Rhododendron borer

The rhododendron borer (Synanthedon rhododendri) belongs to the clearwing moth family (Sesiidae) in the order Lepidoptera. This insect is a significant pest for landscape plantings, as the adults resemble wasps, while the larvae operate as hidden wood-borers inside the host plant.

The primary hosts for this borer are plants of the genus Rhododendron. The larvae bore into the stems, branches, and occasionally the trunk of the shrub, disrupting the vascular system and depriving the plant of essential water and nutrients.

The life cycle of the pest involves the adult moths emerging in late spring or summer. After mating, females lay eggs on the bark. Upon hatching, the larvae burrow into the plant tissue, where they spend one or more years feeding and developing before pupating and emerging as adults.

The damage caused by the rhododendron borer includes several distinct symptoms:

• Wilting of individual branches during the growing season.
• Entry and exit holes visible on the bark surface.
• Accumulation of frass (larval excrement) resembling sawdust at the base of the plant.
• Stunted growth and eventual dieback of infested stems.

Managing this pest requires a combination of cultural and chemical strategies. Cultural control involves pruning and destroying infested wood to eliminate larvae before they emerge. Chemical control focuses on applying residual insecticides to the trunks and limbs during the adult flight period to prevent new infestations from becoming established.