Descriptions of the principal insect pests are largely based on Berryman (1986)
Insect damage to cones and seeds are minor in heavy cone crop years, but in the years following a heavy cone crop, the entire cone crop may be lost as a large insect pest population is concentrated to a much reduced food supply. Due to this, the greatest impact of cone and seed pests is found in seed production orchards, commonly attacked by the pine cone weevil Pissodes validirostris
(Coleoptera, Curculionidae) and the cone worm Dioryctria mendacella
Insects feeding on buds, shoots and twigs may cause substantial long-term damage, affecting plant growth and wood quality, and they may also weaken the tree, facilitating attack by other organisms. They are particularly important in young plantations. Serious infestations seem to be associated with particular site and stand conditions that affect the vigour and exposure of seedlings and saplings. Outbreaks of shoot moths frequently occur in stands growing in dry or poorly drained sites, or in sites that have suffered soil compaction, erosion or nutrient deficiencies. Moreover, tip and shoot insect pests often prefer open conditions which means that damage is most intense in sparsely stocked clear-cut areas and plantations.
The pine shoot moth Rhyacionia buoliana
(Lepidoptera, Tortricidae) is controlled with the use of growth inhibitors (diflubenzuron) in the spring (before the larvae penetrate the buds), by sexual pheromone trapping systems (to lure males) and by biological control (Adalia bipunctata
is a predator of this species).
Leaf-feeding defoliating insects attack forest trees of all ages, but outbreaks are often associated with older stands, overstocked stands, or stands growing on poor sites. The immediate effect of defoliation is a reduction in the vigour and growth of the tree. Reduction in growth may have a significant economic impact on timber production when large areas are affected. Defoliation sometimes results in considerable tree mortality, particularly when the forest has been subjected to other stress factors such as nutrient or water deficiencies, extreme competition, or old age. In addition, the weakened trees often become susceptible to tree-killing insects, such as bark beetles, which frequently cause extensive mortality following defoliator outbreaks. The pine processionary moth Thaumetopoea pityocampa
(Lepidoptera, Thaumetopoeidae) is the most serious pest of P. pinaster
in the Mediterranean region.
A number of insect groups feed on or within the stems of forest trees but, of these, the bark beetles (Scolytidae) are by far the most destructive. Bark beetle adults bore into the bark of living or recently killed trees and lay their eggs in galleries constructed in the cambial region. At the same time, bark beetles may introduce pathogenic fungi, e.g., Ceratocystis
spp., which spread through phloem and xylem, blocking transport systems of the tree. The first adult beetles to attack a tree produce powerful pheromones that, together with resins exuding from the tree, attract other individuals of the same species. It is this combined attack by many beetles, together with the spread of pathogenic fungi that causes death of all or part of the tree. As pheromones concentrate the flying beetle population in an area, dead trees often occur in patches.
Bark beetle outbreaks occur at irregular intervals but are usually associated with stands that are under stress. However, once outbreaks have been initiated, they sometimes spread into relatively healthy stands. This ability of bark beetles to kill normal healthy trees when their populations become large is associated with their tolerance of tree host defensive chemicals, the pathogenicity of fungi associated with them, and/or the attractiveness of their aggregation pheromones. Ips sexdentatus, Tomicus piniperda, T. minor, Orthotomicus erosus, Pityogenes bidentatus
and Hylastes ater
are all bark-boring Scolytidae, and their populations can increase greatly if a stand contains high-risk trees. To avoid such pest population growth, all infested, broken, fallen and burnt trees must be removed, infestation points destroyed and a stand thinning plan implemented. Dioryctria sylvestrella
(Lepidoptera, Pyralidae) is also a bark-boring insect pest, which particularly damages young trees. Low plantation densities stimulate attacks by this insect. Matsucoccus feytaudi
(Homoptera, Margarodidae) attacks weaken trees, making them susceptible to attacks from Pissodes castaneus
and Tomicus piniperda
spp. (Coleoptera, Scolytidae) may flourish in stands which have suffered fire, or in which planting techniques were inadequate, or in which there are many exposed, freshly-felled stumps, or successive drought years. Anoxia australis, Vesperus xatarti
and V. luridus
(Coleoptera, Scarabaeidae) may be particularly damaging in forest nurseries and in young P. pinaster
has shown to be susceptible to Bursaphelencus xylophilus
, the nematode that causes the pine wilt disease, also an invasive species. In other pines, namely Japanese black pine (Pinus thunbergii
), this nematode causes a reduction in hydraulic conductance associated with plant water stress, death of xylem parenchyma cells, and eventually tree death (Kozlowski and Pallardy, 1997
). P. pinaster
is susceptible to the pathogens: Armillaria mellea, Lophodermium
spp. and Cyclaneusma niveum
is very susceptible to fire, especially in pure stands. Abundant understorey is a key factor in determining stand flammability.