Figs. 13, 14, 15, 16, 17, 18, 23, 26, 27, 99, 115, 145, 149, 152, and 153 were copied from prints. Figs. 1, 4, 5, 6, 8, and 10, and Figs. 2, 3, 22, 86, 87, and 100 and frontispiece are original from photos by the author, the photos of the latter numbers being loaned by the Del. Coll. Agr. Exp. Sta.

Chapter I. Injury Done Staple Crops By Insect Pests

Ever since the plague of locusts in the time of the Pharaohs, insects have maintained a continual opposition to agriculture. History is replete with accounts of insect scourges and the enormous losses occasioned by them. And instead of diminishing with the advancement in agricultural methods, injurious insects have undoubtedly become both more numerous and more destructive in modern times. Every now and then we hear of communities assembling for prayer and fasting to appease the Almighty, whose wrath has hurled a new insect plague against them. But a little reflection will show that tlieee scourges are entirely due to natural causes. In fact such injuries are very largely due to man himself, who, in trying to subdue Nature by the clearing and cultivation of the land, has deprived the insects of their natural food. Thus they must needs feed upon that which is substituted by him, and as it is less abundant than the former wild vegetation, the number of insects and the injuries they inflict are more

Z INSECTS INJURIOUS TO STAPLE CKOPS.

apparent. But the native insects are by no means entirelyresponsible for this condition. Foreign insects are constantly being imported in one way or another, sometimes already being established pests in other lands and sometimes only becoming so under their new surroundings. These are even more injurious than those native, for whereas many of our native birds, insects, and diseases constantly prey upon native insects and thus keep their numbers in check, the enemies of imported pests rarely accompany them, and they thus increase at an alarming rate and do enormous damage before they are attacked by the natural enemies of similar native pests.

American farmers have learned from sad experience of the severe losses occasioned by insects, but few realize their enormity when considered as a whole. Several calculations of these losses have been made by competent authorities, who practically agree that one-tenth of the total agricultural product of the United States, or 1300,- 000,000, is but a conservative estimate of the loss annually sustained by this country. But when this statement has occasionally been made by the author it has been met with a look of incredulity which very plainly indicated that he was thought to have a very elastic imagination. A careful collection of such data as may be confided in as accurate shows that the above estimate is entirely correct. Considerably over one-half of this loss is upon the staple crops, the remainder being uj)on truck crops, fruits, domestic animals, and timber.

Growing Cereals. — Probably no other insect does so wide-spread damage as the Hessian Fly, attacking our chief staple, wheat, as well as rye and" barley. One-tenth of the whole crop, valued at 'ii>40,000,000, is generally con-

INJURY DONE STAPLE CROPS BY IKSFCT PESTS. 3

ceded to be destroyed by this pest every year, and in certain sections the loss often amounts to from 30 to 50 per cent. If the loss to rye and barley be put at one-fourth the loss of wheat, it amounts to about $1,000,000. From various estimates made at different times during that period. Prof. F. M. Webster states that 1330,000,000 represents the loss from the depredations of the Chinch-bug since 1850, or 17,000,000 per annum, whicli has been largely confined to the States of the Mississip^^i Valley. Corn has a host of insect enemies. Frequently the Corn Eoot-worm has damaged the crop to the extent of 10 to 20 per cent in many of the largest corn-growing States. The annual loss on this cro23 due to insects is certainly not under 5 per cent, or 137,000,000. Thus with only the above figures we see an annual loss of $85,000,000 u23on growing cereals.

Stored Grain. — But stored grain has its insect pests also, which are especially injurious in the South. Mr. F. H. Chittenden, of the U. S. Department of Agriculture, places the loss on stored corn in the seven Grulf States at $20,000,000, or 20 per cent of their crop. If only one-fourth of this amount, or 5 per cent, of the rest of the country's stored corn were thus lost, it would amount to $40,000,000. Twenty million dollars, or 3 per cent of the value of all other stored grain, certainly no more than cover the loss sustained upon it and other stored products subject to insect pests, which gives an aj^proximate total of $60,000,000 damage to stored products.

Grass and Hay. — A host of grass and clover insects damage the hay crop. Half a million dollars have frequently been given as the loss sustained from the Army-worm alone in individual States. Five per cent of the hay crop, or -^20. 000, 000, fairly represents the loss upon this cro]) and pasture-lands due to insects.

