Popular Science Monthly/Volume 42/March 1893/An Agricultural Revolution
During the last half-century the agriculturists of the United States have constantly suffered from the attacks of two classes of organisms, which have disputed with them the possession of their crops. These organisms are, first, the noxious insects; and, second, the parasitic fungi. To these tiny foes American agriculture yields annually many million dollars' worth of her choicest products. They form an omnipresent host of tax-gatherers, taking possession of the farmer's crops and enforcing their onerous demands without process of law, unless preventive measures are vigorously prosecuted. They are no respecters of persons: like the rain, they fall upon the fields of both the just and the unjust.
The authorities best able to judge have estimated the annual loss in the United States due to these little pests at more than half a billion dollars. Noxious insects, according to Dr. C. V. Riley, the distinguished entomologist of our National Department of Agriculture, occasion losses in the United States which are "in the aggregate enormous, and have been variously estimated at from $300,000,000 to $400,000,000 annually," and parasitic fungi—the rusts, smuts, blights, mildews, rots, and similar maladies of growing plants—according to competent authorities, cause an equal or greater loss. In single States and single seasons the damage is often frightful in extent. During some of the great chinch-bug epidemics the loss in Illinois occasioned by this one insect has amounted to over $73,000,000 a year; and in seasons not marked by an outbreak of such a great crop pest the injury is much more severe than is ordinarily supposed. The official entomologist of the State just named, Prof. S. A. Forbes—after years of careful field observation and statistical study—has recently expressed his belief that "the insects of the State of Illinois derive as large a profit from the agriculture of this great agricultural State as do the farmers themselves."
Fortunately, however, much progress has recently been made in a knowledge of efficient means of preventing this vast drain upon our productive system. By the introduction of a simple mechanical contrivance for the application of insecticides and fungicides, the methods of combating these foes have been revolutionized; and in many localities where the production of special crops had been abandoned new life has been put into their development. This contrivance is commonly called the spraying machine. It consists essentially of a force pump and spray nozzle connected with a reservoir, by means of which certain substances that have a destructive effect upon insect and fungousFig. 1.—Codling Moth: a, injured apple; b, place where egg is laid; e, larva; d, pupa; i, cocoon; g, f, moth; h, head of larva. (After Riley.) life may be rapidly and evenly distributed over the outer surfaces of trees, shrubs, vines, and herbaceous plants.
In America the spraying machine seems to have first come into general use to prevent the injuries of the codling moth or apple worm. This is a very destructive and widely distributed insect, for which there had before been known no remedy that can compare with spraying in cheapness and efficiency. These worms hatch from eggs laid in the calyx ends of the newly formed apples by a small, chocolate-colored moth (represented at f and g, Fig. 1). These eggs are deposited in spring or early summer, from the time the young apples are as large as peas until they attain the size of small hickory nuts. The eggs are placed on the outside of the fruit, and the resulting worms nibble at the skin, finally biting through and eating toward the core. They continue feeding for three or four weeks, when they become three fourths of an inch long, whitish or pinkish-white in color, and of the general form shown in Fig. 1, e. They are now full grown as larvæ, and leave the apples to spin, in some temporary shelter, slight silken cocoons (i), in which they transform to pupæ (d), to change a fortnight later into fall-fledged moths. These moths deposit eggs about midsummer for a second brood of worms.
The earlier preventives of codling-moth injury included such partially effective measures as banding the trees with wisps of hay or loose cloth, to entice the larvæ to spin their cocoons in them, and the feeding of fallen fruit to stock. A serious objection to these methods lay in the fact that the worms were only destroyed after they had done their injury. But now, thanks to the spraying machine, the fruit-grower can place in the calyx ends of the young apples a few particles of poison, which as a rule will kill the newly hatched worms before they enter the fruit. For this purpose three or four ounces of London purple or Paris green are thoroughly mixed with a barrel of water. The apple trees are sprayed with this mixture just after the blossom petals have fallen off. The poison particles are thus distributed over the tree in a fine mist, and when the water in which they are suspended (not dissolved) evaporates, they are left high and dry upon the leaves and fruit, there to remain a menace to insect enemies for several weeks. In the course of time the combined action of rain
|Fig. 2.—Plum Curculio: a, larva; b, pupa; c, beetle—magnified; d, plum showing crescent mark. (After Riley.)||Fig. 3.—Apple injured by Curculio.|
and dew, wind and sunshine dissipate their poisonous properties. By thus destroying the first brood of worms one is saved the trouble of fighting the second brood.
