Tuesday, 18 April 2017

Why are Portugal's Pines and Palms Dying?

Portugal’s Palm and Pine Trees are under attack
Dead Pine (Photo: Steve Andrews)

As you travel around in Portugal you can’t hope but notice the dead palms and dead and dying pine trees, but what is causing this disaster? It is a combination of drought, disease, and in the case of the pines, because of a tiny nematode worm and species of beetles that transport it. The pines and palms of Portugal are under attack. Let’s take a look at the two problems, starting with the pines.

Male Pine Wilt Nematode (Photo: A. Steven Munson, USDA Forest Service, Bugwood.org)

Pine trees, which are a very common tree in the forests of Portugal, are under threat from a very tiny worm known as the pine wilt nematode (Bursaphelenchus xylophilus), which causes “pine wilt,” as well as from a fungus known to science as Fusarium circinatum, which causes pitch canker disease. The worm’s name is often abbreviated to the letters PWN.

The nematode worms are spread to healthy trees by bark beetles and wood borer beetles that have become infested with the worms in a dead and decaying tree. After a tree is infected the needles become brown and the tree can die within a matter of months. The early stage of this nematode does not feed directly on the pinewood but on fungi in the decaying wood. Once the worm is in a healthy tree it feeds within the resin canals and spreads throughout the roots, trunk and branches. It reproduces rapidly in summer. It first caused terrible and noticeable tree loss in Japan in 1905, but is found in and has spread in many other parts of the world, including America, Canada, Mexico, China and Korea. In Europe it has become a problem in Portugal, and there are serious concerns that it could spread to other countries, including the UK, where it would be a threat to the Scots pine (Pinus sylvestris).

Monochamus galloprovincialis (Photo: Siga)

In Europe, the pine wilt nematode is known to be spread by the Timberman Beetle (Monochamus galloprovincialis). In Japan the epidemics have been particularly bad in hot, dry summers, and the problem appears to be spreading in drought conditions experienced in Portugal. Drought weakens the trees and they become prone to attack by beetles.

I have become alerted to the pine wood nematode by witnessing the dead and dying pines near where I live in the Quinta do Conde area. The authorities here are chopping the trees down and then cutting up the trunks in an effort to deal with this threat to the forests. The wood can be fumigated or kiln-dried to kill the worms and beetles or it can be burned or chipped.

Infected pines (Photo: Steve Andrews)

Some species of pine appear to be more resistant to attacks by PWN than others. This can be seen in forests where there are stands of dead trees and others surviving.

Red Palm Weevil

Destroyed Palm Crown (Photo: Kuchenkraut)

The threat to the palms of Portugal, and to those in many other countries around the world, is being caused by an insect known as the Red Palm Weevil (Rhynchophorus ferrugineus). The grub of this weevil eats the crown of the tree and burrows into the palm trunks making a long tunnel and killing its host as it does so. The adult insects are quite attractive and large, reaching a size of between 2 and 4cm.

Red Palm Weevil (Photo: Luigi Barraco)

The females lay their eggs on palms, especially in the crowns, and on recently pruned leaf scars and in lesions in the trunks. They can lay as many as 500 in total and an infestation of these weevil larvae will kill the host palm. After eating their fill the grubs pupate in a cocoon they weave of palm fibres and usually make in leaf litter.

The insects attack many species of palm, although a favourite for this weevil is the Canary Date Palm (Phoenix canariensis), a very popular ornamental palm that is frequently planted in gardens, parks and in city streets and squares. The Date Palm (P. dactylifera) is also a victim of this pest. Fortunately the Washingtonia palms seem to be resistant to attacks by the insect. The weevils can fly and this is how they invade new territory but they can also be transported in infested palms.

In Portugal, most of damage caused by Red Palm Weevils has taken place in the Algarve and south of the country, but over the years this insect pest has spread to other parts as well. In the heart of Lisbon you will see what’s left of palms that have been killed by the Red Palm Weevil.

In some parts of the world, including Borneo, Vietnam and Indonesia, the grubs are regarded as a delicacy, and are eaten alive or cooked. In countries, like Spain and Portugal, they are a very serious problem that are destroying ornamental palms.

There are various ways of treating infested palms, and insecticides injected into the trunk is a method that has often been used. It is possible to save palms under attack by these insects but often the damage has become to severe and a very beautiful tree will die and get removed.

Both palms and pines add to the beauty of Portugal and it would be tragic if they were lost from many parts of the country.

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