Cotton. — The cotton-plant has a number of serious enemies, of which the Cotton-worm, Boll-worm, and Boll-weevil are the worst. In 1880 the United States Entomological Commission valued the annual ravages of the Cotton-worm at 130,000,000, but, thanks to their careful study of the pest, the damage done by it has been greatly lessened in recent years. But the Boll-weevil has now presented itself in Texas. In 1894 it damaged the Texas crop to the extent of '$8, 000, 000, and its injuries are not reported as having diminished, Thus $15,000,000 must be a low estimate for the insect de^^tredations upon cotton.

Tobacco. — The tobacco crop, valued at $25,000,000, has a horde of insect enemies at all stages of its existence, which will easily consume 8 per cent of it, or $2,000,000.

Potatoes. — The Colorado Potato-beetle does not do that crop so serious an injury as formerly, but some new enemies to it have ap^^eared, and a loss of $10,000,000, or about 6 per cent of the value of that crop, is undoubtedly caused by our six-legged foes.

Surely, when we include the injury done to fruits, truck crops, domestic animals, and timber, $300,000,000 is a conservative estimate of the price these aj^parently insignificant little insects are annually costing this country.

Yet there is another aspect to the matter. '' One man's loss is another man's gain '' is never more true than as regards these losses occasioned by insects. For, through wide-spread injury by them, prices rise; while if these injuries were not done and correspondingly large crops were placed upon the market, prices must surely fall.

INJURY DONE STAPLE CJIOPS BY INSECT PESTS. 5

These estimates of losses due to insects are then very largely comparative. Yet, to a large extent, they are still real losses, the same as are those occasioned by fire and storm. For though a small crop may bring better prices, it is usually at the expense of individuals or communities which have sustained exceptionally heavy losses. Were these losses evenly distributed among all those producing a given crop, there would be no real hardship to them; but such is by no means the case.

All this, then, goes to emphasize the fact that the successful farmer — as the successful man in any other trade or profession — is the one Avho is able to overcome obstacles which, though possibly ruining his neighbor, are making a good market for his special crop. And these insect pests can be largely overcome. The millennium will doubtless come before the farmer will be able to stop fighting them, but a large part of the damage by them can be prevented at a cost which renders it profitable. Rational methods of general farm practice with the proper use of apparatus and insecticides, even such as are now known, and in which improvements are being constantly made, if intelligently used by American farmers, would save to them fully two-thirds of this enormous loss.

Chapter II. Structure And Development Of Insects

The more experience the farmer has with insect pests, the more he comes to realize that if he would successfully combat them, he must have a certain amount of necessaryknowledge concerning their structure and growth.

In general, the artificial means which may be effectually used to combat an insect pest will very largely depend upon the anatomical structure of the insect, while control by general methods of culture will depend upon a knowledge of the peculiarities of its life-history.

The value of a proper understanding of these important factors in insect control is therefore apparent.

General Structure of an Insect.

The body of an insect is composed of three separate parts, the head, thorax, and abdomen (Pig. 1), each of which is composed of several rings or segments. To the head are attached the jointed antennae, or feelers, the compound eyes, and the mouth-parts, which are described below. Each of the three segments of the thorax bears a pair of legs, and adult insects usually possess one or two pairs of wings upon the last two segments of the thorax. The abdomen is composed of nine or ten segments, but

STRUCTUKE AND DEVELOPMENT OF INSECTS. 7

bears no appendages save the ovipositor of the females of certain orders.

Harvest-mites, or ^^daddy-long-legs/' sow-bugs, thousand-legged worms, and similar vermin are often popularly called insects, but all of them can readily be distinguished from true insects by their possessing more than six legs,

Fig. 1. — Honey-bee, showing three principal regions of the body of an insect : — 1i, head; th., thorax; ahd., abdomen. (Original.)

the harvest-mites and spiders having eight and the others many more.

How Insects Grow.

With rare exceptions insects hatch from eggs laid by the adult females. Upon hatching they are but little larger than the eggs, and often bear but little resemblance to their parents. Thus the young caterpillar would never be recognized as the immature stage of the butterfly by one unfamiliar with its transformations. Grasshoppers and some other insects, however, upon hatching from the Qgg bear a marked resemblance to the adult form, except that they lack wings.

