The Destructive Effect of the Cotton Weevil.—The damage done by locusts is probably small compared with that due to various insects which attack cotton. It is estimated that insects damage the cotton crop of the United States to the extent of $100,000,000 annually. A little more than half of this is due to insects which live permanently in the cotton area. The remainder is due to a small gray beetle called the boll weevil. Since about 1860 the weevil has been slowly spread ing from its original home in Mexico into the cotton producing region of the United States. (See Fig. 100.) The weevils sting the flower buds in order to lay their eggs, which are deposited at the base of the young bolls or pods. This ruins the cotton. As the weevil occupies new territory year by year new groups of farmers are added to those who suffer. To get rid of the weevil the American farmers have tried many methods such as killing the weeds on which the insect lives part of the year, and introducing new and resistant varieties of cotton, such as that of Guatemala.
How the Phylloxera Ruins the Grapes.—The phylloxera, a kind of plant louse which ruins grape vines, has done even more damage than the boll weevil. Its original home is the United States, but there the vines have become immune to its attacks. That is, in the course of many years, those that were most injured have been killed, and only those that could endure its ravages have lived.
About 1860 the phylloxera was accidentally introduced into Europe through the importation of American vines. It spread at once and did enormous damage. For instance, in 1865-7 the little commune of Graveson near Bordeaux in France obtained its ready money for taxes, clothing, and incidental expenses by producing 220,000 gallons of wine each year. In 1868 the phylloxera reached this section, and by 1873 the production of wine had fallen to 1100 gallons. By 1888 the total loss to France as a whole is estimated at two billion dollars. In some places the consequent poverty of the farmers led to violent political agitation, for people often have the strange idea that troubles due to geographical conditions can be remedied by changes in the laws.
When once the phylloxera is introduced the only remedy is to root up all the vines and start with new ones raised from American stock. Not only France, but almost every grape-raising region in the world has suffered more or less in the same way.
Scales and Moths.—Many other insects, and also certain fungi, do similar damage. The orange scale, for instance, has almost ruined many orange groves. In the northeastern United States the gypsy, brown-tail, and other moths which were introduced from Europe during the latter part of the nineteenth century have greatly injured certain species of trees. Massachusetts, for example, has spent millions of dollars in a single year to get rid of these pests or at least to prevent them from migrating into new territory. The only real remedy seems to be the introduction of parasitic contagious dis eases which spread from moth to moth. When the weather is warm and moist such diseases kill the insects by the million.
Plant Migrations.—The migrations of plants are as harmful as those of animals. The daisy, for instance, was introduced into
America from Europe, and then spread over millions of acres. It diminishes the hay crop by hundreds of thousands of tons, for it crowds out good grass, exhausts the soil, and is itself not eaten by any domestic animals. The Scotch thistle does similar harm.
Even greater damage is done by small forms of yeast-like plants called parasitic fungi, which grow on other plants as mold grows on cheese. Among the worst of these are the wheat rust and the potato blight, which sometimes ruin the crop in unusually wet seasons. In the eastern United States the slowly spreading chestnut blight has ruined many a great tree like that under which stood the village black smith's shop in Longfellow's famous poem. The only known remedy is to cut down all the chestnut trees in a broad belt, as has been done in Pennsylvania, so that the blight may have nothing upon which to live.
Migrations of Disease.—Men and animals as well as plants are often attacked by pests which spread from place to place and hence are variables. For instance, influenza is one of the most dangerous diseases because it is highly contagious and even if its victims survive they are weak for a long time. In this case, as in most variables, two factors are concerned: (1) the bacteria which cause the disease, and (2) man. The bacteria are present in most countries at all times. Occasionally, for reasons not yet understood, they suddenly become extremely virulent and devastating epidemics occur. Man's varia tions are better understood. Under ideal conditions of climate, ventilation, and food, he is able to resist the disease even in its worst form, provided his health is not impaired in other ways. Rarely, however, do these ideal conditions prevail, and hence epidemics break out and cannot be stopped. Every winter there is a mild outbreak because man's power of resistance is at a low ebb. One of the last great epidemics developed in Asiatic Russia in 1889, perhaps because of a severe winter and poor food. It then spread rapidly without apparent regard to weather or climate, following the lines of human intercourse along the world's great trade routes to the remotest coun tries. In 1918 a far worse epidemic broke out, apparently in connec tion with the Great War. More people died than in any epidemic since the notorious Black Death of the fourteenth century. In the United States half a million people died either of influenza or of the pneumonia which often follows it. In the world as a whole about 15,000,000 people, or nearly one in every hundred, fell victims to the disease; in India the number was 6,000,000, or one in 50; in Mexico one in 25; in Yekaterinburg, a Russian city as large as Savannah, a third of the population perished, and in some Indian cities, half. The great ravages of influenza in tropical countries and also in regions like Russia, where the people were especially weakened by war and famine, show that the greatest safeguard against the disease is a general condition of good health. The great mortality from influ enza in the camps of our own army shows how the crowding of people into small areas favors the spread of infectious diseases.