Imágenes de páginas


as a rule, somewhat dwarfed, are close and compact, and stool but little. The leaves, comparatively few in number, are stiff, narrow and erect, with a more or less tough, dry cuticle, often with a glaucous or waxy surface; heads compact and narrow; and grains hard, red, small and heavy.

Varieties likely to prove considerably resistant to rust in the United States, if they are sown early, are, Kharkof, Turkey, Mennonite, Pringles No. 5, Rieti, Odessa and Pringle's Detiance for winter wheats, and Haynes Blue Stem and Saskatchewan Fife for spring wheats.

Durum wheats are much more resistant than other varieties. During the great rust attack of 1904 in the northwest, the maximum loss for durum wheat seems to have been about 10 per cent while that of ordinary wheats was frequently as great as 50 per cent. The different varieties of durum wheat also vary in their power to resist rust, two of the best being lumillo and Velvet Don. "Rerraf'' is one of the best rust resisters in Australia, but is quite non-resistant in the United States.

Other Diseases.—Leaf blight (Septosphaeria tritici Pass.) and Powdery mildew (Erysiphe graminis D. C.) occasionally cause slight losses in certain sections.

1 Carleton, Cereal Rusts of U. S., p. 21.



Species.—It has been estimated that there exist 1,000,000 species of insects of economic importance. About 100 species feed upon growing wheat, and about 50 more are found in granaries. Less than a dozen occasion enough loss to wheat to be of very great importance. Conditions in the United States are most favorable for insects, because the continuous growing of the same grain crops over wide areas, and long, hot summers are very propitious for the multiplication of most species. On account of differences in climatic conditions and in the abundance of parasitic and other enemies, there is a periodicity in the recurrence of grain pests. Since a season favorable to one insect may be unfavorable to another, there is also a more or less marked rotation of different species.

Hessian Fly (Mayetiola destructor Say).—Wheat is the natural food plant of this insect, which is also supposed to be native to Asia. It was introduced into America from Europe. The Revolutionary patriots believed that it was contained in some straw brought over by the Hessian troops, hence its

Some of the ignorant Tory element claimed that General Washington was responsible for its introduction. It was first described technically in 1817.

DISTRIBUTION.— The natural spread of the Hessian fly has been estimated at 20 miles per year. It is now found in nearly all parts of the United States east of the 100th meridian, and on the Pacific coast (since 1884) probably from southern California to British Columbia. In Canada it has been found from Prince Edward Island to Indian Head, Saskatchewan. It also occurs in North Africa, western Asia, Europe and the British Islands. It has been an important grain pest in New Zealand since 1888.

DESCRIPTION AND LIFE HISTORY.—The adult Hessian fly is very fragile, dark-colored, and about 18 inch long. It is about half as large as the mosquito, which it resembles. Even when


comparatively abundant it will escape the notice of the ordinary observer. It can be caught with a sweeping-net, but is easily confused with other insects taken at the same time. The fly seems to be two brooded in all parts of the United States. In the north the broods follow each other in quick succession, while in the south they are widely separated. The egg of the insect is about 1-50 inch long. The newly hatched larva or maggot is slightly smaller than the egg. The fully developed larva is larger, and on account of its resemblance to a seed of flax it is known as the flaxseed. In fall wheat the fly passes the winter in the young plants, principally in the flaxseed stage, but also in the larval stage, not quite full grown. The flies






emerge from the flaxseeds when the wheat is about 2 inches high. The time varies from March in Georgia to May in Michigan. Flies from wintering larvæ appear later. The eggs are deposited in the grooves on the upper surface of the wheat leaves, from 100 to 300 by each female fly. They are difficult of perception, even by one who has good eyesight. In a few days the eggs hatch into a pinkish larvæ that soon turn greenish, and descend to just above the roots, or, if the wheat has jointed, to the base of the particular leaves on which they were hatched. Sucking the juices from the growing wheat plant, these larvæ attain the flaxseed stage in about 4 weeks, the time being dependent on the weather. The prolonged southern summer during which there is little food for the larvæ, is passed in the stubble in the flaxseed stage. In Michigan the fall brood appears about the last of August, while in Georgia it appears about 3 months later. The eggs are now deposited on the young fall wheat, and the life cycle begins over again. In regions far north there may be only one brood, and in the south there may be supplemental broods, both in the spring and fall, this being dependent on the weather. Drought prolongs the flaxseed stage.

In the spring wheat regions the insects winter in the flaxseed stage, chiefly in stubble, but also in volunteer wheat. Egg laying begins late in May and continues to October 1st. Eggs are often deposited on grass and weeds, but the larvæ are not known to survive except on wheat, barley and rye. The fly




is now known to flourish even where spring crops are exclusively grown.

EFFECT OF LARVAE ON WHEAT. -At first the plant seems to be stimulated, and turns a dark green color. Later the infested tillers turn a brownish and then a yellowish color. If the attack comes early, and the plant fails to tiller, death results. If the plant has tillered, some stalks may escape and form the basis for a crop. The larvæ are usually found just above the first joint, but may be found from above the third joint to below the soil. The stalk is usually so weakened that it breaks to the ground, when the wheat is said to be “straw fallen.”

Losses.—The Hessian fly is the worst insect enemy of growing wheat. It is never entirely absent. The minimum annual damage to wheat is thought to average 10 per cent of the crop, that is, over 50,000,000 bushels. In some localities an injury varying from 50 per cent to total failure is not infrequent. In 1901 the loss in New York was about $3,000,000, and the loss in Ontario was nearly as great. In 1900 it was $16,800,000 in Ohio, and nearly two-thirds of the Indiana wheat was not harvested on account of the fly. The outbreak of the Hessian fly in 1900 was the most notable of recent years. The total loss for the United States was estimated at $100,000,000, and milling operations were seriously hampered in the worst affected region. The damage which the fly does is often laid to rust, drought or other causes. In 1904 there was little complaint of damage from the insect, yet many fields in the Ohio valley were injured to the extent of over 50 per cent.

REMEDIES.—There are a number of natural enemies which attack the Hessian fly in the larval and pupal stages. Some are native, and others are being artifically introduced. While they limit the damage, they are useful mainly where other preventives are neglected. The best remedy for a field of wheat severely attacked is to plow deeply, and plant a spring crop. In case of mild infection, the prompt use of fertilizer may increase the tillering of the wheat so as to produce a partial crop. If the crop has a good growth pasturing or cutting in the fall may be beneficial. When injuries from the fly may be anticipated, moderately late planting of winter wheat is perhaps the best preventive. Seeding for this purpose should be about the middle of September in the northern districts, during the first half of October in Kentucky, and during the first half of November in the extreme south. The rotation of crops should be practiced. Burning or plowing under the stubble is of great advantage. The fly can be starved out almost completely over a district of any size by abandoning for one year the culture of wheat, rye and barley. Volunteer grains should also be destroyed. Early plantings of trap or decoy crops will attract the flies, and, after ovipositing, these crops may be plowed under deeply. While no varieties of wheat are absolutely “fly proof,” some tiller more and are less injured than others, such as Underhill, Mediterranean, Red Cap, Red May and Clawson. Preventive measures reduce the annual loss from the Hessian fly by an amount estimated

« AnteriorContinuar »