For many years it has been very difficult to produce plums and other stone fruits on account of the injuries of the plum curculio. The adult of this species is a small, hard-shelled beetle (Fig. 2, c), which appears in the orchard early in spring, and feeds upon the foliage and flowers until the fruit is well formed. It then attacks the young plums, gnawing at them to satisfy its hunger, and cutting crescent-shaped holes (d) in the skin for purposes of oviposition. The eggs hatch into minute grubs that feed upon the pulp for a few weeks, ruining the plums and causing them to fall to the ground. The grubs (a) then leave the fallen fruit, enter the soil a short distance, change to pupæ, and later again change to adult beetles. There is but one brood each season, the insects wintering over mostly as beetles.
Besides stone fruits this insect attacks apples and pears, causing a dwarfing and malformation that are very vexatious to the horticulturist. An apple thus damaged is shown in Fig. 3.
The eggs of the curculio are deposited beneath the skin of the fruit, and the larvæ remain concealed until full-grown. Consequently the early stages of the insect can not be reached by spraying. Fortunately, however, the parent beetles can be killed before many eggs are deposited, by sparsely coating the foliage and fruit with poison from the spraying machine, and thus the plums will be saved. The practicability of this has been proved repeatedly by commercial orchardists.
|Fig. 4.—Canker Worm: e, eggs;, f, larva; g, pupa; a, male moth; b, female moth. (After Riley.)|
In addition to the insects affecting orchard fruits, there are hosts of enemies to the foliage. Nearly all of the latter, fortunately, are also open to destruction by means of the spraying machine. The canker worm is one of the most destructive of these foliage pests; at occasional intervals during the last century it has denuded thousands of orchard and shade trees in many parts of Canada and the United States. Its different stages are shown in Fig. 4. The damage is done by the worms or larvæ which hatch from masses of small cylindrical eggs (e), usually deposited upon the bark of the trees. These larvæ feed upon the parenchyma of the leaves, and sometimes cause a badly infested orchard to appear brown and seared, as if scorched by fire. They continue feeding for several weeks before becoming full-grown. Then they descend to the ground, burrow into the soil a short distance, and spin silken cocoons within which they change to the pupa or chrysalis state, remaining in this condition a few weeks, when the moths come forth. The two sexes of these moths differ greatly: the male (a) has large, well-developed wings, while the female (b) is wingless. The latter is of an ash-gray color. On emerging from the chrysalis she crawls to the base of the tree and ascends the trunk some distance; here the male finds her, and, after mating, she deposits her eggs on the twigs or branches of the tree.
The canker worm, like nearly all similar leaf-eating caterpillars, is so easily destroyed by spraying that, while in years past it was greatly dreaded by orchardists, it now inspires little fear.
The three insects above mentioned are all examples of those species having biting mouth-parts, and which in consequence attack the plant by biting piece by piece the tissues of leaf, stem, or fruit. It is on this account that they are open to destruction by placing particles of poison upon the surface of the food-plant. A large proportion of our injurious insects have such biting mouth-parts, but there is also an important class which have, instead of jaws, a pointed beak that they push into the cells of the plant and suck out the sap. Insects of this kind can not be destroyed by coating the food-plant with particles of poison, because the particles will not be taken into the alimentary system. Consequently one must use against them some insecticide which kills by contact. There are several such insecticides in common use, the most important perhaps being an emulsion of kerosene, soap, and water, called the kerosene emulsion. It is usually prepared by adding two parts of kerosene to one part of a solution made by dissolving half a pound of hard soap in a gallon of boiling water, and churning the mixture through a force pump until the whole forms a creamy mass, which will thicken into a jelly-like substance on cooling. The emulsion thus made is diluted before using with nine parts of cold water, and is then sprayed directly upon the offending insects, killing them by simple contact. Among the more important pests against which this insecticide is used, I may mention the aphides or plant lice, the chinch-bug, the various cabbage worms, the lice of domestic animals, etc.
