If the husk of your walnuts is turning black and slimy and the nuts are not ripening properly, your walnut tree may be infested with the walnut fruit fly. An annoying matter, but one that can be brought under control with consistent measures.
Any affected walnuts should be collected and discarded
Portrait of the walnut fruit fly
The walnut fruit fly belongs to the bore flies and, with its patterned coloring, looks very similar to the cherry fruit fly, also a pest. The adult animals are about the same size as ordinary house flies and also have a very similar physique. The walnut fruit fly has been introduced from the USA, where it poses a major problem for walnut cultivation, which is a major segment of the export trade there.
The walnut fruit fly has a one-year cycle: it overwinters in the soil as a pupated larva and hatches in July of the next year. The flight and mating season is between the end of July and September. The mated females lay about 15 eggs per walnut. There the maggots hatch and feed on the green pericarp for about 3-5 weeks until they pupate.
The characteristics of the walnut fruit fly in brief:
- Belongs to the bore flies
- Size of a housefly, mottled coloring like the cherry fruit fly
- Introduced to us in Europe from the USA
- Flight and mating season July-September
- about 15 eggs (and maggots) per fruit
- Feeding of maggots on green pericarp
signs of infestation
You can recognize an infestation by the blackened, greasy shells of your walnuts. A round, black dot can already be seen at the egg-laying site. The eaten fruit husk is typically difficult to separate from the nut shell, even if there is little left of it.
The nut itself is only indirectly affected because the reduced husk mass has not properly nourished it. The nut kernels are only smaller. Due to the black remains of the fruit husk sticking firmly to the shell, the nut no longer looks attractive and can hardly be sold without extensive cleaning with a high-pressure cleaner (€119.90) and subsequent drying.
countermeasures
The following measures are advisable to limit the annual recurring infestation: If maggot infestation has already occurred, you should harvest as many affected fruits as possible from the tree and pick them up from the ground. Next year, it is advisable to lay a soil fleece under the tree from June. This will prevent the pupated maggots from hatching and getting out of the ground. They may even be killed by overheating.