Is there a way to save the palm trees on the French Riviera? Notes on Rhynchophorus and Paysandisia.
FFor years there have been reports about the two main pests damaging Mediterranean palm trees, the so-called palm moth (Paysandisia archon) and the so-called red palm weevil (Rhynchophorus ferrugineus). Both insects have been featured by various publications, as „immigrated“, i.e. non-endemic, pests: the palm moth originates from Uruguay and Argentina, while the red palm weevil calls tropical regions of Asia his original home.
Unfortunately it is climate change that not only allows us to plant exotic species on the Mediterranean Sea, but that also helps plant pests to spread that were unknown so far.
It is quite urgently necessary to act both, systematically and persistently, against these palm pests, because not only is it highly expensive to dispose of a palm’s old, dead trunk. In some regions on the French Riviera complete, once magnificent, avenues have already been destroyed: whoever is looking for an exceptionally devastating example will make a sad find in Hyères.
A big problem with fighting these two pests consists in the high number of second homes on the Côte d’Azur, where many gardens lie unobserved during autumn and winter, when the larvae of both insects have an exceptionally easy run on their victims.
For simplicity reasons we will confine ourselves to the Latin names hereafter, in order to help with the exact identification among the different languages spoken on the Riviera.
Which palm trees are attacked by the palm moth (paysandisia)?
Chamaerops (all species),
Washingtonia (all species and cultivars, including filifera, slightly less often though).
Which palm trees are attacked by the red palm weevil?
Washingtonia (all species and cultivars, including filifera).
The species that are predominately planted on the French Riviera, Chamaerops, Phoenix, Livistona, Trachycarpus and Washingtonia are attacked by at least one of the two pests. Since for a number of reasons we may not hope that these pests neither will nor can be contained on the Mediterranean, at least not on short notice, we must raise the question of which species should be chosen for new plantings. Secondly, secure, effective, cost-efficient and practicable solutions for the prevention and the reduction of the pests’ distribution rates have to be found.
How to recognise an infestation
The pattern of damage on infested palm trees is quite similar with both insects, although an infestation by the palm weevil will lead to the palm’s death even quicker. While in the case of the palm moth, only its larvae will destroy a palm’s leaves, also the palm weevil itself, and not only its larvae, will eat holes and burrows into the leaf stalk’s base.
Both vermins attack the soft (and only) „growth point“ of the palm tree, the so-called apical meristem or vegetational cone, right beneath the crown. As soon as this is destroyed the palm tree, and this is different from most other plants, cannot keep growing, it will certainly die. For this reason a quick and persistent treatment of all palm trees is necessary.
The older and thus higher, the more bushy or overgrown (Chamaerops) and the more armed with thorns (Phoenix) the specific palm tree is, the more labour- and cost-intensive the control of a possible infestation will be. Brownish outlined small holes in outgrown leaves will indicate that at least a while ago an attack must have happened, and that the next generation of vermins is likely to already have left the palm tree. Quite fine, fibrous material that is thickened by dried palm sap and which looks a little like dark sawdust that has been mixed with tree resin, inside of and beneath the crown and between the old cut off bases of leaf stems along the palm tree’s trunk, are a sure indicator for a massive infestation. Horizontally off-standing leaves, that after getting at first yellowish will turn greyish and then will slowly sink down more and more are another sign of an infestation, especially with the phoenix palm tree.
Due to the increasing extent of the problem, it actually seems quite irrelevant, if a palm tree is already infested or not: unfortunately it is only a matter of time, when an attack will happen, particularly because of the fact that at the beginning of an affection of a palm tree, there will be no visible external signs. A systematic prevention of all palm trees is therefore indispensable.
How can infested palm trees be treated and which prevention measures are possible?
In our latitudes, neither the red palm weevil nor the palm moth are affected by any natural enemy. The oviposition of both species usually happens in late summer or early fall. The palm moth is a quite large and ostentatiously gliding about butterfly, a beautiful but treacherous image between September and November. The flying females of the red palm weevil species are admittedly so fast and inconspicuous that they are rarely noticed. Unfortunately the larvae are also frost-proof, which allows them to have an especially easy run on the palm trees during the home-owner’ absence.
