springtime, while other orchards in quiet places are being damaged by the frost.

An instance occurs near Mapleton, Utah, which is probably not at all an unusual one, where Prof. L. M. Gillilan's Maplewood cherry farm usually bears fruit in safety because of correct canyon air drainage, and which is protected from the occasional violent canyon breezes by being situated on a shelf or ledge at the extreme upper edge of the agricultural section of the valley, yet just a few yards beneath the outlet floor of Maple Canyon, so that all hard winds flow over or above this orchard, leaving it in quiet and safety, while trees below in the lower portion of the valley often have a windfall of fruit due to the stream of air which can be distinctly heard whirring along above the Maplewood cherry farm. These conditions occur when an autumn high-pressure region is so situated as to drain into an adjacent low-barometer area, and cause winds directly through the canyons in question. (Pl. XXIII, fig. 2.)

A STRIKING EXAMPLE.

Another interesting natural phenomenon occurs at the outlet of Spanish Fork Canyon (the route of the Denver & Rio Grande Railroad), where there are several hundred acres of good soil lying in a most delightful temperature and precipitation climate that does not even grow good pasturage because of the strong canyon winds, yet this region is surrounded by as fine orchards and farms as are found anywhere in the West. This canyon is long and drains a large, high region from the mountains out onto the broad, open Utah Valley, and the winds run at velocities estimated at from 30 to 50 miles per hour at the canyon's mouth all night long, even in bright, fair weather, when the surrounding regions are resting in comparative quietude.

Orcharding has been and is being tried there in a limited way, but so far has not appeared to be profitable. The scanty vegetation that gets hold on this region leans far out toward the valley and appears to have foliage on but one side of the short stems. A house was once built on this bench, but it was blown from its foundation one clear night. This wind flat is a delta from Spanish Fork River which was formed in prehistoric times on the shore of Lake Bonneville; Spanish Fork town, located just beyond and beneath this bench, from 40 to 60 feet lower, enjoys a splendid climate and is protected admirably from frosts by a reasonable amount of wind.

These canyon breezes are the one great primary problem of the frost fighter, and while fighting frost with fire had its beginning in the favorably situated, mountain-protected orchard, it has also had

its finish in the other mountain orchard that is fanned nightly by a 15 or 20 mile breeze which carries the smoke and heat away in a very thorough manner. And even where the smoke and heat are not carried away so completely, the heating problem varies in intricacy with the wind velocities prevailing.

CONCLUSION.

Thus we find that the mountains are often perfect barriers against evil climatic influences and often actually augment and multiply the influences for good. The bugaboo of a treacherous, stormy, frigid, or furnace-like climate has receded far beyond the regions of agricultural possibilities and up into the very mountain tops to remain forever.

1

INSECTS INJURIOUS TO THE ONION CROP.

By F. H. CHITTENDEN, Sc. D.,

Bureau of Entomology.

INTRODUCTION.

The onion and other bulb crops of similar structure are very seriously affected by insects when growing in the field. About six species of plants are included in this group-the common onion, Welsh onion, leek, garlic, chives or sives, and shalląt. Of these only the common onion is grown to any extent in North America. Comparatively few insects appear to be especially attached to onions, but of these several are very important pests. All are of foreign origin. The list includes forms such as the onion thrips, the root maggots, and such general pests as cutworms, army worms, wireworms, white grubs, and a few other species such as the strawberry thrips. Those listed as general pests are all more or less omnivorous. Doubtless were it not for the pungent odor of the onion and its kind it would be resorted to for food by many insects other than those which have been mentioned. The most important of all of these insects is the onion thrips (Thrips tabaci Lind.).

A census of the years 1908, 1909, and 1910 shows a steady increase in acreage devoted to onion growing in different regions. In one centered about Stark County, Ind., the increase has been great. In 1910, 1,500 acres were planted to this crop, and in spite of serious injury sustained from the thrips and some other insects the growers realized such a high percentage of profit that the following year the acreage was doubled. As an example of the profit from onion growing in this region it was claimed by one prominent grower who farms in Indiana as well as in Illinois that his income on onions was 15 times as great as on wheat and corn. The damage due to the onion thrips in the Stark County (Ind.) region was estimated at $54,000 in 1910, and with double the acreage for 1911 this would have caused a loss of $108,000 for this region alone. Fortunately, however, this loss was not realized, since the insects were not so numerous as in the previous year.

