The Palm Weevil and the killer butterfly

Rhynchophorus ferrugineus and  Paysandisia archon in Portugal

In Portugal we have two insects killing palm trees, both the weevil : Rhynchophorus ferrugineus and the butterfly: Paysandisia archon. Understandably; as the weevil is an ally of the butterfly. The adult weevil is a sort of armoured tank whose larvae pierce the crown of the palm (or new shoots) the holes are then used by the butterfly who is less able to penetrate the base of the palm fronds. It is not uncommon to see palms attacked by several generations of insects and the larvae of both the weevil and the butterfly. Global research is ongoing to try and find a way to destroy the two killers.

A sad sight on the Algarve, a row of dead Phoenix canariensis.

As with the Mediterranean fruit fly, a lack of coordinated action at an overall level on the peninsula means that the recommendations of the Ministry of Agriculture may delay the end of the war, but will not win it.

We must learn to live with it. Prevention is essential, since the enemy is discreet and almost always deadly.

Preventive measures

The preventive methods essential to recommend are:

• Never cut, prune, or injure a palm tree during the period of adult insect activity (temp. +15°)
• Both insects are attracted by the smell of sap from the wounds. It is a key olfactory cue for them. So any wound not immediately cauterized is a sure signal to the enemy, "Come on over."
• Do not prune or trim except during the winter months: January to March (last August the Valencian Institute for Agricultural Research confirmed that Rhynchophorus ferrugineus and Paysandisia archon eggs don’t hatch below 15 °). Its the best advice to give to your  neighbours and for yourself.
• Carefully prune palms during a waning moon, in January and February, when vegetation has slowed.
• Take extra care with the Phoenix canariensis, (generally avoid planting these palms).
• Keep a look out. If you detect young palm leaves with an abnormal colour, defects or holes aligned in the palm leaf itself or holes at the base of the leaves or if there is a seeping liquid - immediate alert !

Paysandisia life cycle 

Ask for confirmation from a specialist if available (other larvae like the Cetonia aurataor or common rose chafer may be confused with the killers). Treat with a systemic insecticide (ex. water the crown with “Confidor” (every 30 days in the late summer ). You must act quickly, do not hesitate to check your adult palm trees with binoculars, watch out for sawdust at the foot of the tree. You must be sure to remove flowers and fruit before treating as systemic insecticides kill bees. (and bees are adore Phoenix flowers). Its one good reason not to generalize the use of systemic insecticides.

It is prudent however to use systemic insecticide (after removal of flowers and fruit) if there is a palm showing visible signs of attack in the neighborhood (the larvae can take up to two years to kill a palm tree, generations of larvae develop into adults 2 to 4 times per year).

The number of adult insects that will hatch and survive to attack other palms is between 25 and 100 per generation.

Nematodes are deemed to be an effective organic insecticide but they are very expensive and must be implemented by a qualified professional (which is rare), the November 2011 issue of “Phytoma” indicates that their effectiveness is decreasing while research on insect eating fungus seems promising.

Rhynchophorus life cycle 

Preventive organic treatments (nematodes, bacilius thuragiencis, Beauvais bassiana)  are not really economically viable, and the nematodes tend to pollute swimming pools. Eygpt and Saudi Arabia have published research on preventive or curative treatments with plant extracts. The latest publication concerns jojoba oil. They have tested many species of plant-based insecticides.
For the moment we haven’t found any plant based treatements that are sufficiently inexpensive as they have to be applied often and for the lifespan of the palm tree.
The least expensive is Biopalm (manufactured by a small french company, Provalis) is a latex spray that creates a protective film and only needs to be applied once a year. Though it has proven very effective against the butterfly its effectiveness has not yet been demonstrated against the weevil.

Traps don’t seem to work. The traps do not attract all of the insects all of the time so some will lay on the palm that they can easily identify visually. There are no known repellents that work.
In summary, we must add that most preventive techniques currently available require a  difficult implementation that in many cases is not respected.

Rhynchophorus ferrugineus - this is the female

Precautionary measures

According to Javier Segura (Palmasur) no palm trees are safe. However, there are palm trees that are at a higher risk from both killer insects:

• Phoenix canariensis : one of the most common palm trees in the Algarve (already considered dangerous because of  the spines at the base of the fronds)
• Phoenix dactylifera : the date palm
• Chamaerops humilis : the only native palm to Portugal, its young off-shoots are specifically targeted by the butterfly.
• Do not plant them. Diversify your palm trees, plant Syagrus romanzoffiana or the Livistonia, or the Cariot if you don’t have too much wind

We have helped to spread the killers by the globalization of the trade in palm trees but also by planting in excess Phoenix canariensis. The least we can do is stop attracting the enemy.

J. P. Brigand - 2012