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Scale Insects — Part 2

The majority of species of scale insects at present found in the United States are not indigenous to this country, but have been introduced from abroad, most of them from European regions, on plants and young trees and on fruit carried to this country in the course of commercial interchange.

Life History of the Oyster-Shell Bark-Louse of the Apple.

(Mytilaspis pomorum Bouche.)

This insect is probably the commonest and most widespread, and consequently the best-known, of any of the orchard scales. It is found all over the world. It was probably an European insect originally--at all events, it was known in Europe during the last century--and was probably imported into this country on nursery stock by the early settlers. It is found in the United States practically wherever apples and pears are grown, more abundantly at the north than at the south, and has often received treatment at the hands of writers on injurious insects. It was certainly known in this country as early as 1794, and it unquestionably followed apple culture as it progressed to the west. It was known in the districts bordering along Lake Michigan in 1840, and in 1868 it invaded Iowa and northern Missouri. Today it is present even in California. It is by no means confined to apple and pear, but is also found upon quince, hawthorn, buckthorn, rarely upon raspberry and currant, but also upon linden, hop tree, horse chestnut, maple, water locust, honeysuckle, ash, elm, hickory, cottonwood, willow, poplar, wild grape, rose, fig, bitter-sweet, red maple, black ash, white ash, white birch, red birch, and very abundantly upon the lilac, so that it will be an easy form to collect and to study. In the winter time, if one of the oyster-shell shaped scales is lifted gently with the point of a needle, it will be found to contain at the narrow front end the shriveled body of the female with from 50 to 100 yellowish white eggs packed closely together behind the body. Sometimes the eggs will be found to be very few in number, but then the larva or pupa of a little chalcidid parasite will be found under the scale, which will account for the destruction of the eggs. In most of the northeastern states the young hatch from these eggs during the latter part of May or early in June (at an earlier date farther south), and wander out upon the twigs and settle at once. The young twigs are the only parts of the tree which seem to be seriously affected. Older twigs, however are also attacked, and many specimens of the insect may be found upon the trunk. As soon as the young insect finds a suitable position it gradually inserts its beak and commences to grow through the influence of the healthy food sap which it pumps up. Almost immediately, also, the secretion of scale begins. This is first noticed in the form of a white, waxy powder which first is seen in the form of threads, but which soon becomes homogeneous. In a few days the first molt takes place, not as in the ordinary manner with insects by a series of contractions and extensions which work the old skin to the end of the body, from which it is finally freed, but by a sort of loosening and shrinking of the body underneath, all the parts except the proboscis being shed and abandoned with the skin. Strengthened by the secretions from the body, this skin forms the first scale. Presently the skin is shed a second time and mingles with the second secretion, which forms the second scale, which may be noticed at the anterior end of the completed scale of the old individual. As growth continues this second scale becomes too small and is lifted up into the third portion, while the shield proper is secreted by a series of increments, more being constantly added on behind as the insect beneath grows. When full grown the female is ready for reproduction. The male scale is a perfectly distinct object, as indicated in the accompanying figure, and the adult male, which makes its appearance at the time when the females are almost ready for oviposition, is shown at Figure 151. There is but one annual generation in the northern states, and, owing to this fact, the leaves are not attacked, for if the insect were to go on the leaves it would be lost when they fall in the autumn. Upon the fruit it is almost equally rare, although occasionally a specimen is found in such a location. In the south, however, the insect is two-brooded, and the adults of the first generation are found upon the fruit and leaves without danger to the perpetuation of the species, since their offspring crawl back to the permanent portions of the plant before autumn. As a matter of fact, however, even in the south the insect is very seldom seen upon either the leaves or the fruit.