These examples will perhaps suffice to illustrate how valuable an adjunct the spraying machine has become in preventing the injuries of the hordes of destructive insects that overrun our farms, orchards, and gardens. Turning now to the other class of noxious organisms—the parasitic fungi—we shall find that it plays an equally important rôle in their subjection.
As the first illustration under this heading we will take the downy mildew or brown rot of the grape, a disease which for many years has troubled the vineyardists of the Eastern half ofFig. 5.-Section of Leaf showing Mycelium of Fungus. Magnified. (After Farlow.) the United States, and has proved especially destructive in the great fruit belt of northern Ohio, along the southern shore of Lake Erie. It has often destroyed nearly the entire crop, and several times has threatened to ruin the vineyard industry over a wide area. Fortunately however, this disaster has been averted by the timely introduction of the spraying machine. The brown rot of grapes is a diseased condition of the fruit caused by the presence of a minute parasitic plant—a fungus—that develops by absorbing the tissues of its host. It attacks not only the fruit but also the leaves and young shoots, on which it often appears as a whitish, mildew-like covering, which has given the disease its other common name of downy mildew. This fungus reproduces by means of minute particles called spores, corresponding in function to the seeds of higher plants. If one of these spores lights upon a moist leaf, its inner contents divide into a number of distinct particles, which soon escape through an opening in the spore wall; then each particle swims about in the filmFig. 6.—Fruiting Branches. Greatly magnified. of water for a short time—resembling a little animal—when it becomes quiet and sends out a minute tube which penetrates the skin of the leaf. It then continues to develop inside the leaf, pushing about between the cells, and forming the mycelium or vegetative portion of the fungus. As there is little nourishment to be found between the cells, this mycelium develops minute processes, which push through the cell walls and absorb the cell contents. A small section of an affected leaf, greatly magnified, is represented at Fig. 5, the unshaded double-walled spaces showing the leaf cells, the shaded part between the walls the mycelium of the fungus, and the projections a, a, the processes or suckers that penetrate the cells. When these vegetative portions of the fungus have developed in the leaf to a certain extent, they send out through the breathing pores or stomata their fruiting branches, which bear upon their tips the small oval spores (Fig. 6). These fruiting branches form the so-called mildew on the plant, and, as they only appear under certain atmospheric conditions, the mycelium may exist in the vine for some time before this outward manifestation of its presence is seen. On this account a whole vineyard sometimes appears to be invaded by the mildew in a single night.
From the above description it will be readily seen that this fungus can not be successfully combated after it has established itself within the tissues of the host. To prevent its injuries one must also prevent its ingress to the plant. Fortunately, this can be done by coating the green parts of the vine with some substance having a destructive effect upon the spores of fungi. The salts of copper have such an effect, and in consequence they have come into general use as fungicides. They were first experimented with on a large scale in the vineyards of France, and gave such satisfactory results that they were adopted in a practical way by many growers. In America this use for them has hardly been known for more than a decade; yet, thanks to the remarkable series of investigations and experiments carried on by the Division of Vegetable Pathology of the Department of Agriculture—at first under the direction of Prof. F. Lamson-Scribner, and later that of Mr. B. T. Galloway—assisted to a considerable extent by several of the State experiment stations, their efficacy is already well attested, and they are in practical use over a large territory. The fungicides most commonly employed are the Bordeaux mixture, a combination of copper sulphate, lime, and water; eau céleste, a combination of copper sulphate, ammonia, and water; and various solutions of carbonate of copper. These fungicides are sprayed upon the plant early in the season, before the ingress of the disease-producing fungus, in such a way that after the water evaporates the leaves and stems will be coated with minute particles, usually crystals, of a salt of copper. These tiny sentinels stand guard over the plant; when a fungus spore falls upon the leaf and sends out its germinating tube, the latter comes in contact with the copper crystal and is destroyed.
