Walnuts (Juglans regia) infected by walnut husk fly. When the walnut husk fly (Rhagoletis completa) lay their eggs in the pericarp of the walnut the larvae allow fungi and bacteria pathogens into the fruit. If they reach the edible kernel then it shrinks and rots, but even if they only reach the husk it will turn black and stick to the shell. This is a serious pest problem for commercial growers. The fly originated in North America and has now spread to Europe as an alien species. Normally, as the walnut ripens the drupe splits and the nut can be easily removed. (The green walnut drupe has not yet split.) It is then washed, sorted and dried, either in the air or by special machines to reduce the humidity level.