The adult weevils puncture the young flower-buds and deposit
eggs; and as the grubs from the eggs develop, the bud drops.
They also lay eggs later in the year in the young bolls. These do
not drop, but as the grubs develop the cotton is ruined and the
bolls usually become discoloured and crack, their contents
being rendered useless.
No certain remedy is known for the destruction on a commercial scale of the boll weevil, but every effort has been made in the United States to check the advance of the insect, to ascertain and encourage its natural enemies, and to propagate races of cotton which resist its attacks. Special interest attaches to the investigations made by Mr O. F. Cook, of the U.S. Dept. of Agriculture, in Guatemala. The Indians in part of Guatemala raise cotton, although the boll weevil is abundant. Examination showed that although the weevil attacked the young buds these did not drop off, but that a special growth of tissue inside the bud frequently killed the grub. Also, inside the young bolls which had been pierced a similar proliferation or growth of the tissue was set up, which enveloped and killed the pest. Probably by unconscious selection of surviving plants through long ages this type has been evolved in Guatemala, and experiments have been made to develop weevil-resistant races in the United States. Mr Cook also found that the boll weevil was attacked, killed and eaten by an ant-like creature, the “kelep.” Attempts have been made to introduce this into the infested area in Texas; but owing to the winter proving fatal to the “kelep” its usefulness may be restricted to tropical and subtropical regions.
The cotton boll worm (Chloridea obsoleta, also known as Heliothis armiger) is a caterpillar. The parent moth lays eggs, from which the young “worms” hatch out. They bore holes and penetrate into flower-buds and young bolls, causing them to drop. Fortunately the “worms” prefer maize to cotton, and the inter-planting at proper times of maize, to be cut down and destroyed when well infested, is a method commonly employed to keep down this pest. Paris green kills it in its young stages before it has entered the buds or bolls. The boll worm is most destructive in the south-western states, where the damage done is said to vary from 2 to 60% of the crop. Taking a low average of 4%, the annual loss due to the pest is estimated at about £2,500,000, and it occupies second place amongst the serious cotton pests of the U.S.A. The boll worm is widely spread through the tropical and temperate zones. It may occur in a country without being a pest to cotton, e.g. in India it attacks various plants but not cotton. It has not yet been reported as a cotton pest in the West Indies.
The Egyptian boll worm (Earias insulana) is the most important insect pest in Egypt and occurs also in other parts of Africa. Indian boll worms include the same species, and the closely related Earias fabia, which also occurs in Egypt.
The cotton worm (Aletia argillacea)—also called cotton caterpillar, cotton army worm, cotton-leaf worm—is also one stage in the life-history of a moth. It is a voracious creature, and unchecked will often totally destroy a crop. In former years the annual damage done by it in the United States was assessed at £4,000,000 to £6,000,000. Dusting with Paris green is, however, an efficient remedy if promptly applied at the outset of the attack. The annual damage was in 1906 reduced to £1,000,000 to £2,000,000, and this on a larger area devoted to cotton than in the case of the estimate given above. It is the most serious pest of cotton in the West Indies. The Egyptian cotton worm is Prodenia littoralis.
The caterpillars (“cut worms”) of various species of Agrotis and other moths occur in all parts of the world and attack young cotton. They can be killed by spreading about cabbage leaves, &c., poisoned with Paris green.
Locusts, green-fly, leaf-bugs, blister mites, and various other pests also damage cotton, in a similar way to that in which they injure other crops.
The “cotton stainers,” various species of Dysdercus, are widely distributed, occurring for example in America, the West Indies, Africa, India, &c. The larvae suck the sap from the young bolls and seeds, causing shrivelling and reduction in quantity of fibre. They are called “stainers” because their excrement is yellow and stains the fibre; also if crushed during the process of ginning they give the cotton a reddish coloration. The Egyptian cotton seed bug or cotton stainer belongs to another genus, being Oxycarenus hyalinipennis. Other species of this genus occur on the west coast of Africa. They do considerable damage to cotton seed.
Fungoid Diseases.—“Wilt disease,” or “frenching,” perhaps the most important of the fungoid disease of cotton in the United States, is due to Neocosmospora vasinfecta. Young plants a few inches high are usually attacked; the leaves, beginning with the lower ones, turn yellow, and afterwards become brown and drop. The plants remain very dwarf and generally unhealthy, or die. The roots also are affected, and instead of growing considerably in length, branch repeatedly and give rise to little tufts of rootlets. There is no method known of curing this disease, and all that can be done is to take every precaution to eradicate it, by pulling up and burning diseased plants, isolating the infected area by means of trenches, and avoiding growing cotton, or an allied plant such as the ochro (Hibiscus esculentus), in the field. Fortunately the careful work of the U.S. Department of Agriculture and of planters such as Mr E. L. Rivers of James Island, South Carolina, has resulted in the production of disease-resistant races. In one instance Mr Rivers found one healthy plant in a badly affected field. The seed was saved and gave rise to a row of plants all of which grew healthily in an infected field, whereas 95% of ordinary Sea Island cotton plants from seed from a non-infected field planted alongside as a control were killed. The resistance was well maintained in succeeding generations, and races so raised form a practical means of combating this serious disease.
In “Root rot,” as the name implies, the roots are attacked, the fungus being a species of Ozonium, which envelops the roots in a white covering of mould or mycelium. The roots are prevented from fulfilling their function of taking up water and salts from the soil; the leaves accordingly droop, and the whole plant wilts and in bad attacks dies. It has yearly proved a more serious danger in Texas and other parts of the south-west of the United States, and the damage due to it in Texas during 1905 was estimated at about £750,000. No remedy is known for the disease, and cotton should not be planted on infected land for at least three or four years.
“Boll rot,” or “Anthracnose,” is a disease which may at times be sufficiently serious to destroy from 10 to 50% of the crop. The fungus which causes it (Colletotrichum gossypii) is closely related to one of the fungi attacking sugar-cane in various parts of the world. Small red-brown spots appear on the bolls, gradually enlarge, and develop into irregular black and grey patches. The damage may be only slight, or the entire boll may ripen prematurely and become dry and dead.
Many other diseases occur, but the above are sufficient to indicate some of the principal ones in the most important cotton countries of the world.
Improvement of Cotton by Seed Selection.
In the cotton belt of the United States it would be possible to put a still greater acreage under this crop, but the tendency is rather towards what is known as “diversified” or mixed farming than to making cotton the sole important crop. Cotton, however, is in increasing demand, and the problem for the American cotton planter is to obtain a better yield of cotton from the same area,—by “better yield” meaning an increase not only in quantity but also in quality of lint. This ideal is before the cotton grower in all parts of the world, but practical steps are not always taken to realize it. Some of the United States planters are alert to take advantage of the application of science to industry, and in many cases even to render active assistance, and very successful results have been attained by the co-operation of the United States Department of Agriculture and planters. With the improvement of cotton the name of Mr Herbert J. Webber is prominently associated, and a full discussion of methods and results will be found in his various papers in the Year-books of the U.S. Department of Agriculture. The principle on which the work is based is that plants have their individualities
