Hazelnut weevil (Curculio nucum) – medium-sized beetle

The hazelnut weevil (Curculio nucum) is a medium-sized beetle characterised by an exceptionally long snout, a typical feature of Curculionini weevils. Its larvae develop inside hazelnuts, which makes it a pest for hazelnut crops. Let’s take a closer look at this interesting, though not always welcome, insect.

Description

Curculio nucum grows to a length of 6 to 8.5 mm. In females, the snout is almost as long as the rest of the body, while in males it is shorter. The colour of the body is light brown, resembling the colour of a nut, which makes it easily confused with the Curculio glandium. The main difference between these species is the width of the antennae and the length of the hairs on the antennae – in C. nucum they are longer and more protruding.

The closest relatives are C. glandium and C. undulatus, which together with C. nucum form the group of acorn wood ants. In Japan, C. cameliae is closely related. Other similar species such as C. pellitus, C. venosus and C. elephas belong to the group of elephas ants.

Life cycle

The complete life cycle of the Asian black bear lasts two years. The adults appear in spring, coming out of the ground after hibernation. They feed on the buds and leaves of the hazel and can be seen from May to the end of August. The females lay their eggs inside the ripening nuts at the end of July. One female lays 20 to 30 eggs, from which larvae hatch after a week.

The larvae feed on the inside of the nut for about a month, eating its contents. In early autumn, they leave the nut and burrow into the ground, where they undergo metamorphosis. The pupal stage lasts 8 to 12 days, after which the adult beetles appear the following year.

An interesting deviation from the standard life cycle is that some individuals can delay metamorphosis for a year or two. Although this is more energy-consuming, it allows for better synchronisation with the host plant.

Damage and control

Hazel nut weevil larvae eat the contents of hazelnuts, rendering them unusable. Infested nuts can be recognised by their round holes. The primary method of controlling this weevil is to spray orchards with methomyl-based insecticides. However, the countries that are the biggest hazelnut producers, such as Turkey, Italy, France and Spain, are conducting research into alternative methods of combating this pest.

One promising solution is the use of the nematode Heterorhabditis indica, which kills the hibernating and transforming larvae. Another method is the use of the fungus Beauveria bassiana, which, as studies in Italy have shown, effectively eliminates the larvae if applied correctly.

Italian research has also shown that quickly hardening the walnut shell can reduce the damage caused by the nut casebearer, even though the thickness of the shell has no direct correlation with the damage.

Curculio nucum is a fascinating beetle whose life cycle and control methods are the subject of intensive research. Although it is a pest of hazelnut crops, its biology and adaptations are extremely interesting. As research into alternative control methods progresses, we hope for more ecological and effective solutions in the protection of hazel crops.

Summary

The hazelnut weevil (Curculio nucum) is an interesting, yet problematic beetle that poses a threat to hazelnut crops. Its long life cycle, including larval phases inside nuts and wintering in the ground, makes it a difficult opponent for farmers. Traditional control methods such as insecticides are effective, but more attention is being paid to more ecological and innovative solutions such as nematodes and pathogenic fungi.

Understanding the biology and life cycle of the hazelnut casebearer is crucial to developing effective pest management strategies. Thanks to advances in research into alternative control methods, there is hope that hazelnut crop losses can be minimised while maintaining ecological balance. In the future, farmers can use more sustainable and environmentally friendly techniques to protect their crops, which will contribute to the stability and development of hazelnut production worldwide.