Why they are a Pest
The female fly lays her eggs in a wide range of fruits. The eggs hatch into maggots that feed on the fruit as they develop, causing it to rot from the inside. Evidence of fruit having been stung is visible as small brown marks that may be raised lumps.
In places where the cucumber fly has stung the fruit there is browning off around the area within the fruit.
The cucumber fruit fly seems to have a more limited range of fruits it attacks, mainly cucurbits (cucumber, pumpkin, marrows, zucchini) as well as tomatoes, papaw, guava, and passion fruit.