The Codling Moth

Teacher's Story

T is difficult to decide which seems the most disturbed, the person who bites into an apple and uncovers a worm, or the worm which is uncovered. From our standpoint, there is nothing attractive about the worm which destroys the beauty and appetizing qualities of our fruit, but from the insect standpoint the codling caterpillar (which is not a worm at all), is not at all bad. When full-grown, it is about three-fourths of an inch long, and is likely to be flesh color, or even rose color, with brownish head; as a young larva, it has a number of darker rose spots on each segment and is whitish in color; the shield on the first segment behind the head, and that on the last segment of the body, are black. When full-grown, the apple worm is plump and lively; and while jerking angrily at being disturbed, we can see its true legs, one pair to each of the three segments of the body behind the head. These true legs have sharp, single claws. Behind these the third, fourth, fifth and sixth segments of the abdomen are each furnished with a pair of fleshy prolegs and the hind segment has a prop-leg. These fleshy legs are mere makeshifts on the part of the caterpillar for carrying the long body; since the three pairs of front legs are the ones from which develop the legs of the moth. The noticing of the legs of the codling moth is an important observation on the part of the pupils, since, by their presence, this insect may be distinguished from the young of the plum curculio, which is also found in apples but which is legless. The codling moth has twelve segments in the body, back of the head.

The codling larva usually enters the apple at the blossom end and tunnels down by the side of the core until it reaches the middle, before making its way out into the pulp. The larva weaves a web as it goes, but this is probably incidental, since many caterpillars spin silk as they go, "street yarn" our grandmothers might have called it. In this web are entangled the pellets of indigestible matter, making a very unsavory looking mass. The place of exit is usually circular, large enough to accommodate the body of the larva, and it leads out from a tunnel which may be a half inch or more in diameter beneath the rind. Often the larva makes the door sometime before it is ready to leave the apple, and plugs it with a mass of debris, fastened together with the silk. As it leaves the apple, the remnants of this plug may be seen streaming out of the opening. Often also, there is a mass of waste pellets pushed out by the young larva from its burrow, as it enters the apple; thus it injures the appearance of the apple, at both entrance and exit. If the apple has not received infection by lying next to another rotting apple, it first begins to rot around the burrow of the worm, especially near the place of exit.

A wormy apple.  Photo by M. V. Slingerland.

The codling caterpillar injures the fruit in the following ways: The apples are likely to be stunted and fall early; the apples rot about the injured places and thus cannot be stored successfully; the apples thus injured look unattractive and, therefore, their market value is lessened; wormy apples, packed in barrels with others, rot and contaminate all the neighboring apples. This insect also attacks pears and sometimes peaches. It has been carefully estimated that every year the codling moth does three million dollars worth of injury to the apple and pear crops in New York State. Think of paying three million dollars a year for the sake of having wormy apples!

The larva of the codling moth, much enlarged.  Photomicrograph by M. V. Slingerland.

The larvæ usually leave the apples before winter. If the apples have fallen, they crawl up the tree and there make their cocoons beneath the loose bark; but if they leave the apples while they are on the trees, they spin silk and swing down. If carried into the storeroom or placed in barrels, they seek quarters in protected crevices. In fact, while they particularly like the loose bark of the apple trees, they are likely to build their cocoons on nearby fences or on brush, wherever they can find the needed protection. The cocoon is made of fine but rather rough silk which is spun from a gland opening near the mouth of the caterpillar; the cocoon is not beautiful although it is smooth inside. It is usually spun between a loose bit of bark and the body of the tree; but after making it, the insect seems in no hurry to change its condition and remains a quite lively caterpillar until spring. It is while the codling larvæ are in their winter quarters that our bird friends of the winter, the nuthatches, woodpeckers and chickadees, destroy them in great numbers, hunting eagerly for them in every crevice of the trees. It is therefore good policy for us to coax these birds to our orchards by placing beef fat on the branches and thus entice these little caterpillar hunters to visit the trees every day.

The pupæ and cocoons of the codling moths.  Photo by M. V. Slingerland.

It is an interesting fact that the codling caterpillars, which make cocoons before August first, change immediately to pupæ which soon change to moths, and thus another generation gets in its work before the apples are harvested.

