Giant palm weevils—Rhynchophorus spp.
Various palm species and especially Canary Island date palm are fed upon by several Rhynchophorus species weevils. The South American palm weevil, R. palmarum, is well established in Mexico and has spread to at least neighboring San Diego County where it is killing Canary Island date palms. The palm weevil, R. vulneratus, was found in limited portions of Orange County and apparently eradicated.
Other exotic species of concern include the palmetto weevil, Rhynchophorus cruentatus, which is native to Florida and the Caribbean, and red palm weevil, R. ferrugineus, which is native to Southeast Asia. Whether any Rhynchophorus species in addition to South American palm weevil will become established in California is unknown.
Identification
Adults are large snout beetles (family Curculionidae) about 1-1/2 inches long. Their color is variable and includes black overall, black with a red stripe, orange with black spots, or dark with colored blotches on the pronotum (topside behind the head).
Larvae grow up to 2 inches long and are legless, plump, and yellowish to white with a brown head. Mature larvae (prepupae) and pupae occur in a chamber of coarse palm fibers in tissue where they fed as larvae.
Unless the trunk and especially the growing point at the top of the trunk are regularly examined (e.g., from a lift-bucket truck) infestations can be easily overlooked until palms are irrecoverably damaged. Periodically inspect trunks and use binoculars to observe fronds and detect any chewing damage that can indicate infestation by giant palm weevil. If dieback of the apical (center, newest, uppermost) leaves is observed, carefully inspect their base for larval mines and frass (excrement). When trimming palms, direct workers to look for weevil life stages and signs of their presence.
Indications of giant palm weevil infestation include
- a fermented odor from the upper trunk
- empty pupal cases or dead weevil adults around the base of palms
- gnawing sounds from trunks, caused by the large larvae chewing inside
- truncated or cut-off leaf tips, which may be visible from the ground using binoculars
- tunnel openings on the trunk or base of fronds that exude chewed plant material or viscous liquid
- weevil adults, which can be caught in special traps
Be sure to distinguish introduced Rhynchophorus weevils from the native giant palm borer, Dinapate wrighti, Bostrichidae; their adults and larvae can be the same color and length. Giant palm borer is a secondary pest that infests palms after they become severely stressed or are already dying. Adults of giant palm borer are cylindrical with a bulbous head that lacks the elongated mouthparts of weevils. Unlike weevil larvae, larvae of giant palm borer have prominent legs.
Life cycle
Weevils develop through four life stages: egg, larva, pupa, and adult.
Adults live about 2 to 3 months feeding on palms and going through several cycles of mating and egg laying. Adult females use their long mouthparts to chew a hole into palm tissue and lay eggs into the hole. Egg laying commonly is done in cracks and crevices and wounds in the trunk, from the collar region near the roots up to the base of frond petioles and the leaf crown. Each female lays several dozen to a few hundred eggs, which hatch in about 1 to 6 days.
Emerging larvae chew and feed on palm tissue and bore into the center of the palm. Their tunnels fill with frass (excrement) and plant sap. Larvae develop through 3 to 7 increasingly larger instars for about 2 months. Mature larvae pupate in cocoons of palm fibers in the trunk or base of palm fronds. The pupal stage lasts from about 11 to 45 days.
One generation from egg to adult takes 45 to 140 days, the shorter period when temperatures are warmer. Palm weevils have several generations per year.
Damage
Adults chew and feed on fronds, giving them a ragged appearance that reduces the aesthetic value of palms. Chewing damage weakens plants if feeding is extensive. The most serious damage is from larval feeding. Larvae chew and feed in the apical growing point of Canary Island date palm and in the trunk and root collar (basal trunk) of various date palms. Larvae damage or kill fronds by chewing in tissue where fronds attach to the trunk. This causes a sparse canopy of few fronds, seriously weakens the trunk structure, and can kill date palms and cause collapse of the crown. Heavily infested date palms may break at the trunk. When Canary Island date palm is heavily fed upon, the entire leaf crown may drop.
Solutions
Do not bring palms or other plants into California unless you know they were inspected by agricultural officials or certified to be pest free. Report any suspected findings of giant palm weevils to the local office of the county agricultural commissioner.
Use good sanitation to prevent palm weevils from spreading from infested palms. Where palms are found to be infested with giant palm weevils, remove the plant and burn, chip, or deeply bury the infested material. Plant resistant species, such as the native California fan palm (Washingtonia filifera), European fan palm (Chamaerops humilis), or non-palm species.
For more information see The Biology and Management of Landscape Palms, The South American Palm Weevil Invasion in San Diego County, California, and The South American Palm Weevil A New Threat to Palms in California and the Southwest (PDF).
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
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Adult chewing damage to palm fronds.
Adult red palm weevil.
Palm weevil larva exposed.
Adult, larva (bottom), and pupa (right) of red palm weevil.
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