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Species of fly

The apple maggot (Rhagoletis pomonella), also known as the railroad worm (but distinct from the Phrixothrix beetle larva, also called railroad worm), is a species of fruit fly, and a pest of several types of fruits, especially apples. This species evolved about 150 years ago through a sympatric shift from the native host hawthorn to the domesticated apple species Malus domestica in the northeastern United States. This fly is believed to have been accidentally spread to the western United States from the endemic eastern United States region through contaminated apples at multiple points throughout the 20th century. The apple maggot uses Batesian mimicry as a method of defense, with coloration resembling that of the forelegs and pedipalps of a jumping spider (family Salticidae). The adult form of this insect is about long, slightly smaller than a housefly. The larva, which is the stage of this insect's lifecycle that causes the actual damage to the fruit, is similar to a typical fly larva or maggot. Caterpillars, especially the larvae of the codling moth, that infest the insides of apples are often confused with the apple maggot. However, these organisms generally feed in the apple's core while apple maggots feed on the fruit flesh. The apple maggot larvae are often difficult to detect in infested fruit due to their pale, cream color and small body size. The adult fly lays its eggs inside the fruit. Larvae consume the fruit and cause it to bruise, decay, and finally drop before ripening. The insect overwinters as a pupa in the soil. It only emerges after metamorphosis into a relatively defenseless fly. Adults emerge from late June through September, with their peak flight times occurring in August.

Eggs are fusiform and pearly white when laid in fruit, but after a short period of time in the fruit, they turn a cream color. Developing larvae can be clearly seen within the translucent eggs. The larva is white or cream colored, but if it eats green pulp near the fruit skin, the green shows through the skin. Larvae range in length from 7 to 8.5 mm and in width from 1.75 to 2 mm. California quarantine inspection records show apple maggot infest fruit have been intercepted at border patrol stations since around 1950. A misidentified apple maggot, originally thought to be the snowberry maggot Rhagoletis zephyria, was found in the Oregon Department of Agriculture (ODA) tephritid fly collection, and the fly was collected in 1951 in Rowena, Oregon. The ODA conducted a survey in 1980 to measure the distribution of the apple maggot, and traps in southwestern Washington showed apple maggot distribution in and around Vancouver, Washington, a suburb of Portland, Oregon. This theory is supported by lack of R. pomonella infestation on C. douglasii in Washington, Oregon, and Idaho, implying that the fly in these regions is not native on hawthorn. There are recorded sightings of R. pomonella from Southern Utah and New Mexico, as well as in the Sierra Madre Oriental Mountains and the Altiplano central highlands of Mexico. Peach, pear, cherry, plum, chokeberry, cranberry, dogwood, and fruits of the Japanese roses Rosa rugosa and Rosa carolina can also host apple maggots. However, apple maggots are not usually a serious pest of plums, cherries, and pears; larvae have been found in pears but not adult flies. The fly cycles through one generation a year with adults living up to four weeks. The male springs onto the female and positions his forefeet on her abdomen and his second pair of feet on her wings. The male waits and uncoils his spring like penis, quickly entering the opening of the ovipositor when the female extends it. Mating occurs on the host plant and averages thirty minutes, during which the flies are attached and can fly about. Then, when the flies separate, the ovipositor and penis are quickly retracted. The maggot stage has other predators including several braconid wasps: Utetes canaliculatus, Diachasmimorpha mellea, and Diachasma alloeum.

A 1920 study showed that apple maggot eggs were parasitized by Anaphoidea conotrecheli, which is a common egg parasite of plum curculio and grape curculio. The study states that the abundance of Anaphoidea conotrecheli parasitization of apple maggot eggs would depend on the abundance of the eggs of its principal host, the plum curculio. In addition, the hawthorn and apple host races of R. pomonella are able to produce viable offspring in a lab setting, but in nature, flies maintain their genetic integrity partly because of allochronic premating isolation from differently timed adult eclosion. Flies that infest apples eclose before flies that infest the hawthorn, which fruits later than apples.

Rhagoletis pomonella is a significant pest for apple crops because the species feeds on apples and lays eggs within the fruit.