The walnut husk fly (Rhagoletis completa) is of American origin. It occurs in Europe since the 1990s and is now established in several European countries where it a pest of walnut.
Life cycle and appearance of Walnut husk fly
The adult walnut husk fly is 4-5 mm long. It has a reddish brown to yellowish brown body with a creamy yellow scutellum. The wings are clear and marked with broad, blackish crossbands. The larvae are up to 6 mm long, whitish to yellowish white, and translucent.
Adult walnut husk flies occur form early June onwards. The females lay their eggs beneath the skin of developing walnuts. The eggs hatch after 3-7 days and the larvae feed inside the developing fruits for up to 5 weeks. Fully fed larvae drop to the ground and enter the soil to pupate and the puparia overwinter.
Infested fruits bear a small, back oviposition scar. Later the surface of the fruit blackens and the underlying tissue is destroyed and turns black as well. When the fruit is opened the larvae can be seen; there can be up to 25 larvae in a fruit.