common name: palmetto weevil
scientific name: Rhynchophorus cruentatus Fabricius (Insecta: Coleoptera: Curculionidae)

Introduction - Distribution - Description - Life Cycle - Host Range - Damage - Associations with Metamasius - Chemical Ecology - Management - Selected References


Introduction

Weevils are a type of beetle that have their mandibles at the end of a sometimes very long rostrum (a snoutlike projection of the head). In fact, the rostrum of some weevils (i.e., nut weevils) is as long as their bodies. These modified mouthparts are used for feeding and to prepare holes in plant material in which eggs are laid. Weevils are a large, diverse and important group of insects. Most feed on plant material, and many are considered to be economic pests. While adults feed outside the plant, the larvae (or grubs), which are legless, feed within the host plant.

weevil rostrum

head view

lateral view

The largest weevil in North America is the palmetto weevil, Rhynchophorus cruentatus Fabricius. The palmetto weevil is native to Florida and is the only species of palm weevil in the continental United States. Worldwide, there are ten described species of palm weevils. Until recently, the palmetto weevil was considered a minor pest, attacking only severely wounded and dying trees. However, it is starting to gain status as a pest of stressed nursery and transplanted palms.

Distribution

Worldwide, there are ten described species of weevils in the genus Rhynchophorus that feed on palms. The palmetto weevil has been reported from coastal South Carolina south through the Florida Keys, and west into coastal Texas. Fossil records suggest that the palmetto weevil was present in Florida during the Pleistocene (about 1 million years ago).

Description

Adults of the palmetto weevil vary in color from solid black to almost completely red with a variable black pattern. Their total length from the tip of the rostrum to end of the pygidium (the tergum of the last visible segment of the abdomen) ranges from 1.9 to 3.0 cm. Males and females can be distinguished by the surface of the rostrum. The rostrum of males are covered with tiny bumps while females have a smooth, shiny rostrum.

relative size of adults

color variation

male and female snouts

The larvae, or grubs, are legless and creamy to yellowish in color. Their prominent head is dark brown and very hard. Mature larvae can be quite large, some with a mass close to six grams. Although we are aware of no human consumption of palmetto weevil in the United States, larvae of palm weevils are considered a delicacy in other locations.

grub in palm leaf base

grubs

Life Cycle

The palmetto weevil has a complete life cycle: with an egg, several larval instars, prepupal, pupal, and adult stages. Eggs are laid in the bases of leaves or in wounds in a dying host palm. In the laboratory, a palmetto weevil female will lay an average of 207 eggs in her lifetime. Eggs hatch in about three days and begin to feed on palm tissue. As they molt (grow) the larvae have an increasingly large appetite and tend to feed primarily in the soft tissue surrounding the apical meristem. Mature grubs migrate to the periphery of the stem or petioles and prepare a cocoon from palm fibers. After surrounding themselves with the cocoon, the larvae enter a prepupal stage, then a pupal stage. After a few weeks, an adult emerges from the pupal case and may immediately break free of the cocoon or wait within the cocoon for several days before emerging. The entire life cycle, from egg to adult, takes about 84 days. Adults may live for several weeks (up to 26 weeks in captivity).

grubs feeding

cocoons

pupal case

Palmetto weevil adults are active fliers and can be found throughout the year in Florida. However, adult activity is usually more noticeable in the late spring and early summer months. When not flying in search of a host palm, adults hide between the leaf bases and stems of healthy palms presumably to conserve water within their bodies.

Host Range

The palmetto weevil is closely associated with the cabbage palmetto (Sabal palmetto (Walker) Loddiges ex J.A. et Schultes), a palm native to the southeastern U.S. In addition, the native saw palmetto (Serrenoa repens (Bart.) Small) appears to be an acceptable alternate host. Several other palms, most of them introduced species, have been observed with larval infestations. These include the Canary Island date palm (Phoenix canariensis Hortorum ex Chabaud), P. dactylifera, Pritchardia sp., Washingtonia sp., royal palms (Roystonea sp.), Latania sp., coconut palm (Cocos nucifera Linnaeus), and Caryota sp.

