In May 2008, Manuel Rodrigues of the Parks and Landscaping division in Silves, spent half a day showing the author the damage caused in that concelho by the “Gorgulho Vermelho” or Red Palm weevil. This article is the result of research undertaken since that experience. Almost nothing has been done to warn the public of a serious threat and how to tackle it, past and present official policies must change radically and much needs to be learnt about controlling the insect sustainably.

Background.

From Porto on the northern coast to the Algarve in the south, palm trees, and particularly the Canary Island Date, Phœnix canariensis, a sturdy, showy plant which once established could almost be forgotten about, constitute an important part of Portugal's landscape.  (Figure 1)

Figure 1.  Canary Island Dates in Silves.

With the arrival of the Red Palm Weevil, Rhynchophorus ferrugineus, in the Algarve less than two years ago, that has begun to change.  In the first eight months of 2008 alone, over 10,000 palm trees have been destroyed because of the weevil just in neighbouring Andalucía. This should warn us about what Portugal can expect.  Since then the insect, which almost certainly entered the country in trees imported from Egypt or Spain early last year, has probably penetrated hundreds of palms in the Algarve and scores have already been killed. (Figure 2)


Figure 2. The weevil's legacy in a garden in Figueira.

Flourishing exotics.

The problem is still new to Portugal but it is just another stage in a process which began in the 1980s when the beetle, native to Southeast Asia and Melanesia, started to spread west and north. Moreover, this new arrival in the Algarve is only a small part of a much larger picture the naturalist E.O. Wilson recognised long ago as a major challenge of our age - the invasion of exotic species.  Portugal has certainly not been exempted and most of us would be astonished at how much foreign flora and fauna has become naturalised here.  On its journey, the weevil has attacked coconut palms in India and ravaged date groves across the Gulf and parts of North Africa before spreading to much of the remaining Mediterranean.

As well as the common Agave and sugarcane, over 30 palm species including both of Europe’s two native palms are known victims.  But the insect is particularly fond of date palms and, for reasons which remain a mystery, it is especially drawn to males of the Canary Island Date.  According to a knowledgeable Spanish source, about 80% of trees lost to the weevil there are Canary males.

Official incompetence.

This is bad news for Portugal.  Although it has no harvestable crop (apart from a little ‘honey’ tapping in its habitat), this handsome tree is the most visible and significant palm in the local landscape and it has been extensively planted, especially in the Algarve, in parks, hotels, villa developments and private gardens as a beckoning suggestion that the tropics aren’t so far away.  Much of Portugal will look very different without its most common palms and there is every indication we will lose these exotics as well as true dates and Washingtonias in addition to other more unusual members of the family if serious, national and community-wide measures are not taken.

            Brussels’ reaction has been too little, too late.  The Commission adopted “emergency measures” to keep the insect out 14 years after it first arrived. By that time the weevil was well established across most of southern Europe. The response of different countries ranges from the exemplary - Israel overcame the weevil threat with a no-nonsense strategy in a few years - to deplorable - Spain identified Egyptian date palm imports as the source of the weevil, closed its borders but then reopened them. At present, the Portuguese authorities appear, charitably, to be relaxed about the threat. The weevil’s arrival here was identified by Spanish investigators over a year before its presence was officially disclosed. Lisbon hasn’t managed to take a single concrete protective step since. Moreover, the focus in both countries remains, wrongly, one of eradicating victims rather than treating them. (This is beginning to change in Spain and parts of the Algarve.) Correspondence to the Ministry of Agriculture proposing measures and urging dissemination of information about the problem is routinely ignored. The national media have said almost nothing and repeated appeals to the serious dailies, weeklies and magazines are greeted with silence. Indeed, the few articles published in the foreign community’s mainly lightweight papers are more than all the Portuguese press together has mustered.

The insect.

Like every beetle, the Palm Weevil undergoes several remarkably different stages during its roughly 4-month-long life cycle which begins on breaking out of an egg similar to a small polished rice grain and culminates in an extraordinary metamorphosis from almost featureless grub into a complex and conspicuous adult.  (Figures 3 & 4)  Although the insect can be quite large with an adult body 3-4cm long and a larva of 5cm, it is rarely seen because almost all its life is spent hidden among frond bases or within the trunk itself.

Figures 3 & 4. Larva, chrysalis and adult (one cocooned) Palm Weevil stages.

