Japanese weevil

Systematic position. The Japanese weevil (Phyllobius intrusus) is a member of the Coleoptera order and the Curculionidae family. It is recognized as a significant invasive pest that, while native to Japan, has established itself in various global regions, posing a persistent threat to ornamental nursery stock and landscape plantations.

Host plants. This species is primarily associated with coniferous trees. Major host species include various varieties of arborvitae (Thuja), false cypress (Chamaecyparis), and juniper (Juniperus). The pest's ability to attack different parts of the plant throughout its life cycle makes it particularly difficult to manage in dense commercial plantings.

Biology and life cycle. The life cycle of Phyllobius intrusus is univoltine, meaning it completes one generation per year. Adult weevils emerge in late spring or early summer to feed on foliage. Following mating, females lay eggs in the soil around the base of host trees. The larvae develop in the root zone, feeding on fine roots throughout the summer and autumn, and overwinter in the soil as larvae before pupating in the spring.

Damage and economic significance. Foliage feeding by adults results in notched leaves and thinning of the canopy, which is mainly a cosmetic issue. In contrast, larval feeding on roots causes severe physiological stress to the plant. Damaged roots lead to nutrient and water deficiencies, resulting in yellowing, stunted growth, and potential death of the trees, especially in young saplings or weakened nursery stock.

Control measures. Integrated pest management (IPM) is essential to combat the Japanese weevil.

• Apply soil-drench insecticides to target larvae in the rhizosphere.
• Foliar applications of registered insecticides during the adult emergence period to prevent egg-laying.
• Regular inspection of root systems when moving or transplanting stock to prevent spreading.
• Maintaining healthy soil conditions and plant vigor to enhance the trees' natural resilience against larval damage.