Apple maggot injury

Apple maggot injury, externally visible tunnels

Apple maggot injury, note cone-shaped pits

Market Diseases of Apples, Pears, and Quinces
Apple Maggot Injury

Rhagoletis pomonella Walsh

Occurrence and importance
The apple maggot, or railroad worm as it is sometimes called, may be found in all major apple-growing sections of eastern and midwestern States.

Apple maggot is destructive in poorly sprayed commercial orchards and difficult to control in well-managed ones, if neglected apple trees are close by. This insect pest is most damaging in home orchards.

Thin-skinned, sweet, and sub-acid varieties of summer and fall apples are most readily infested, but certain winter varieties may also be injured. Injury has been observed on Wealthy, Cortland, Delicious, Northern Spy, Golden Delicious, Stayman, Rome Beauty, and York Imperial fruit grown on or near neglected trees.

Symptoms
The symptoms caused by the apple maggot depend on the time and severity of infestation, and to some extent on the kind of apple.

If infestation is early and heavy, the egg-deposit wounds and the extensive tunneling of the flesh by the maggots will cause the apples to be severely deformed, with numerous, deep depressions giving a knobby appearance. Internally, numerous narrow, brown tunnels are found running through the flesh (top photo). Apples with fewer eggs deposited may show no deformity or knobbiness but will have tunnels through the flesh. Some of the tunnels are just beneath the skin and show through as fairly long, somewhat winding tracks, hence the name "railroad worm." These tunnels are especially noticeable on green varieties (middle photo).

In many instances, especially in rapidly growing, hard-fleshed varieties, the maggot is crushed by the pressure of the flesh. This may occur soon after the egg hatches. Few if any maggots survive in late-maturing varieties. Apples injured by the egg deposit, but with no injury from the maggot, form a small cone-shaped pit on the surface of the apple (bottom photo) which is unlike any other apple injury. In some winter varieties the maggot is able to make a short tunnel before death. There may be no external evidence other than the cone-shaped pit, while the flesh may show a brown streak where the maggot fed. These last two symptoms are probably the only types of maggot injury that would be found on apples on the market.

Apples that are severely affected and knobby are sometimes mistaken for apples with boron-deficiency cork. If an adequate sample of apples is cut, however, the maggot tunnels will readily be seen. Maggot tunnels are somewhat discolored, but the affected tissue is not corky and does not radiate from the core area as is typical of boron-deficiency cork.

Causal factors
The apple maggot is a footless white or cream-colored larva, usually-about 1/3 inch long when full-grown. The adult form is a fly, somewhat smaller than a housefly, which emerges from the soil during late June, July, or early August, depending on the locality. The flies make tiny punctures in the skin of the fruits and place their eggs just underneath. The eggs have a short period of incubation; in hot weather they hatch in 4 to 6 days. The period spent in the apple by the maggot varies greatly but may be as short as 2 weeks. When mature, the maggots leave fruits which have fallen to the ground, enter the soil, and form puparia which resemble grains of wheat. Within the puparia, the insects transform to the adult stage. They emerge the following summer or sometimes the second summer.

Control measures
The apple maggot is controlled by spraying trees with effective insecticides. The recommendations of the State agricultural experiment station or extension service should be followed regarding the best methods and materials to use.