Although adults are found above treatment thresholds in a third of the commercial orchards in Pennsylvania, use of broad-spectrum insecticides in July and August prevents injury.
Description and life cycle
The adult fly is black, about the size of a house fly, with three or four white stripes across the body in the males and females, respectively, and has a prominent white spot in the middle of the back. The wings are clear, with four black bands shaped somewhat like the letter "W." Maggots are white and legless and reach about ¼ inch at maturity. Pupae resemble a grain of wheat.
Host range includes apples, cherries, and hawthorns. Early maturing and thin-skinned apples are often most severely infested. In many apple cultivars more than 90 percent of the maggots in the fruit fail to survive if the apple remains on the tree.
Apple maggots have only one generation per year. Pupae overwinter in the soil. Adults emerge during the summer (mid-June), with peak emergence in July and August. After a 7- to 10- day mating and preoviposition period females begin depositing eggs singly just under the skin of the fruit. Eggs hatch in a few days and the young maggots start to feed, working their way through the fruit. Injury is characterized by pitting and dimpling on the apple surface, and brown, winding trails caused by maggot tunneling and excrement along with an associated bacteria. Infested apples drop prematurely and the mature maggots leave to pupate in the soil.
Adult apple maggot flies are monitored most effectively by sticky red spheres baited with apple volatile lures or less effectively with baited yellow sticky traps. Three traps are recommended per block, near the border, one to two rows in from the edge. Traps should be placed in the orchard around mid-June, about head height, positioned so they are surrounded by fruit and foliage but not touched by them or obstructed from view. Traps should be inspected and cleaned weekly. If no insecticide residue remains, a contact insecticide application is recommended immediately when an average of five flies per trap are captured. Capture of flies for up to 14 days following an insecticide application can be discounted if the appropriate insecticide is chosen.
Remove abandoned apple trees and alternate hosts from 100 yards around the orchard. Infestation may be reduced in small orchards by trapping out adults using unbaited sticky red traps at the rate of one trap per 100 to 150 apples. Insecticide-treated spheres have been developed to provide control and reduce pesticide use. Frequently picking up and destroying dropped apples may reduce the problem the following year, but it will not be effective if infested, abandoned apple trees are nearby.
Effective control of apple maggots requires spray coverage when trap thresholds are exceeded. Insecticides are directed against adult flies before eggs are laid. In areas with a history of apple maggot problems, regular application of broad-spectrum contact insecticides in July, August, and possibly early September, with thorough coverage of all foliage, should provide adequate control of this important pest.
Specific chemical recommendations for home gardeners are in Fruit Production for the Home Gardener, and recommendations for commercial growers are in the Penn State Tree Fruit Production Guide.