CITIZENS OF THE APPLE-TREE
Many years ago, my old friend, the late Dr. J. A. Lintner, State Entomologist of New York, compiled a list of 356 insects that feed on the apple-tree. Later authorities place the number at nearly five hundred species. It must be a good plant that has such a host of denizens. The number of fungi is also large; and the tree often supports lichens, algæ, and other forms of life.
The apple-tree is not single in its denizens. No plant lives alone. It has association with its fellows, perhaps contest for space and nourishment. It provides habitat for many organisms, many of which live on its bounty. I have never seen a bearing apple-tree that was not a colonizing place for other living things. We accept these things as matters of course, as being in place, living their part in nature. Therefore, one cannot understand the apple-tree unless one knows something of its citizenry.
Probably the most prominent citizen of the apple-tree is the codlin-moth. Its larva is the apple-worm, the one that makes "wormy apples," the burrows going to the core and out again. The insect is native in Europe, but has been known in North America nearly two hundred years, and is widespread in the apple countries of the world.
If one has screens in the apple cellar, one is likely to find small moths on them in the spring, larger than a clothes moth, about three-fourths inch in spread of the soft gray watered-silk wings. This is the imago or mature form of the insect known as the codlin-moth (it lives on codlins or apples). The larvæ or "worms" were brought into the cellar in the apples; some of them crawled out, spun themselves in a cocoon and pupated; in due season the moth emerged, ready to lay the eggs for other larvæ. Ordinarily the fruit-grower does not see the moth, for it is a small object amidst the foliage of apple-trees; the larva or apple-worm he knows well.
There may be two or more broods of apple-worms, depending on the length of the season. In the northern apple regions of North America there is usually only one brood, with a partial second brood. The first brood is hatched from eggs laid by moths that emerge in spring. The moths come from larvæ that have lain in cocoons all winter, hidden under bark on the trunks and main branches of the apple-tree, in crevices in nearby posts and fences, and sometimes in the ground. The pupæ are the transformed larvæ or worms that left the apple of the previous year, usually before it fell, and crawled down the tree to find a place to spin the silken brown cocoons in which they wrapped themselves to undergo the wonderful transformation.
So is the cycle complete: egg laid in early spring, mostly on the leaves; larva hatched in about one week, crawling to the young apple to feed, where it lives for perhaps a month; larva departed from the fruit to form a cocoon and to remain quiescent till it pupates the following spring (if there is no second brood) when it transforms into a moth; the moth alive for one week or ten days, laying perhaps as many as one hundred eggs or even more. If there is a second or third brood, the pupa resurrects in ten days or so into the moth; eggs are laid; larvæ are hatched; pupæ again are formed; and thus is the process continued. But the winter stage is the larva, although perhaps in store-houses the moths may emerge earlier and survive till spring.
The eggs of the first brood are commonly laid on the leaves and fruit. The young larva or worm eats very little on the foliage. It usually crawls into the blossom end of the apple. The young apple stands erect, with the calyx open (Fig. [6]); later the calyx closes and protects the larva that hatched there, forming a good cover for its operations (Fig. [7]). The worm drives for the core, where it eats the young seeds and burrows extensively; then, when nearly grown, it sets out for the surface, eating a straight burrow; an opening is made through the skin of the apple, but this exit is plugged until the animal is ready to leave the place and to crawl down the tree to pupate. The larvæ of later broods may enter at the side of the apple, where a leaf affords protection or where two fruits come together; but the life-history is the same, varying in its rapidity.
This account discloses the vulnerable point in the life-history, if one is to destroy the insects and to grow fair fruit; if poison is lodged on the erect open-topped little apple, the young larva will get it before he injures the fruit. If the application of the poison is delayed until the calyx closes (Fig. [7]), there will be small chance of reaching the worm. The best way to reach the second brood is to destroy all the first brood. The standard practice, therefore, is to spray the trees soon after the petals fall, with the idea of depositing arsenic in the blossom end.
But the season of egg-laying is long, often extending over a period of three or four weeks, for the moths do not all emerge from the cocoons simultaneously. It is customary, therefore, to spray again about two weeks after the first application, with the hope of catching the young worms on their way to the fruit.