The codling moth is a beautiful little creature with delicate antennæ and a brown, mottled and banded body; its wings are graced by wavy bands of ashy and brown lines, and the tips of the front wings are dark brown with a pattern of gold bronze wrought into them; the hind wings are shiny brown with darker edges and little fringes. The moths issue in the spring and lay their eggs on the young apples just after the petals fall. The egg looks like a minute drop of dried milk and is laid on the side of the bud; but the little larva, soon after it is hatched, crawls to the blossom and finds entrance there; and it is therefore important that its first lunch should include a bit of arsenic and thus end its career before it fairly begins. The trees should be sprayed with some arsenical poison directly after the petals fall, and before the five lobes of the calyx close up around the stamens. If the trees are sprayed while blossoming, the pollen is washed away and the apples do not set; moreover, the bees which help us much in carrying pollen are killed. If the trees are sprayed directly after the calyx closes up around the stamens the poison does not lodge at the base of the stamens and the little rascals get into the apples without getting a dose. (See Lesson on the Apple).

Method—The lesson should begin with a study of wormy apples, preferably in the fall when the worms are still within their burrows. After the pupils become familiar with the appearance of the insect and its methods of work, a prize of some sort might be offered for the one who will bring to school the greatest number of hibernating larvæ found in their winter quarters. Place these larvæ in a box with cheese-cloth tacked over its open side; place this box out of doors in a protected position. Examine the cocoons to find the pupæ about the last of April; after the pupæ appear, look for the moths in about five days.

It would be a very good idea for the pupils to prepare a Riker mount showing specimens of the moths, of the cocoons showing the cast pupa skin, and of the caterpillar in a homeopathic vial of alcohol; pictures illustrating the work of the insect may be added. The pictures should be drawn by the pupils, showing the wormy apple, both the outside and in section. The pupils can also sketch, from the pictures here given, the young apple when just in the right condition to spray, with a note explaining why.

3. How many segments are there in the body? How many of these bear legs? What is the difference in form between the three front pairs of legs and the others?

4. Look at a wormy apple. How can you tell it is wormy from the outside? Can you see where the worm entered the apple? Was the burrow large or small at first? Can you find an apple with a worm in it which has the door for exit made, but closed with waste matter? How is this matter fastened together? If the apple has no worm in it, can you see where it left the apple? Make a sketch or describe the evidence of the caterpillar's progress through the apple. Do you find a web of silk in the wormy part? Why is this? Does the worm eat the seeds as well as the pulp of the apple?

5. Take a dozen rotting apples, how many of them are wormy? Do the parts of the apple injured by the worm begin to rot first? In how many ways does the codling moth injure the apple? Does it injure other fruits than apples?

6. How late in the fall do you find the codling larvæ in the apple? Where do these larvæ go when they leave the apple?

Work to be done in March or early April—Visit an orchard and look under the loose bark on old trees, or along protected sections of fences or brush piles and bring in all the cocoons you can find. Do not injure the cocoons by tearing them from the places where they are woven, but bring them in on bits of the bark or other material to which they are attached.

1. How does the cocoon look outside and inside? What is in the cocoon? Why was the cocoon made? When was it made?

2. Place the cocoons in a box covered with cheese-cloth and place the box out of doors where the contents can be frequently observed and make the following notes:

3. When does the larva change to the pupa? Describe the pupa. How does the cocoon look after the moth issues from it?

4. Describe the moth, noting color of head, thorax, body, front and hind wings.

5. If these moths were free to fly around the orchard, when and where would they lay their eggs?

6. When should the trees be sprayed to kill the young codling moth? With what should they be sprayed? Why should they not be sprayed during the blossoming period? Why not after the calyx closes?

7. How do the nuthatches, downy woodpeckers and chickadees help us in getting rid of the codling moth?

8. Write an essay on the life history of the codling moth, the damage done by it, and the best methods of keeping it in check.

References—The following bulletins from the U. S. Dept. of Agriculture: Farmers' Bulletin 247, "The Control of the Codling Moth and Apple Scab;" Bulletin 35, New Series, Bureau of Entomology, "Report on the Codling Moth Investigations," price 10 cents; Bulletin 41, "The Codling Moth," 105 pages, 15 cents, by Special Field Agent, C. B. Simpson; Bulletin 68, Part VII, "Demonstration Spraying for the Codling Moth," price 5 cents. The Spraying of Plants, Lodeman, Macmillan Company; Economic Entomology, Smith.