Damage characterization of Chrysobothris mali Horn (Coleoptera: Buprestidae) in California walnuts

The Pacific flatheaded borer, Chrysobothris mali (Coleoptera: Buprestidae) has recently emerged as a notable pest of walnut orchards in California. Understanding the pest’s biology and infestation behavior is crucial in developing successful Integrated Pest Management (IPM) practices. We conducted a series of studies to characterize the damage caused by C. mali in walnut orchards. We collected flatheaded borer-infested branches in-season and during the winter and recorded important parameters, such as branch diameter, wound length, feeding channel length, and the number and position of different stages of its life cycle in the branch throughout the year. Branches were collected in February to represent the overwintering period, and from May through November to represent the seasonal period. Inactive larval stages were found predominantly during the overwintering period in February. For the seasonal period, predominantly pre-pupal larvae, followed by the pupae, were found in the earlier part of the season (May to mid-June), while predominantly adults were recovered during the peak summer (late May to late July). Actively feeding larvae were observed at the beginning of late August. Up until mid-September, 72% of the larvae were found primarily feeding on the cambium layer. However, since late September, most larvae were found deep in the pith. To better understand the damage caused by C. mali, the distribution and direction of damage within the tree were evaluated. The highest number of damages was observed on the southeastern side of the tree, and branches with a 1.5–2.5 cm diameter had the highest number of damage symptoms.