Complete Met amor pilosis. — But let us return to the caterpillar and follow it through its short but interesting life. Upon hatching from the egg it at once commences to feed and grows very rapidly. But before long an obstacle to further growth arises. Unlike higher animals, insects possess no internal skeleton or framework for the organs of the body, but the outer skin becomes hardened and to it the muscles and ligaments are attached. This hardening of the skin is best seen in the horuy wing-covers of the beetles and is due to the secretion of a hard substance called chitin. This chitin is secreted by all parts of the skin in greater or less degree, and thus forms a sort of shell for the whole body. Though this hardening is not so apparent in larvae as in adult insects, it is always present, and it is for this reason that when the youngcaterpillar has made a certain growth it is forced to shed its skin, which refuses to expand further, in order to develop more fully. Thus the skins of insects are shed several times (see Fig. 2, h), — usually five or six, but sometimes as many as twenty, — this process being known as ^^ molting." During its life as a caterpillar, which is called the "larval stage," and during which it is called a "larva," it is an elongate, worm-like creature, with six short, jointed legs on the three thoracic segments, a pair of fleshy false legs or jiro-legs on the last abdominal segment, and probably several pairs of pro-legs between these and the true legs. No traces of wings can be seen, but the body is often covered vvith hairs, spines, or warty tubercles.

But with the next molt the insect changes in appearance most radically, becoming a pupa, or chrysalis as this stage is termed for butterflies. Duriug the pupal stage the insect remains dormant either in a small cell slightly under

STRUCTURE AND DIOVELOTMENT OF INSECTS. D

the surface of the earth, or in a silken cocoon spun by the caterpiHar, or merely attached to the food-plant by a

Fig. 2. — Complete Metamorphosis. The different stages of the Corn Ear-worm {Heliotliis armiger Hiibn.X a, eggs on corn-silk; h, the first three larval stages; c, pupa from below; d, same from above; e, adult moth — all enlarged; 5, about twice natural size. (Original.)

strand of silk or the cast larval skin. In many of the Diptera, — the order including flies, mosquitoe's, gnats, etc., — however, the last larval skin is not shed, but hardens and forms a case — called a puparium — within which the pupal stage is passed.

The typical pupa (Fig. 2, c, d) of a butterfly or moth is of a more or less oval shape, rather resembling the adult insect than the larva, with the wings and antennae tightly folded at the sides, the legs drawn up snugly together under them, and the head and mouth-parts bent upon the breast, or sternum. But all of these parts are not always recognizable, the legs and mouth-parts being sometimes lacking. Gradually the adult insect develops, and at last the pupal skin is broken open and the airy butterfly emerges to enjoy a short life and perpetuate the species. Such a series of transformations is that commonly found among butterflies and moths (Lepidoptera), beetles (Coleoptera), flies (Diptera), and bees (Ilymenoptera), and is known as a complete metamorpliosis. All of these insects normally pass through four stages, the Qgg, larva, pupa, and aduit.

Incomplete MettDnorpliosis. — In contrast to this mode of development is that of the grasshoppers (Orthoptera), bugs (Hemiptera), and some other insects. As already stated these are much like the adult upon emerging from the Q-gg. With each molt they become larger and small winglike pads gradually appear on the sides of the thorax. There is no dormant or pupal stage, the adult insect differing from the previous stages in having fully developed wings, being larger, and often by an accompanying change of markings. The immature stages of such insects are called nympJiSy and this development an incomplete metavwrpJwsis, having but three stages, the egg, nymph, and adult (Fig. 3).

The time occupied by the complete life-cycle of an insect varies from a week or ten daj^s for the plant-lice to thirteen or seventeen years for some Cicadas, and is entirely dependent upon the habit of the species and the climate.

STRUCTURE AND DEVELOPMENT OF INSECTS.

A correct knowledge of the exact time and conditions under which the transformations occur for each individual

Fig. 3. — Incomplete Metamorphosis of a Bug (Brachymena Ji-'puatulaia). a, eggs; h, adult bug; c, different stages of young bugs or nymphs. (Original.)

insect pest is therefore often most essential when seekingmeans for its control.

How Insects Feed.

The material to be used in cumhating a given insect is almost entirely dependent upon the structure of its mouth-

INSECTS INJURIOUS TO STAPLE CROPS.

parts. Much Paris green is wasted upon insects unable to eat it and which it will, therefore, never kill.