The fungicide generally used for the downy mildew of grapes is eau céleste. It is first applied in spring, a few days before the vines blossom. One or two, and rarely three, additional applications are afterward made at intervals of about two weeks. In 1890 I made a special investigation of the results of spraying against this disease in northern Ohio, visiting many of the vineyards personally and sending out numerous letters of inquiry. As a result I was able to publish in the Bulletin of the Ohio Experiment Station (Vol. III, page 262) the following paragraph:
"The early part of the season of 1890 was peculiarly favorable to the development of downy mildew, and consequently an excellent opportunity was offered to test the value of eau céleste as a preventive. It has stood the test in a remarkable manner, and the efficiency of the preventive, when properly applied, has been proved beyond question. All accounts agree in this respect, and show that while the crops on the unsprayed vineyards averaged from one half a ton to a ton per acre, the sprayed vineyards yielded two to three tons per acre. Such results need no comment: they speak for themselves."
For many years pear trees, both in the nursery and orchard, have been seriously affected by a fungous disease that causes the leaves to drop during the summer prematurely, sometimes as early as June or July. It also often develops upon the young pears, causing a spotting and cracking of the fruit. The mycelium of this fungus grows between and through the cells of the pear leaf, causing circular brown spots to appear upon the surface; these spots gradually enlarge as the mycelium spreads through the tissues, and, finally, the whole surface being affected, the leaf withers and falls off. By means of fungicides and the spraying machine, horticulturists are now able to prevent this disease completely. A large number of experiments in controlling it have been remarkably successful, and the difference between sprayed and unsprayed trees has been graphically illustrated by Mr. Galloway at Fig. 7. the engravings being faithful reproductions of photographs from Nature. Recent experiments have proved that the so-called apple scab—a disease which ruins a large percentage of the apple crop every year—may also be prevented by spraying
|Fig. 7.—Pear Leaf-blight Experiment|
with fungicides; and many other of the most destructive plant diseases are already under control, while experiments and investigations are continually progressing, with a view of bringing into subjection those which are yet out of reach.It was naturally to be expected that the fruit-consuming public would object at first to purchasing fruit which they knew had been sprayed with poison. This is shown in the recent "grape scare" in New York city, and the present attitude of certain English journals toward the importation of American apples. But when the spraying, with either the insecticides or fungicides now
From annual report of Connecticut Agricultural Experiment Station for 1890.
commonly in use, is done with proper reference to the time, methods, and conditions of treatment, there is no danger to the consumer. Both practical experience and chemical tests have repeatedly shown that apples sprayed early in the season with Paris green or London purple retain none of the poison at the time of ripening. The most recent demonstration of this appears in the last report of the experimental farms of Canada. A peck of Rhode Island greening apples that had been sprayed twice with Paris green (one pound to two hundred gallons of water) were carefully gathered, without rubbing, and tested for arsenic. "The process to which they were submitted is one that affords extremely accurate results, and is considered the most delicate of all for the detection of arsenic. It is capable of revealing the presence of one fifty-thousandth part of a grain of arsenic. If twenty-three thousand bushels of apples contained two and a half grains of arsenic, the minimum fatal dose for an adult, the poison could have been detected by this method." Notwithstanding the most careful analysis no traces of poison were found; and, in conclusion, the chemist states: "I am of the opinion that further experiments of this nature would only serve to corroborate this negative result, and to prove that there are no grounds on which to base a suspicion that our sprayed apples are poisonous. The insoluble character of this poison precluding its assimilation by the apple, if such were possible, the infinitesimal part of Paris green that can remain on the apple, the frequent rains subsequent to the spraying, . . . all go to substantiate the argument that there is not the slightest danger of poisoning in using sprayed apples."
There is abundant evidence of a similar nature concerning the use of copper salts on grapes. In France, where a large proportion of the grape crop is converted into wine, elaborate investigations have shown that practically none of the copper salts are present in wine from sprayed vineyards. Prof. B. Fallot, of the School of Agriculture at Montpellier, in recording the results of one of these investigations, says: "The figures obtained have proved once more that wines, after the grapes have received numerous treatments with large quantities of salts of copper, contain scarcely a trace of this substance, and are entirely harmless."
Such is a meager and imperfect outline of this most recent improvement in the art of agriculture, which I have ventured to call an agricultural revolution. This improvement has been brought about by the combined efforts of the entomologist, the botanist, the chemist, the mechanician, and the agriculturist. Every step forward has been the result of careful study and experiment, and the whole subject is a striking illustration of the practical benefit agriculture may derive from scientific investigation and systematic experimentation.