THE ONION THRIPS.

(Thrips tabaci Lind.)

Our most serious onion pest is of almost microscopic dimensions, generally known as the onion thrips or "thrip." It is also called the

66

onion louse." It causes injury to the onion crop practically throughout the country, producing a condition somewhat generally known as "white blast,' ," "white blight," and "silver top." It is also the cause of "scullions," or "thick-neck"-undeveloped and unmarketable bulbs. In aggravated cases whole fields, and sometimes large areas, are rendered unproductive, and in extreme cases are completely destroyed. The whitened appearance of the onion leaves and tops is due to the extraction of the vital juice, first by rasping, followed by suction. In a short time after attack begins the leaves become peculiarly curled, crinkled, and twisted, and finally die down prematurely. (See Pls. XXIV and XXV, showing the difference between normal and thripsinfested onions.)

The importance which this thrips has assumed since about 1904 is such that a considerable proportion of those who have been engaged in investigation of truck-crop insects in the Bureau of Entomology have devoted more or less time to its investigation and in the practical application of remedies. This work has to date cov

FIG. 1.-The onion thrips (Thrips tabaci):

a, Adult; b, enlarged antenna of same; c, small nymph; d, older nymph. All enlarged. (Reengraved after Howard.)

ered five years. The principal work in the field has been done by Mr. H. M. Russell in Florida, by Messrs. D. K. McMillan and H. O. Marsh in Texas, by Mr. Marsh in Colorado, and by Mr. M. M. High in Texas and Indiana.

DESCRIPTION.

The general appearance of both sexes of this thrips, which are very similar, is shown in figure 1, a, highly magnified. The adult insect is pale yellow in color, with the thorax somewhat darker. The wings are still paler yellow, with dusky fringes and bristles. A fullgrown nymph or larva is shown at d, and a younger one at c. The egg is bean-shaped, semitransparent, and is deposited by the female just beneath the epidermis of a leaf.

HISTORY AND HABITS.

Onion thrips may now be found in practically all cultivated fields in the United States, as well as in many uncultivated areas where

suitable food plants for its sustenance are growing, so that there is always danger of infestation to onions and other susceptible crops, whether grown in new or in old land.

Observations tend to demonstrate that in some localities, at least, it makes little difference as to the previous crop. Nevertheless there can be no doubt that, taking the country at large, there is always grave danger of infestation to onion fields where crop rotation is, not practiced and where onions follow onions or other susceptible plants, and where culls and other refuse from onion beds are allowed to accumulate in and near fields to be replanted in onions.

There is little evidence available that the quality of the soil has in itself much bearing on the degree of infestation.

Owing to the minute size of thrips, it is a matter of some difficulty to investigate their full life histories, and it is particularly difficult to generalize without knowing more of the habits of the important groups. The following, however, is approximate:

The parent thrips is usually found on the lower side of leaves or embedded in flowers. The female, by means of a tiny saw-like organ with which she is provided near the end of the abdomen, cuts a slit, in a leaf or stem usually, and in this deposits an egg, generally inserting it under the epidermis concealed from view. Here the egg hatches in a few days, and the young thrips works its way out and begins to feed. The thrips larvæ suck the juices of the plants in the same manner as do the adults, and, since they feed continuously, their growth is rapid. In one or two weeks, depending upon the temperature, they cease feeding and seek a suitable location in which to transform to the final stage of the nymph and from that stage to the adult. The life cycle from the time of deposition of the eggs until the maturing of the adult has been found to require, under the most favorable conditions that is, in a warm temperature-about three weeks. Half a dozen or more generations might thus be produced during a season.

It should be added in regard to the life history of this thrips that infestation may be complicated by the attacks of other insects, such as the red spider, when growing in greenhouses (see Pl. XXVI, middle figure) or by cutworms and wireworms in the field (see Pl. XXXII).

FOOD PLANTS.

Besides onions and related plants, this thrips attacks cabbage, cauliflower, parsley, cucumber, melon, pumpkin, squash, kale, turnip, tomato, seed beets, blackberry, and strawberry.

Of ornamental plants it does much injury to carnations and roses and more or less injury to aster, blanket flower (Gaillardia), honeysuckle (Lonicera), daisies, nasturtium, narcissus, mignonette, candy73029°-YBK 1912-21