Apple maggot

Apple maggot, (Rhagoletis pomonella), species of fruit fly that is a serious apple pest in North America. The adult female lays its eggs beneath the skin of apples, leaving pits and dimples on the fruit. The larvae then tunnel through the flesh, causing it to become spongy, discolored, and prone to internal decay. The resulting damage reduces the marketability of the affected apples and causes fruit loss. Some populations of the insect use hawthorn fruits, rather than apples, to complete their life cycle and are not considered significant agricultural pests.

Physical description

Slightly smaller than a common housefly, apple maggot adults are about 6 mm (0.25 inch) long. They have black bodies, yellowish legs, and pale markings on their abdomens. Females are generally somewhat larger than males and have a pointed, protractable ovipositor (egg-laying organ) at the tip of their abdomens. The head is yellowish to brownish and bears a pair of large brown-red compound eyes. Distinctive dark bands across their transparent wings form a characteristic zigzag or F-shaped pattern.

The creamy white larvae are maggots and have no wings, wing buds, or legs on their soft, tapering bodies. A pair of dark mouth hooks acts as a fork that helps the larvae feed as they tunnel through fruit flesh. They reach about 7–8 mm (0.3 inch) long when fully grown.

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Life cycle

Female apple maggot flies lay 300 to 500 eggs over their 30-day life span. Each egg is inserted into a developing fruit through a puncture in its skin. After about five days the larvae hatch and begin feeding and tunneling throughout the fruit, making winding, brownish trails. Because of these characteristic trails, apple maggots are also known as railroad worms (the name is more commonly applied to the larvae and wormlike adult females of Phrixothrix beetles). Infested fruits often drop from the tree prematurely, and apple maggot larvae eventually tunnel out to pupate and overwinter in the soil. Adult flies begin to emerge the following June, and most populations produce one generation per year.

Distribution and host plants

The apple maggot is native to North America and is most widespread in the eastern United States and southeastern Canada; its range extends westward across parts of the northern United States and southern Canada. The pest was detected in Portland, Oregon, in 1979 and subsequently spread through parts of the Pacific Northwest, where it continues to threaten major commercial apple-growing regions.

Before the introduction of domesticated apples to North America, apple maggots fed primarily on native hawthorns (Crataegus). Following the apple’s arrival with European colonizers, however, some apple maggot populations shifted to the new host plant. Because apples generally ripen earlier than hawthorns, apple-associated flies emerge and reproduce earlier in the season than hawthorn-associated flies, a temporal isolation that reduces the opportunities for the two groups to interbreed. In addition, the two groups tend to mate on or near their respective host plants, further reducing interbreeding. These differences have made the apple maggot an important model for studies of host specificity and the possibility of speciation (the evolution of new species) without geographic isolation. The insects also develop in quince and are reported to be incidental in plums, pears, tart cherries, and certain other members of the rose family.

Control

Apple maggot flies are closely monitored in susceptible regions, and small outbreaks can sometimes be managed without conventional pesticides. Growers commonly use sticky traps, often tapes or red spheres that resemble ripening apples, along with chemical or food-based lures, to attract adult flies. Such traps can reveal when adults become active and help growers determine whether and when additional control measures are needed. When deployed in sufficient numbers, traps may also help reduce adult populations.

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In addition, orchard sanitation can interrupt the apple maggot’s life cycle. Infested and fallen apples can be collected and destroyed before mature larvae leave the fruit and enter the soil to pupate. In home gardens, small orchards, and organic operations, individual developing apples can be enclosed in protective bags that prevent female flies from reaching the fruit and laying eggs. These approaches can reduce reliance on insecticides, although they may be impractical for large commercial orchards.

In commercial orchards apple maggots are commonly controlled with insecticides targeting adult flies. Growers in infested areas typically begin treatment as adults begin emerging from the soil and before they have mated or started to lay eggs. Predictive models of apple maggot emergence are available in many places to help farmers efficiently target the pests.