The apple maggot, Rhagoletis pomonella, is a serious apple pest that often damages homeowners' fruit. Careful management, however, including treating with insecticides and trapping adults before they lay eggs can reduce the chance of injury.
The adult fly is black with three or four white stripes across the body and a prominent white spot in the middle of the back. The wings are clear and have four black bands shaped somewhat like the letter "F." The maggot is white and legless, and can be 1/4 inch long when fully grown. Apple maggot fruit damage is characterized by egg-laying scars that cause pitting and dimpling on the apple surface and brown, winding, bacteria-laden trails under the apple's skin caused by the maggot's tunneling and excrement. The apple maggot is sometimes called the "railroad worm" because of the speed at which the maggots dig through the apple and the tunnels they create.
Adult flies start to emerge in mid-June, and most have emerged by the end of August. Peak emergence can occur in mid- to late July. The adults spend a feeding period of 1 to 2 weeks before the female begins to lay her 300 eggs over her 30-day life span. The female lays eggs singly in small holes cut in the apple skin. Each egg hatches in 2 to 10 days, and the young maggot starts to feed, working its way through the fruit. The most severe infestations occur in early maturing and thin-skinned apples.
After the infested apple has dropped to the ground, the mature maggots leave the fruit and burrow up to 2 inches into the soil. The maggots pupate and remain in the soil until the following summer. Only one generation occurs each year in Pennsylvania, although a small percentage of the pupae do not emerge for 2 or more years.
Insecticide treatments based on trap thresholds are one method of controlling the apple maggot. Two or three sticky red spheres baited with an apple volatile are hung in the edge of each orchard block and monitored twice a week. The grower should apply an insecticide treatment when the average number of flies per trap reaches five. After the insecticide residue has declined, captures in the trap are accumulated until the threshold is exceeded again. This method can prevent unnecessary insecticide sprays if maggot density is too low to result in significant injury. Effective control in the absence of trap monitoring can require spray coverage during July, August, and early September. If pesticide treatments are necessary, products containing broad-spectrum insecticide (e.g., spinosad, pyrethroids, or carbaryl) can be applied.
Trapping alone also can serve as a control measure for apple maggots in small plantings. Baited red spheres hung at a rate of 1 per 100 to 150 fruit are used to "trap out" females before they have a chance to lay eggs. These traps should be placed in mid-June. Other cultural control measures include frequently picking up dropped fruit and removing nearby abandoned trees.