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White Flies, or Aleyrodids

(Family Aleyrodidae.)

This family, more closely allied to the Aphididae and Coccidae than to any others, and yet widely different in important features, is a puzzling and abnormal assemblage of insects. They are very small insects, frequently minute, and infest the leaves of plants, usually on the lower side. In their early stages they are scale-like, and much resemble some of the Coccidae. Unlike the Coccidae, however, the development of the two sexes is practically parallel, and both males and females are active and winged. Differing again from the scale insects, there are two pairs of wings instead of one pair. All four wings are covered or dusted with a whitish, meal-like secretion, which has given to the group the popular name of white flies. In the immature stages the body may be more or less covered by a secretion of wax, but the most distinctive character is the presence of an opening on the dorsum of the last abdominal segment which is known as the vasiform orifice. In the adults the antennae are seven-jointed, and the eyes are usually somewhat constricted near the middle, and may be even completely divided. The wings when at rest are nearly horizontal, and are broad and well rounded. The wings may be unspotted, or variously spotted or banded. The Aleyrodidae do not constitute a large group, and but two genera and not more than 150 species are known. Of these both genera (Aleyrodes and Aleyrodicus) are known in the United States, and rather more than fifty species occur within our geographic borders. Doubtless many of these are imported, since they occur more commonly upon greenhouse plants than upon wild indigenous plants. They are found upon both herbaceous plants and upon forest trees. They are very seldom serious enemies to vegetation, although the species occurring commonly upon the orange does considerable damage, which arises not alone from the actual loss of sap and consequent withering of the leaves from the sucking of the insect, but also from the profuse quantity of smut fungus, the spores of which find their nidi in the honey dew secreted by the insects. Their natural enemies are practically the same as those of the scale insects, but the very minute Hymenopterous parasites of the family Mymaridae seem to be their specific internal parasites. In fact, they are too small to harbor any other true internal parasites except the members of this family, which, as a matter of fact, includes the smallest true insects known.

Typical Life History of a White Fly

(Aleyrodes citri, Riley & Howard.)

This species, which is the only form of any great economic importance in the group and the one to which we have just referred, occurs abundantly in some of the orange groves in Florida and in northern greenhouses and made its appearance about 1890 in the orange groves of Louisiana. It is not known whether it is an indigenous or imported species. It will probably be found in any northern greenhouse in which orange or lemon trees occur. It is found in the south also upon the china-berry tree, upon viburnum, cape jessamine and the water oak. The following life history account is drawn up from observations made in greenhouses in the city of Washington. The insect passes the winter in the full-grown larval condition. The adult insects issue during April and begin to lay their eggs about or before the middle of the month. About twenty-five eggs are laid by each female. The eggs are very delicate, pellucid, and each possesses a slender petiole or foot-stalk about one-third the length of the egg proper. In two weeks the young larva hatches. It is comparatively active and crawls a short distance from the egg-shell before beginning to feed. In from two to four weeks it molts and by the middle of June three skins have been cast. In molting the insect curves the abdomen upwards at more than rightangles, moving it occasionally up and down. It shrinks away from the side margin until it occupies only about one-third of the original lateral space. The skin then splits at the anterior end or underneath the head. Then the head and prothorax are pushed out and the skin is gradually worked backwards. At the end of June the adults begin to issue again in numbers, remaining alive for about twenty days. There are three generations annually. When the perfect insect issues from what may be termed the pupa shell, the wings are rolled up and make their appearance with extreme slowness. Just before the adult is ready to issue the pupa shell becomes transparent so that the contained insect, shrunken away from the skin, is plainly seen in all of its stages. The adult just after issuing is at first milk-white except for bright lemon-yellow thoracic lobes. The adult male is readily distinguished from the female by the anal claspers and by the waxy tufts issuing from the head and from the abdomen. All stages are well represented in the accompanying figures.