Walnut twig beetle

Pityophthorus juglandis

1. Identification

• Common name: Walnut twig beetle
• Scientific name: Pityophthorus juglandis Blackman
• Order: Coleoptera
• Family: Curculionidae (subfamily Scolytinae)

2. Pest description

• Adults: Small bark beetles of 1.5–2 mm; cylindrical dark brown body; elytra with fine striations; intense activity during spring and summer.
• Larvae: Legless, whitish, curved body, brown head; develop in galleries beneath the bark.
• Pupae: Whitish, formed in pupal chambers inside the bark.
• Galleries: Complex system of subcortical galleries, frequently associated with the presence of pathogenic fungi.

3. Main hosts

• Walnut trees of the genus Juglans, especially J. nigra and J. regia.
• May also occur in hybrids and other species of Juglans.

4. Symptoms and damage

• Borings in the bark with expulsion of fine frass.
• Subcortical galleries that interrupt the flow of sap.
• Dark spots on the bark and exudation of sap.
• Progressive decline of the crown, with yellowing and premature drop of the leaves.
• Strong association with the fungus Geosmithia morbida, agent of the disease known as “Thousand cankers”, which causes multiple necroses in the cambium and may lead to the death of the tree.
• Significant reduction in the vitality and productivity of walnut trees.

5. Biological cycle

• Several generations per year, depending on temperature.
• Adults emerge in spring and begin excavating egg-laying galleries.
• Eggs laid beneath the bark; larvae excavate radial galleries.
• Pupation inside the bark; new adults emerge and may start new generations.
• Development favoured by hot summers, prolonged droughts and weakened trees.

6. Monitoring

• Observation of recent borings and expulsion of frass.
• Inspection of branches and trunks with dark spots or exudation.
• Assessment of galleries beneath the bark on suspect trees.
• Use of attractant traps for reinforced monitoring in orchards with a history of the pest or in regions with confirmed presence of the “Thousand cankers” disease.

7. Management measures

• Cultural: Removal and destruction of infested wood; avoiding transport of walnut firewood between regions; maintenance of tree vigour.
• Preventive: Avoiding wounds to the bark; reducing water stress; regular monitoring.
• Integrated protection: Intervention only when necessary; practices that reduce the availability of attractive wood; rigorous phytosanitary surveillance due to the risk associated with the fungus Geosmithia morbida.

Bibliographic references

• CABI Invasive Species Compendium – Pityophthorus juglandis.
• EPPO Global Database – Pityophthorus juglandis.
• Tisserat, N. et al. (2011). Thousand Cankers Disease of walnut.
• Seybold, S. J. et al. (2016). Biology and management of walnut twig beetle.