Insects may be rouglily divided into two classes, those which bite and those which suck their food. Among the former are the beetles, grasshoppers, the larva? of butterflies and moths, and the larva? of saw-flies; and among tl:e

Fig. 4. — Front- view Face of Grasshopper {Schiztocerca americana). ant., an'^enna; oc, ocellus; ey., eye; cL, clypeus; Ibr., labrum, or upper lip; mx.p., maxillary palpus; lab. p., la'oial palpus; g<(l., galea, lobe of maxilla; lab., labium, or uuder lip. (Original.)

latter are butterflies, flies, bees, and bugs, while the ]arva3 of most flies and bees do not possess mouth-parts homologous with those of the above.

Biting Mouth-parts. — Mouth-parts typical of those of biting insects are easily seen in the grasshopper (Figs. 4, 5, and 6). In brief, they consist of an upper and a lower lip, between which are two pairs of jaws which work transversely. The upper pair of jaws, or mandiJjhs (md.), are stout, sliort, and horny, usually sharpened at the tip,

STRUCTURE AND DEVELOPMENT OF INSECTS.

slightly serrated at the margins, and flattened at the base. The lower pair of jaws, or maxillce {mx.), are longer, not so strong, and to each of them is attached an accessory lobe, and a jointed style called a palpus or feeler. At each

Fig. 5. — Mouth-parts of Grasshopper, separated to show position and relation, a, from above the mouth; b, lookiuo; into the month; c. from below tlie month. Ihr., labrum, or upper lip; md., mandible or biting-jaw; mx., maxilla, or second jaw; lab , labium, or under lip: hyp., hypopharynx, or tongue; mx.j)., maxillary palpus. (Original.)

side of the lower lip is another palpus, these palpi beingsensory organs.

Sucking Moidli-parts. — In the sucking insects these mouth-parts are prolonged into a tube through which the juices of the food plant — or animal — are sucked. In the plant-lice and other bugs the lower lip is elongated so that it forms a tube, and the maxillae and mandibles consist of long hair-like bristles, or seta?, enclosed within this tube (Fig. 7). The tip of this beak is rested upon the surface of a leaf into which the set^ are thrust, lacerating the tissue, and by a pumping process of the mouth the juices are sucked up through the beak. The structure of the mouth-parts of the various orders of sucking insects varies

INSECTS INJURIOUS TO STAPLE CROPS.

considerably, but all agree in that their food must be sucked up in a liquid state. Any application of a poison -

Fig. 6. — Cicada, showing Mouth-parts of a Bug, a Sucking Insect. a, seen from below, beak or rostrum {ro. G. ) reposing between forelegs; b, head removed; e, eye; lb7\, labrum; md., mandible-setae; mx., maxillary setae ; lab., labium. (Original.)

ous spray to the surface of foliage will be of no avail against them, though sure death to most biting insects

STRUCTURE AND DEVELOPMENT OF INSECTS.

wliich chew the leaves, and sucking insects must therefore be killed by other means.

How Insects Breathe.

In the side of one thoracic segment and each abdominal segment except the last, of a caterpillar or larva, is a small

Fig. 7. — Mouth-parts of a Plant-louse; a, the jointed beak; h, the lancets, much enlarged; c, antenna; d, foot. (After J. B. Smith. )

oval spot, in the centre of which is a slit closed by two membranous lips. These apertures are called sj^iracles or stigmata (Fig. 8, st^-st^^), and are the openings of the respiratory system. Similar openings are to be found in all insects, though not so easily seen in the adults. Connecting these spiracles is a pair of tubes on each side of the body, throughout its length, from which branch off

IG

INSECTS INJURIOUS TO STAPLE CROPS.

smaller tubes to all of its organs and tissues. Fresh air is thus inhaled to all parts of the body through these tubes (Fig. 8, tr).

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1 1 wsy

Fig 8, — Diagram of tracheal or breathing system of an Insect. .s-^j-6'^jQ, the ten pairs of spiracles; A, head; B^_^, the three segments of the thorax; 7V., the two main tracheal trunks; trs., trachea leading from the main trunk to the spiracle; ti^u., trachea connecting the two main trachea; tri., visceral trachea; tro., ventral trachea; t7\, the anterior termination of the trachea; g.-g., nerve-cord with ganglia to which go branches of the visceral trachea; au., eyes; a, antennae; p, palpi; 7n, mandibles; Pi-p^, bases of the legs. (After Kolbe.)

