Identification & Control of the Red Palm Weevil

The Red Palm Weevil Rhynchophorus ferrugineus has become a serious pest of palm trees in Iberia.  Originally from tropical Asia, this beetle reached Spain in the mid-1990s.   Its effect on palms in areas it has invaded has been devastating.  Although originally localised in Spain, there has been a population explosion during the past few years.  This has led to the loss of thousands of palms, at great expense.

Red Palm Weevils are large insects.  Adults may reach a length of 5cm.  As with most weevils, a prominent rostrum is present, with the mouthparts located at the end of this.  This rostrum makes the Red Palm Weevil appear to have a large snout.  Given its size, rostrum and bright red colouration with variable black markings, the Red Palm Weevil is unmistakeable.  Adults have diurnal habits, making their detection easier.  Larvae live within palms and can reach a large size.  The grubs can be separated from larvae of other beetle species that live in palms such as chafers (most notably the black Potosia opaca) because the larvae of the Red Palm Weevil lack legs, whereas the larvae of chafers have six small legs.  Unlike the Red Palm Weevil, these chafers feed on rotting material and will not damage healthy palms. 

The Red Palm Weevil is an unmistakable insect (K. Bensusan)

Red Palm Weevils can be present on a palm all year round.  As long as sufficient live plant material is present on a palm, the weevils will continue to feed on a plant and lay their eggs on it, so that infestations within single palms can continue to grow for several generations until they reach hundreds of beetles.  Once palms can no longer offer enough food, adults will disperse to find new plants to infect.  Gravid females will prefer to lay their eggs on palms that are diseased or in a bad condition.  Although Red Palm Weevils are active all year round, adults are most active and numerous during the summer months.  They have a considerable ability to disperse and are capable of flying in excess of three kilometres a day.

Which palms are affected?

A range of palm species have been reported as affected by the Red Palm Weevil.  In Spain, this beetle is most commonly reported infesting Phoenix species, with records of weevils on Washingtonia being less regular.  Infestations are common in the Canary Palm Phoenix canariensis and the Date Palm Phoenix dactylifera, but the Canary Palm seems to be the most seriously affected species.  Mature palms are most susceptible, making the damage incurred more costly.  The Red Palm Weevil causes a high incidence of mortality in all species that it infests and has caused the death of thousands of palms in Andalucia over the last few years.  Common targets are palms that have been pruned recently (especially if the pruning has not been carried out to a high standard and has left scars), or which have been weakened due to transplantation.  Since adults are most active during the summer months, this period presents the highest probability of new infections.

Can infections be prevented?

Recently pruned, damaged and recently transplanted palms are most susceptible to attacks by adult weevils.  Pruning and transplantation of palms should be avoided during the hotter months of the year (April to October), as this increases the probability of infection due to the increased activity of adult weevils.  Treatment of palms can begin before infection has been proven if there are infected palms in the vicinity.  Treatment methods for plants without symptoms are outlined in the Table.

How can infected palms be identified?

Diagnosing infections with Red Palm Weevils is not easy at first.  During the initial stages of infection, the weevil occurs at low densities and causes limited damage, which may in addition be masked by the state of the injured or sickly palms that they prefer to attack.  However, symptoms become obvious once palms become infested.  The time taken before symptoms become apparent, as well as the symptoms themselves, depend on factors such as the point of entry of weevils, level of infestation and health of the palm in question, with unhealthy palms showing symptoms more quickly.

The greatest damage is caused when weevils enter palms via the crown.  In such cases, weevils will damage growth of new fronds and ultimately kill the palm.  Initial symptoms in such cases are new leaves in the centre of the crown that show signs of having been eaten, or that are leaning or appear weak.  These new fronds that have been attacked can be pulled off easily.  At a more advanced stage, the centre of the crown will collapse entirely.  If palms are attacked via scars and damaged tissue away from the crown, damage is normally less severe.  Symptoms in such cases will include mature fronds losing their normal orientation, giving the palms a disordered and asymmetrical look.  When infected fronds are pulled off, tunnels and emergence holes produced by weevils will be visible, along with detritus produced by feeding larvae.  Other symptoms include (i) a smell of rotting and fermenting plant material emitting from affected parts of the trees, and (ii) the large exit holes of adult weevils.  The larvae and pupal cases of these beetles can also be found in affected plants.

Infestations of the Red Palm Weevil may cause the crown of palms, such

  as that of this Canary Palm, to collapse entirely (courtesy Alex Torres Ruíz).

Larva (above) and pupal case (below) of the

Red Palm Weevil (courtesy Alex Torres Ruíz).

What can be done about infected palms?

Reaching a decision on whether to treat or remove a palm will depend on a number of factors, such as (i) age of the palm in question, (ii) monetary value of the palm, (iii) the palm’s importance to the landscape, and (iv) the probability of infection of nearby palms.  Decisions should be made quickly to ensure effective containment of infestations and palms that are not treated should be removed.  Use of the insecticide ‘Imidacloprid’ is recommended from June to September, when the weevils are most active and abundant.  The use of entomopathogenic nematode worms is recommended all year round.  A range of species of nematode have been used successfully against the Red Palm Weevil, such as Heterorhabditis indica, Heterorhabditis bacteriophora, Steinernema carpocapsae and Steinernema abbasi.  Guidelines for treatment of palms with different grades of infection are given in the Table. Controlling an existing infestation of Red Palm Weevils on a single palm is estimated at €250-300 Euros per annum in Spain (although treatment of multiple specimens reduces costs). 

Populations of Red Palm Weevils can also be controlled with pheromone traps.  These traps should be placed in areas where weevils are known to occur, but away from palms.  They are highly effective when the density of weevils is low and when used in conjunction with pesticides and nematodes.  They will not, however, eradicate the problem on their own.  When densities are higher, they will still cause reductions in weevil populations and lower infestations on palms.  Baiting traps with attractants such as dates or sugarcane will result in a significantly higher incidence of capture.  The pheromone employed is an aggregation pheromone produced by males that will attract both sexes, although a higher ratio of females is normally captured than males.  Insecticide should be placed in these traps to kill trapped weevils.  In more heavily infested parts of Spain, single pheromone traps may remove thousands of beetles.  Studies have shown that traps that are placed on the ground are more successful than those that are suspended, and that red or camouflaged traps attract more beetles than white ones.  Experts advise not to use pheromone traps in an area unless the presence of the Red Palm Weevil has been proven, since they attract weevils from a considerable distance and could result in the infection of sites that are still weevil-free.

                Recommended treatment for the Red Palm Weevil, adapted from advice given by the Consejería de

                Agricultura y Pesca of the Junta de Andalucía.

How should infected palms be disposed of ?

Remains of infested palms should be buried immediately after removal. Infected plant material should be covered in insecticide and placed in polythene bags.  The pesticide ‘Clorpirifos’ could be used for this purpose.  Care should be taken when applying the pesticide.  Properly trained personnel should carry this out, wearing protective gear.  Burning of remains is not effective due to the high level of moisture present in rotting palms.  Disposal should take place close to the area of infection, as transportation of infected plant material into as-yet uninfected areas may result in inadvertent dispersal of adult weevils emerging from the felled palm.

Any further inquiries should be made to weevil@gibraltargardens.gi .