Forest pests have to be managed and controlled

William Merivale

August 20 2013 04:03 AM

As an island cut off by some distance from our nearest neighbour and with prevailing south-westerly winds blowing in clean air from the Atlantic, we have long enjoyed the highest plant health status in Europe.

Admittedly, the perception of that status has taken a bit of a battering in recent years, first with the outbreak of Phytophthora ramorum, which resulted in a rethink of the policy of planting Japanese larch, followed by the first discovery just a year ago of the ash dieback disease Chalara fraxinea. Nevertheless, we can still count our blessings.

Moreover, despite all the cutbacks, we can still be thankful for the diligent work carried out at all our ports of entry to ensure that all timber coming into the State is bark-free, and therefore, as far as possible, pest-free.

That said, on many grounds we should all have an issue with the continuing importation of cheap, throw-away plywood of Chinese manufacture. Aside from its dubious and at times downright illegal provenance, it is on occasion still found to contain quantities of bark hidden within the leaves of ply and who knows what pests may be freed once the plywood disintegrates?

We do of course have a number of forest pests which must be managed and controlled.

The smaller browsing mammals such as bank voles, rabbits and hares can cause a considerable amount of damage during the early years of a plantation's establishment. Of these, rabbits are probably the worst as the damage caused by the other two species tends to be limited to just a certain number of trees.

Where rabbits are a major problem, often the only solution is to erect a rabbit-proof fence, either around the entire perimeter or, in the case of large areas, just along the boundaries of certain sections is sufficient.

Spiral plastic tree guards are another quite effective solution, though like fencing they come at a cost: a rabbit fence will probably cost at least an additional €2.50-3.00/m over a conventional stock-proof fence, and the current cost of spiral guards with supporting canes is about 50c each.

The bank vole likes to ring bark young Sitka spruce, though the damage is usually not significant.

Hares mostly only damage young broadleaves where they will sometimes travel an entire row of trees, nipping off the leader as though with a pair of secateurs.

While not of concern to the owner of a first-rotation woodland, the most significant pest of conifers on reforestation sites, not just in Ireland but throughout Europe, is the large pine weevil Hylobius abietis.

The adult weevil is attracted by the smell of recently cut timber and the female lays her eggs in the stumps of recently cut conifers and the immature weevils develop under the bark.

It is the adult that causes the damage by feeding on the bark of the tree. If left unchecked in young plantations they are capable of killing every tree on the site.

To date, the only effective treatment has been with insecticide.

With the drive towards limiting the use of chemicals in the interests of sustainable forest management, a considerable amount of research is being devoted towards finding an alternative, natural means of control.

William Merivale is national secretary of PEFC Ireland and a forestry consultant based in Cork. Email: william@cjandco.net