The blood of insects does not circulate through any system of tubes as it does in the higher animals. Along

STRUCTURE AND DEVELOPMENT OF INSECTS.

the middle of the back, above the alimentary canal, is a long tube po^Kilarly called the heart (Fig. 9, Ir, Fig. 10, dr). This heart is composed of a number of chambers each of which is funished with side valves for admitting blood from the body-cavity. The blood coming into the heart from the body- cavity is propelled forward toward the head, where it again flows into the body-cavity. Thus various currents of blood are maintained throughout the

Fig. 9. — Ideal section througli an insect, a, alimentary canal; h, heart; n, nerve-cord; s, stigmata: t, tracheal tubes; I, legs; w, wings. (From Riverside Nat. History.)

body, but other than the heart there is no system of bloodvessels, the blood merely filling the body- cavity around and through the various organs and tissues. Constantly flowing around the respiratory tubes or tracheae, the blood is quickly and thoroughly purified, though the exact manner in which this is done is not definitely known. The respiratory system has absolutely no connection with the mouth or pharynx (Fig. 10, ^j/i), as have the lungs of the higher animals, and if an insect is to be suffocated, it must be done by closing the spiracles. It is in this way

INSECTS INJURIOUS TO STAPLE CROPS.

that tobacco-dust, lime, ^^yrethruni, and similar insecticides kill sucking insects by penetrating the spiracles and choking the tracheal system. Whale-oil soap, kerosene emulsion, and the other ^'contact" insecticides, or *^ irritants,'* may also stop u}) the spiracles and thus cause death, but they act chiefly as "irritants,'' penetrating the

Fig. 10. — Internal Anatomy of Silkworm. A, the upper, or dorsal, body-wall seen from within; B, the back of the silkworm removed, showing alimentary canal; C, alimentary canal removed, showing nervous system and tracheal trimks; t?'., trachea; d.v., dorsal vessel or heart; p7i., pharynx or mouth; su., supracesophageal ganglion; sp. sp., spiracles or breathing-pores; n, nerve-cord; ^?\<., tracheal trunk; (?6S., oesophagus or throat; C7\, crop; s.g., silk-gland; pro., proventriculus or grinding-stomach ; St., stomach ; h.L, hind-intestine. (Photo, by author from Azoux model.)

skin and thus killing the insect. When insects are killed by means of a gas such as carbon bisulfide or hydrocyanic acid gas, they are truly asphyxiated by a substitution of these gases for air, the same as are higher animals by the use of anaesthetics,

Though arsenical poisons are generally used as sprays for biting insects, soft-bodied caterpillars and similar larvae are often killed by the use of contact insecticides, which affect them the same as sucking insects.

The reader will observe that, almost without exception, the remedies advised for different insect pests in the following pages are determined by some peculiarity, either of structure or development, of the insect to be combated.

CHAPTER III. GENERAL FARM METHODS AGAIjNST INSECT PESTS

In the following pages artificial means of combating insect pests, such as spraying with insecticides, are not as often the remedies or preventives given as those which consist of some method of general farm practice. That such should be the case is but natural, for the staple crops, being cultivated in large areas, can hardly be treated with sprays or mechanical devices, in many instances, with any degree of profit. The best methods to employ against most of the insects affecting the staple crops are what might be termed cultural methods, consisting of some mode of culture or handling the crop which fatally interferes with the develoj^ment of a given insect pest. Such treatment is far less simple in many instances, however, than the use of a spray-pumj) or powder-gun. In the latter case the farmer merely waits until he observes a crop being injured and then with a liberal a2:>plication of poison destroys his insect enemies; but in using the former method he must have a more or less accurate knowledge of the life-history of the insect which he wishes to combat. It will also be necessary for him to observe or ascertain the usual dates of the transformations of various insects for his particular locality, as they vary considerablv for different latitudes and altitudes, and to make due allow-

GENERAL FAUM METHODS AGAINST INSECT PESTS. 21

auce for any variation of these dates on account of the peculiarities of the individual season.

Looking Ahead. — Few farmers, in planning the management of their land and crops for the coming season, consider the effect which any given procedure will have upon the injurious insects with which they may have to contend. A field which has for several years been in wheat, corn, or tobacco may be sown with some other crop for the sake of soil improvement, or may even be favored with a greenmanuring of rye, crimson clover, or cow-peas; but how often is it considered necessary to rotate crops in order to lessen insect pests ? In most cases the answer would doubtless be, " Not until some noticeable loss has been suffered from their injuries." That this is a mistake may be seen from a brief survey of the best methods for combating our worst insect pests.