On emerging from their fibrous cocoons - the second insect (head visible) in Figure 3 - adults usually remain on the same host.  But they can fly several kilometres to locate another suitable tree where males produce a highly specific scent (pheromone) to attract females as well as males.  If the palm is injured, because of burrowing insects or freshly cut fronds, for example, this attractiveness is enhanced by a kairomone, another smell emitted from the tree’s wounds.  After mating the female will lay 200 to over 300 eggs which hatch within a week and drill into the tree.  This is the stage when almost all the damage occurs.  The hatchlings, only a few millimetres in length at birth, enter the palm, feed on its internal tissues, grow dramatically and in the course of about 4-12 weeks turn into 5cm-long larvæ before emerging to pupate and repeat the cycle.  Every stage of the weevil can be present at the same time on the same tree.

Warning signs.

For all the damage already inflicted across much of the Old World by an almost invisible, seemingly unstoppable insect, several protective treatments exist and are definitely effective.  The first, essential, step is to identify signs of attack as soon as possible.  Holes the thickness of a finger, Figure 5, from which the grubs emerged can be found anywhere on the trunk, but usually near the crown, or in frond bases.   There is sometimes an unpleasant vomit-like odour of rotting vegetation and brown ooze from the wounds or trunk surface.  It should be noted that these injuries expose victims to potentially-lethal fungi.  But the most obvious signal is one or more mutilated leaves or even a broken frond emerging at the top and centre of a frequently asymmetrical or incomplete crown. (Figures 6 & 7)  

Gnawing rats can cause similar if tattier frond damage.  The contrastingly clean-cut weevil incisions may remind observant, well-travelled readers of Rhinoceros beetle damage to coconuts.  (With a little continuing help from the authorities and global warming, an issue with which they have been equally remiss, this is a new exotic we can look forward to.)  N.B.  In rare cases, larval damage of the Canary Date is confined to areas beneath &/or around the growing tip and no alerting leaf damage will appear.  But warning signs such as displayed in Figure 7 are usual although they may be more concealed within denser healthy fronds than illustrated here.  Check carefully.


Figure 7.  Weevil-damaged emerging fronds on a palm in Odelouca.

By the time these broken stems or incomplete leaflets which look as if they’ve been trimmed with scissors appear, internal cavities may well be extensive.  But a weevil-damaged tree is by no means a doomed tree.  As severely fire- or lightning- injured palms in the tropics attest, half a trunk may be lost and yet sufficient vascular bundles remain (since this circulatory system occupies the whole of the trunk, not just its exterior as with regular trees), to sustain healthy crowns.  Thus the only standing survivors of burned tropical forest are usually palm trees because they can incur considerable damage and still survive. If one is vigilant and acts quickly, major losses will be avoided and in warm weather recuperation can be quick.  It should not be assumed, however, that because the weevil remains undetected in an area or even because a garden is isolated that trees are safe.  Monitor them!

Control.

What should be done as soon as damage is detected?  Currently, two types of insecticide must be applied in different ways to affected trees.  Contact insecticides (such as Diazinon) are sprayed against insects, eggs and pupæ that the chemical reaches on the surface.  And, most important, systemic insecticides (like Imidacloprid or Tiametoxan) which plants absorb and incorporate in their tissues (or system) to kill whatever eats them are injected into the trunk to tackle concealed larvæ. This involves drilling the trunk and inserting a tube with a small bladder attached that squeezes insecticide into the tree.  Four such injections are normal with a Canary Date. (Figures 8-11)

Figures 8-11. Drilling into the trunk; full injection with and without protective plastic envelope; depleted injections in Royal Palm trunk.

Small or contained palms can incorporate sufficient systemic agent through their roots and may simply be watered with the product.  But adequate uptake is unlikely with the larger specimens preferred by the weevil.  And because of the volumes required to treat mature trees and the risk of groundwater contamination systemic irrigation is limited in Europe to two applications annually.  Injections, therefore, are a far more efficient and environmentally sound method.  Nevertheless, not all larvæ may be killed with the first injections.  And the chemicals, like antibiotics in our bodies, are gradually metabolised.  So for maximum security the treatment should be repeated at least a second time 2-3 months later.

Especially for big trees, these procedures are onerous and can be expensive - in areas of extreme infestation eleven sprays and injections are recommended yearly costing up to 450€ for a single tree.  But in most cases larvæ can be eliminated with one or certainly two injection/spray treatments.  These will cost about 75€ for a single tree’s dual treatment or ~65€ each for several palms.  To the person who buys a few plants and some fertiliser occasionally, these figures may appear considerable.  But to the home-owner who appreciates the landscaping merits of a large specimen, or merely recognises its contribution to a property’s value, they will be worth it.  And to the hotelier, developer or local authority aware of what it will cost to remove, destroy and then replace large palms, the response should be obvious: save your investment.  (Figure 12.)


Figure 12. Injections being administered at a hotel's gardens.  (Tree on far right already treated.)