Of course there is always the theoretical possibility of fighting pests like these with insecticides with agents like Chlorpyrifos 48%, Dimethoate 40%, Phosmet 50%, Imidacloprid 20% or Thiamethoxam 25%. Other possibly effective insecticides are banned within the EU for very good reasons. All named insecticides are extremely harmful to bees and do have highly negative impacts on other useful creatures, on bird life and the ground water. But even more importantly, since a persistent prevention has to be put in place, the application of these insecticides can’t be stopped, which makes them on a long time scale just too environmentally hazardous, and – given the number of palm trees on public and private land – simply too expensive.
For the treatment and prevention of these pests we would recommend applying nematodes of the Steinernema carpocapsae, which can easily be ordered via Internet. Cooled down (2-6 °C) they will be storable as a powder for about a week. The whitish transparent nematodes (active phase: 0,1 mm) will look for the vermins, they will percolate into them through their body opening, they will propagate within them and will discharge a bacterium which will kill the weevil or moth within only a few days.
In order to ensure a successful treatment the temperature of soil and air should at least be around 12°C, otherwise the roundworms are inactive. During winter the nematodes will usually die off. Therefore the nematodes will need to be distributed again during the next season. The treated palm crowns should be kept damp during the following 6-8 weeks after a treatment to maximise a lasting effect. Unfortunately nematodes are sensitive to light and ultra-violet rays. Consequently the treatment should be executed during dusk/dawn or at times with a slight overcast! The ideal temperature in regards to the most effective application would be around 15 – 20 °C.
The costs will be around 10,- to 30,- EUR (30 to 100 m2), depending on the ordered amount. This on the other hand will be sufficient for quite a high number of palm trees, plus the necessary man hours for the application of the nematodes.
Because of the amount of work needed and due to the severity of the problem, we would now recommend, especially with higher palm trees, which cannot easily be treated from a standing position or from a small ladder, to permanently install a spray nozzle with opening holes of at least ½ mm, which are then connected to a small (irrigation)pipe down and along the palm’s trunk. The small nozzle only has to be readjusted every now and then, according to requirement, e.g. after heavy storms or after the palm crown has been maintained by your gardener. From the lower end of the pipe, which stays permanently on the respective palm, one can then easily pump the prepared nematode lotion up into the crown by using a manual water pump. Irrigate the crown amply with the nematode lotion – and ideally monthly. With smaller palm trees, which are easier to reach, you can simply use customary pressure sprayers. Please pay attention to sufficiently wide enough holes in the spray nozzle, so the nematodes can pass them(!), and please also take care to never use the same sprayer, which you already have been using for the application of any poison, like insecticides or herbicides.
Another possibility for fighting the larvae of the two pests lies within the application of the entomopathogenic fungi Metarhizium anisopliae or Beauveria bassiana, which deploy their environmentally friendly effectiveness also with other species of insects. They too can be comfortably ordered over the Internet.
Which other precautions will help to save your palm trees?
Please examine your palm trees on a regular basis in regards to above named patterns of damage and please have a qualified gardener peel, skin and maintain them regularly. A Chamaerops humilis for instance, which is not maintained permanently, will overgrow and get bushy within one season; it will develop toward a hotbed for both insect species.
Never leave freshly cut palm leaves lying around for a longer period. If there is no other way than leaving them on your property for a while then please wrap the ends into heavy duty plastic bags or put the cuttings into quite airtight barrels, because the palm sap’s scent is extremely attractive to both species and will be detected even from quite far distances.
Please also instruct your gardener to never enter your property with palm cuttings from other gardens, but to dispose of all palm cuttings beforehand. This may sound a little alarmist but, more often than not, this way these pests are distributed very effectively to properties that had been not-infested before.
Even if more and more arborists and botanists share the (right) opinion that with tree surgeries wound protection lotions should not be applied any more and thus cut surfaces should be left open and unsealed, we would highly recommend to refrain from this otherwise good practise in the case of palm trees and start using wound protection lotions again with palm trees in order to minimise the effect of attraction of the insects through emerging tree sap.