THE PALMS OF WESTERN LIGURIA
There are forms of heritage that we are used to seeing everyday and that we pay little attention to. It is only when this heritage goes missing or when we run the risk of losing it that we realise how important it is. I think this is also the case of the palm trees of Western Liguria.
The Red Palm Weevil, a phytophagous insect that feeds on palm trees, is a well-known pest to palm trees. We always more frequently see distressing images of decapitated palms, sometimes with the stipe still erect but already dead. It must not be forgotten that Bordighera today still boasts, with legitimate pride, an xtraordinary palm grove of Phoenix dactylifera, whose origin dates back to the Middle Ages. Throughout the centuries, the palms have earned the curiosity of travellers and inspired artists and writers, soon recognised as an icon of this area. Date-palms certainly arrived in Liguria thanks to the work of man; their primary origin is still doubtful. Legends, myths and cultivation techniques such as the harvesting of leaves for Jewish and Christian religious traditions, increase the cultural value of this extraordinary heritage.
Losing these plants would also cause serious damage to the landscape. Indeed, many things have changed since the arrival of the Red Palm Weevil. Before arriving in Western Liguria and infesting its palm trees, the famous Rhynchophorus ferrugineus, better known as Red Palm Weevil, had lived quietly and in balance with nature for millennia in its countries of origin in Asia. The Red Palm Weevil still feeds on palm trees in the tropical forest – which is home to an extraordinary biodiversity – but it most certainly does not alter the forest’s environmental balances. In the hot, dark and muggy forests a multitude of limiting factors live together and contribute to preserving a natural ancestral order to which the Red Palm Weevil is also subject.
But what induced this insect to move out of its environment and reach our gardens? The reason, as often happens, must be found in human behaviour. When the first intensive cultivations of coco-palms started during the last century, the biological balance resulting from the long co-evolution between animals and plants, broke. After moving away from an environment that limited and confined it, the Red Palm Weevil advanced towards west, covering more than 12,000 km in around a hundred years and reaching Bordighera in 2007.
From a phytopathological perspective, the Mediterranean basin contains particularly rich exotic phytophagous species, which were introduced accidentally from other continents. Some are extremely aggressive, while others show acceptable cohabitation. In this list, in addition to the Red Palm Weevil (Rhynchophorus ferrugineus), the greatest danger for palm trees is posed by the Argentinian butterfly (Paysandisia archon). The edaphic action of these two insects, combined with their highly effective reproductive system, is such that it is able to destroy thousands of palm trees.
Regarding the strategies to be taken for combating this situation, numerous attempts and tests are being carried out. However, despite the strong wish to save the palm trees, attention must be paid to the protection of the environment as a whole and to biodiversity. In this regard, research has been committed greatly to seeking biological solutions. The results achieved using nematodes and fungi, for example, are interesting, however, it seems that we must wait further before being able to have access to preparations that can truly solve the situation.
From this perspective, the Municipalities of Sanremo and Bordighera, in cooperation with the Regional Plant Health Service, the CRA-FSO (Council for Research and Experimentation in Agriculture) of Sanremo and the Centre for Studies and Research on Palm Trees began work by using entomopathogenic nematodes-based authorised products. These natural organisms are able to effectively and naturally control insects that damage agriculture. We particularly tended towards products that contain – in addition to nematodes – also chitosan, a substance obtained from the exoskeleton of crustaceans which is able to shelter them from abiotic agents and enhance their infectious powers. Our allies in the fight against the Red Palm Weevil today, therefore, are entomopathogenic nematodes such as the Steinernemasthat, which in the commercial formulation, are at an infectious stage and are able to locate the insect’s larvae and penetrate their body through natural openings. After entering the larvae, nematodes release bacterial symbionts that quickly kill the larvae.
As an alternative to pesticides, biological control techniques have various advantages. Firstly, the organisms used are non-toxic, have no preharvest interval, do not pollute, do not develop resistance and, in many cases, have the same costs as chemical control (sometimes even lower). Unlike chemical products that are not selective towards a great number of other living beings, antagonists have more specific preferences and, therefore, do not damage all “non-targeted organisms” (man, domestic animals, fish, birds, etc.). “Zero tolerance” towards the Red Palm Weevil, consisting of the prompt elimination of the infested palm trees by the Municipalities of Sanremo and Bordighera, has shown considerable results in the past years.
The municipalities have chosen not to carry out chemical treatment in urban areas, both on the foliage and by endoxylematic injections. In the latter case, this would mean causing wounds to the palm trees and forcing an active substance inside the trees together with its adjuvants and coformulants. This would be highly unnatural, even though this technique is very frequently practised by chemical business operators today. The doubt (from a physiological perspective) as to whether we can exclude that the complex xylematic upstream system is not subject to cavitation alterations when the capillary columns are cut by a perforation must also be considered. It is also true that vessels communicate with each other, however, the tests conducted during the DIPROPALM project (financed by MiPAAF) have shown limited mortality among the larvae of the palm trees treated. Other tests carried out using tracer dyes in healthy adult palm trees, variety Phoenix canariensis, have shown insufficient transmigration of products towards the foliage. Another aspect that must not be forgotten is the mechanical damage to the stipe that could appear later, caused by an over 30 cm. drill bit (needed to introduce the cannula), in addition to the risk of fungal or bacterial infections. Lastly, channelling the chemical formulation by using the already existing hole made a few months earlier, gives rise to even more doubts.
In the face of these remarks, the request submitted by a number of promoters to the Municipality of Bordighera to subject 1500 palm trees of the municipal palm heritage to preventive endotherapic cures is inappropriate and incautious! It is worrying that someone could think of coming to Bordighera and exploiting the historical date-palms to perform a pilot experiment. Experimental tests should be carried out on sample plants by public research institutes or test centres, acknowledged by MiPAAF and performed in pre-established places. We should also consider that, in case of problems arising from endotherapic treatment, Bordighera would not be safeguarded. I believe that precautionary duty should never be underestimated.
Ultimately, the palm heritage of the Riviera is subject to a serious threat, but we have sufficient reason to believe that the Red Palm Weevil will not be able to destroy it beyond a certain measure. We will probably lose a great part of the palm trees of the Canaries, but the wealth of different species that grow or could grow along Mediterranean coasts provide us with an extraordinary reservoir of biodiversity.
In this light, the Centre for Studies and Research on Palm Trees, the Municipalities of Bordighera and Sanremo, Aicardi School and the historical Allavena Garden are cooperating in the GERICO project which will produce 15,000 new palm trees to be planted in the gardens of the Western Riviera. The aim of the initiative is to reconstruct the palm heritage in this geographical area over the next 10-15 years, by donating young palm trees of different species to the coastal municipalities and to owners requesting them.
Sanremo Centre for Studies and Research on Palm Trees