Irish forests are recognised under the European Union Plant Health Directive 77/93/EEC as being among the healthiest in Europe, with relatively few serious forest pests or diseases. This is mainly due to Ireland’s island status, the relative newness of the forest estate, and the enforcement of forest plant health regulations. However, the species composition of Ireland’s largely ‘man-made’ forests, which are comprised mainly of exotic conifers, makes them inherently susceptible to introduced harmful organisms. To date there have been no outbreaks of destructive insect pests from abroad but the growing movement of forest plants and wood products (e.g. logs, sawn timber, pallets, packing wood and ship’s dunnage) between countries increases the risk of potentially very damaging forest pests and diseases spreading to Ireland.
The principal forest insect pest which occurs in Ireland is the large pine weevil(Hylobius abietis). Other significant insect pests include green spruce aphid (Elatobium abietinum), pine shoot moth (Rhyacionia buoliana), pine beauty moth (Panolis flammea) and the European pine sawfly (Neodiprion sertifer). These insects are all native to Ireland and affect forests in different ways.
LARGE PINE WEEVIL ( Hylobius abietis)
The large pine weevil is the most destructive insect pest in Irish forests. It lives in the canopy of coniferous trees and breeds in the cut stumps of trees following clearfelling. The emerging adults feed on the bark and underlying tissues of newly planted trees. Damage is often so severe that it results in major losses in plant survival and significantly increasing re-planting costs. Routine protective measures are essential if reforestation is to be successful. Various methods are used to protect plants from the weevil which include:
- Fallowing – leaving sites unplanted for 2-4 years following clearfelling by which time the emergence of weevil larvae from the stumps will have passed.
- Cultivation –Weevils do not like to cross bare soil as they vulnerable to attack by birds and other predators. Mounding is particularly effective.
- Insecticide application – protecting newly planted trees through the pre-treatment in the nursery with an approved insecticide; followed by spraying of the planted trees in the second, or maybe third year depending on the level of outbreak.
- Physical protection –using stem collars, fine meshed nets, gels etc. To date none has been successful in Ireland.
- Biological control – application of nematodes (microscopic roundworms) which specifically attack the larvae of the weevil. Still in the experimental stage, this non-chemical method of control shows promise and has been researched by the National University of Ireland Maynooth and applied on a pilot scale by Coillte.
Further information at: Controlling the Large pine weeil using natural enemies
GREEN SPRUCE APHID (Elatobium abietinum)
Green spruce aphid is a sap-sucking insect that infests spruce trees (Picea species).
Unlike most aphids, the green spruce aphid is active and causes damage to trees over the winter months. The effects of the green spruce aphid are particularly noticeable on Sitka spruce after a mild winter. If weather conditions are favourable, the aphid can rapidly reproduce to immense numbers through parthenogenesis, a form of asexual reproduction where growth and development of embryos occur without fertilization. Old foliage develops a pale mottled discolouration during the winter and many of these needles will fall off in the spring. A black sooty mould may be noticeable on the stem joints. Badly affected trees can give the appearance of having been burned or dead. New growth produced in spring, however, is unaffected, and its bright green appearance contrasts strongly with the discoloured and sparsely foliated older stems.
The loss of foliage effects the growth of the infected trees and it can take several years before trees fully recover. Aphid outbreaks tend to be more severe in the milder parts of the country such as in the south and south west.
Natural enemies such as ladybirds, lacewings, hoverfly larvae and parasitic wasps help to limit infestations during the summer. Outbreaks of the aphid are accepted as a normal occurrence in forests and while there may be a loss of increment, control measures are not employed as the trees can recover.
Further information at: The Green Spruce Aphid – a pest of spruce in Ireland
PINE SHOOT MOTH ( Rhyacionia buoliana)
Pine shoot moth can be a pest in pine plantations, due to the habit of its larvae burrowing into the needles and feeding inside developing buds.
The larvae will burrow or mine into the base of pine needles, feeding and over-wintering in the lateral buds and shoots and destroying their interior and resulting in the deformation or death of leading shoots. Terminal and/or lateral buds will also fail to flush on infected shoots. Where a leading shoot survives it will usually be bent (like a Dutch pipe) and where they do not, multiple leaders will form. Either way significant stem deformities will occur. As young plantations are the most vulnerable, early damage and deformities will remain in the lower part of the tree as it matures. As this is the most valuable part of the tree, badly affected crops are often only fit for pulp or fuel wood.
Infestation is visible in spring and autumn, as spun silk ‘tents’ and resin appears between buds and at the bases of affected shoots. The larvae may be visible in these areas. Adults usually fly during June, July and August.
In Ireland, pine shoot moth is a problem with lodgepole pine stands and notable outbreaks have occurred in forests on the midland bogs and the Ballyhouras in the south west. As lodgepole pine is mostly used for pulpwood, which is chipped rather than sawn, control measures are not employed.
PINE BEAUTY MOTH (Panolis flammea)
Pine beauty moth has caused significant damage to forests in the north of Scotland where it has defoliated large areas of lodgepole pine. The insect is also indigenous to Ireland but while outbreaks have been recorded in the west, it has not developed into the destructive pest that it has in Britain.
Pheromone trapping, which has been used to monitor populations of the insect, has shown that the species is capable of occurring in the high numbers that have caused outbreaks in Scotland. However, serious outbreaks have not occurred in Ireland. It has been observed that larvae which develop in the litter under the trees are prone to infestation by a fungus, and this coupled by wet conditions maybe the controlling mechanism that has prevented the insect from becoming a seriously destructive pest in this country.
On-going monitoring of pine beauty moth populations is regularly carried out but no control measures are employed. Further information at: Pine beauty moth ; its biology , monitoring and control
EUROPEAN SAWFLY (Neodiprion sertifer)
Pine sawfly larvae resemble caterpillars and are18-24mm long when fully-grown. They emerge in April and May and have distinct black heads with greyish-green bodies with lighter stripes down their backs and sides.
The larvae feed on pine needles and can defoliate a complete stand. The bark on newer shoots may also be eaten. As a result, the shoots may be deformed or die back: trees are rarely killed but heavy infestations may remove all but the current season’s needles resulting in reduced height and diameter growth.
Outbreaks of pine sawfly occur sporadically in lodgepole pine crops in Ireland but can also affect other pines such as Scots pine.
No control methods are employed to control outbreaks.