Apple maggot fly

What you are looking at

An apple that looks close to sound from the outside, marked with only a faint dimple or two on the skin where an egg was laid just beneath it, and that turns out to be rotting from the inside when cut open. Brown, winding, thread-like trails run through the flesh, and by the time they are extensive the interior has turned to an unpleasant brown mush even where the surface still looks presentable. Affected fruit often drops well before normal ripening.

The fly itself is small, about 5 mm long, black with clean white markings and dark banded patterns on wings held out to the sides at rest. It is often seen sitting on fruit or nearby foliage through mid to late summer rather than actively flying.

Not the same insect as codling moth

Casual conversation sometimes uses “apple maggot” as a loose stand-in for whatever grub or maggot happens to be in an apple, and that habit obscures a real and useful distinction. Codling moth, the insect responsible for most maggoty apples in Europe and much of the rest of the world, is a moth whose caterpillar bores a single neat hole straight to the core. This species is an entirely different, unrelated insect, a fly whose larvae tunnel throughout the flesh from just beneath the skin rather than driving for the core, and it is confined to North America. Where a reader elsewhere sees “apple maggot” used online, it is worth checking which of the two is meant, since the symptoms, timing and controls differ.

A fly caught mid-evolution

This species is native to northeastern North America, where wild hawthorn is its original host and remains a common one today. It took roughly a century after apple orchards were established in the region for the fly to begin using the new fruit at any scale, a shift well documented enough to serve as a standard example in evolutionary biology of a population splitting into host-specific races within a genetically observable number of generations. Having adapted to apple, it has since spread through most of the apple-growing regions of the eastern and midwestern United States and Canada, and reached parts of California, Oregon, Washington and Utah following an accidental introduction there in the late 1970s.

It has never established outside North America, and both Europe and Australasia treat a suspected finding as reportable rather than routine.

What to do about it

Hang red sphere traps, coated in sticky adhesive and ideally apple-scent baited, in the outer canopy from mid-June, and treat five or more flies on a trap as the point to act.

Collect and destroy fallen fruit at least twice a week through late summer.

Manage or remove wild hawthorn growing near a valued apple tree, since it is this fly’s original host and a likely source of reinfestation each year.

Bag individual fruitlets on a garden-scale tree where the crop is small enough to make this practical.

What it gets mistaken for

Codling moth, present nearly everywhere apples are grown including within North America, tunnels straight to the core through one obvious round hole plugged with dry frass, a different pattern from this species’ network of internal trails starting from a barely visible surface dimple. The two are different insects on different biological paths and only this one is the “apple maggot” of North American usage.