Giant palm borer—Dinapate wrighti
Dead, dying, and severely stressed California fan palm (Washingtonia filifera) and date palm (Phoenix dactylifera) can become infested with giant palm borer. This beetle is a secondary pest and not the original cause of unhealthy palms.
Adults are blackish to dark reddish-brown beetles (family Bostrichidae) up to 2 inches long. Like other bostrichids the head points sharply downward and the mouthparts are not visible from above. From above the head appears bulbous.
Eggs are oval, white to yellowish, and about 1/16 inch long. They occur in a chamber in wood chewed by the female, commonly just below the leaf crown.
Larvae are stout, yellowish, and occur hidden in trunks. They can grow up to 2-1/2 inches long and have a C-shaped posture when disturbed or exposed. Pupae occur in a chamber in wood within about 1 inch of the surface of trunks.
The adult female chews an entrance tunnel and excavates a chamber beneath the leaf crown of hosts. Males are attracted to enter the tunnel and mate. The adult female then lays up to about 500 eggs. The emerging larvae each bore a tunnel and feed for several years on the inner stem of host palms. The larval stage lasts from about 3 to 9 years if infested plants survive that long.
Mature larvae pupate just under the surface of the trunk in about April and May. Adults emerge about two months later, leaving holes in trunks about 1/2 inch in diameter. Adults fly mostly during summer and infest other hosts or reinfest the palm where they fed as larvae.
At least several years are required to complete one generation. Generations overlap, and all life stages can be found in a host during much of the year.
Giant palm borer can infest California fan palm and date palm. Adults and larvae chew, feed, and tunnel in the main stem (trunk) and base of the leaf crown. At the base of the leaf crown females excavate a chamber that becomes partly filled with soft, thick, moist material composed of chewed fibers, excrement, and sap. Larvae form tunnels in wood that can occur up and down the length of trunks. Emerging adults leave holes in trunks.
Fronds of infested palms turn yellow and the plants commonly die prematurely. Weakened palms may fall over in the wind.
Giant palm borer is a secondary pest. It attacks hosts that are dead, dying, or severely stressed, such as from transplanting.
Provide palms with good growing conditions and proper cultural care to keep them healthy. Dispose of dead and dying palms, in which the beetles breed. No control other than good preventive care of palms is known.
For more information see Observations on the Biology of the Giant Palm-Boring Beetle, Dinapate wrighti Horn (Coleoptera: Bostrichidae) and The Biology and Management of Landscape Palms.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).