In undisturbed locations, palms are rarely observed with palmetto weevil infestations. Trees stuck by lightening have been observed with subsequent weevil infestations. The palmetto weevil has, in recent years, caused considerable damage to some newly transplanted or otherwise stressed palms.

Damage

The symptoms of a palmetto infestation vary, but commonly involve a general, often irreversible decline of younger leaves. In palm species with upright leaves, such as the Canary Island date palm, the older leaves begin to droop during the early stages of infestation but quickly collapse thereafter. As the infestation progresses, the larval feeding damage and associated rot is so severe that the integrity of the crown is compromised and the top of the palm falls over. This condition is termed "popped neck". If the palm is pulled apart at this stage, larvae, cocoons, and even adults may be found within the crown region. Early detection of weevil infestation is difficult, and treatment even in the early stages of infestation may be too late to save the tree. This area needs more study, however.

damage to Canary Island date palm

damage to Sabal palm

Associations with Metamasius

In the early 1980's, the silky cane weevil (Metamasius hemipterus sericeus (Oliver)) was accidentally introduced into Dade County, Florida. This insect is an important pest of sugarcane and other plants, including palms, in the Neotropics. Adults are attracted to and lay eggs in palm sheaths, petioles, or stems. Within the host, larvae develop into adults in less than two months.

adult silky cane weevil

Silky cane weevil adults appear to be attracted to palms by odors emanating from small wounds, such as those created by pruning leaves. While initial infestations of palms by the silky cane weevil is not usually lethal, we believe that the stress created by the infestation makes these palms susceptible to successful attack by the palmetto weevil. This association also needs further study.

Chemical Ecology

Volatile odors emanating from dying palms are attractive to palmetto weevil adults. The exact number and ratios of volatile compounds released from wounded trees are unknown but several compounds, known collectively as "palm esters" have been found to be attractive. In addition, an aggregation pheromone (5-methyl-4-octanol, or "cruentol") produced and released by male weevils attracts other male and female palmetto weevils. Neither the "palm esters" or cruentol are very attractive by themselves. But when put together, they are synergistic, attracting many adult palmetto weevils.

As the story may go, a palmetto weevil adult male flying amongst many palms locates an attractive odor… that given off by a dying palm. He flies upwind towards the odor source and eventually lands on the potential host palm. As he begins to feed he releases the aggregation pheromone which is attractive to other weevils at long distances. Other weevils fly towards the pheromone source, and as they get closer, the pheromone/"palm esters" mixture takes over to guide them to the host. As more males land on the host, they also release pheromone attracting even more males and females. Once a population has gathered on the tree, mating and egg laying take place.

Management

Insecticidal treatment of trees infested with the palmetto weevil is futile. The best recourse is to cut down infested palms and destroy them before adults emerge from the tree. Prophylactic treatment of recently transplanted palms with insecticides is an option but the costs can quickly become prohibitive unless only a few trees are to be protected.

Growers managing nursery plantings of palms may have the greatest potential to control the palmetto weevil by an integrated program. First, trees should be grown using cultural practices that promote vigor. This means following proper fertilization and irrigation guidelines. Trees such as the Canary Island date palm are not adapted for the south Florida climate. Great care should be taken to ensure the health of these trees. Secondly, wounding of trees, such as by pruning, should be avoided. Following these two steps will help to prevent an infestation. If trees are infested with palmetto weevils, there is little if any chance of saving them. Therefore, sanitation, as in removing and destroying infested plant material is crucial in preventing or reducing subsequent infestations.

Finally, mass trapping of palmetto weevils in buckets baited with cruentol and "palm esters" may have great potential in reducing infestations. Mass trapping, combined with sanitation has been shown to reduce American palm weevil populations in Central America. We are currently evaluating mass trapping as a tool for management of the palmetto weevil.

bucket trap

Selected References

Authors: Thomas J. Weissling and Robin M. Giblin-Davis, University of Florida
Photographs: Robin M. Giblin-Davis; and Michael Thomas, Division of Plant Industry
Drawings: Divison of Plant Industry
Project Coordinator: Thomas R. Fasulo, University of Florida
Publication Number: EENY-13
Publication Date: August 1997. Latest revision: January 2007.
Copyright 1997-2007 University of Florida

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Department of Entomology and Nematology
Division of Plant Industry
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