Canker Worm, the caterpillar of a nocturnal lepidopterous insect, or moth, of the family geometra, Linn, (or phalcenites, Lat.), of the group hyberniadce, and the genus anisopteryx. In the moths from which canker worms are produced the females are wingless. The males have antennae with a downy edging on each side; the wings are large and silky, and when at rest the fore wings entirely cover the hind wings; the fore wings are ash-colored, with a whitish spot on the front edge near the tip, and two irregular white bands crossing them, with black dots along the sides and outer margin; the hind wings are pale ash, with a blackish dot near the middle; the expanse of wings is about 1 1/4 inch. This is the common American species, which is smaller and darker than the European, and is the A. vernata (Peck); there is a smaller species, without the whitish bands and spots, the A. pometaria (Harris). These moths usually come out of the ground about the middle of March, sometimes a little earlier or later, according to the season, and continue to rise for about three weeks; in mild winters they have been seen in every month from October to March; the females are most numerous in autumn and winter, and the males most abundant in the spring.
The wingless females creep up the trunks of the nearest trees, and are followed in a few days by the males, when the pairing takes place; the eggs are placed on the branches in clusters of 60 to .100, the number usually laid by each female, and are attached by a water-proof varnish; soon after this the insect dies. The eggs are hatched from the 1st to the middle of May, at the time when the red currant blossoms and the young leaves of the apple begin to start; the young worms gather upon the tender leaves, and creep into the buds and flowers; at first they make but small holes, but at last devour all the pulpy part of the leaves of the apple, elm, cherry, plum, lime, and other native and cultivated trees. The worms vary considerably in color within the limits of the same species; when very young they are of a blackish brown color, with a yellowish stripe on each side, the belly whitish, and two bands of this color across the head; when fully grown they become ash-colored on the back, black on the sides with a pale yellowish line below it; some are dull greenish yellow, others green with two white stripes on the back, and others clay-colored; when full-grown they are nearly an inch in length.
After eating for about four weeks, they begin to quit the trees on which they have fed; some creep down, but most let themselves down from the branches by threads; they burrow immediately into the earth, from two to six inches in depth, according to the nature of the soil; they make little cavities in the earth by turning themselves round, and are changed into chrysalids within 24 hours, those of the females being the largest; the chrysalis state may continue till the following spring, or it may cease in mild weather in the autumn. They come out of the ground mostly in the night. Nature seems to desire to confine the canker worms to a limited space, as the females have no wings, and bury themselves within the spread of the trees from which they descend; but accidents have extended them to remote localities. The canker worm has ten legs, six anterior jointed ones, and four fleshy prop legs behind; they are called span worms and loopers from their singular mode of progression; from the absence of the three intermediate pairs of prop legs in the centre of the body, when creeping they arch up the back and bring forward the hind part of the body, and then, resting on their prop legs, they stretch out to their full length in a straight line, and so repeat the spanning process. - The ravages of canker worms are not very apparent till June, when they are most voracious; but then the leafless and apparently withered fruit trees and elms afford a melancholy spectacle.
The best way of destroying the canker worm is to prevent the females from ascending trees to deposit their eggs; various methods have been resorted to for this purpose, consisting in the application of viscid substances to the trunk, immediately on the bark or on strips of cloth, paper, or board; tar is generally used, and it should be applied from November, and renewed daily as long as the insects come forth; tin troughs filled with cheap oil a few feet above the ground have been tried with success on a small scale; melted india rubber has been recommended in England. When the worms are on the leaves, they may be destroyed on small and choice trees by dusting air-slacked lime on them when wet with dew. Showering with a mixture of whale-oil soap in water, in the proportion of one pound to seven gallons, will kill the worms without injuring the leaves or the fruit. By jarring the limbs many worms will descend with their silken threads and may be easily killed. After they have entered the ground, they may be killed by digging or ploughing under the trees, scattering a few grains of corn, and turning a few hogs into the orchard; these animals will root up and devour the chrysalids, and will crush many with their feet.
Canker worms are eaten by many species of birds; ground beetles also devour them; the potter wasp fills her clay cells with them as food for her young; ichneumon flies deposit their eggs in them, and the little maggots thence arising feed upon their substance; even their eggs are pierced by a small four-winged fly, sometimes every one in a cluster being thus rendered abortive.
Canker Worm. - 1. Adult Male. 2. Larva. 8. Adult Female. 4. Cluster of Eggs.
No doubt beast, bird, and insect would be enough to keep the canker worms within their natural limits; but since the felling of the forests in which they naturally dwell, and the persecution of insectivorous birds which devour them, the worms seem to have increased in spite of all man's destructive ingenuity. In our northern cities the best remedy for them yet found has been the introduction of English sparrows, which seem to have almost exterminated them. - For fuller information on these pests, and the best means of destroying them, see Harris's " Insects injurious to Vegetation".