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Boring by Pine Shoot Beetles
Examples of insects feeding on the canopies of trees are the pine shoot beetles, Tomicus spp. This bark beetle is the
first one to invade dying or felled pines to breed in spring. The progeny emerging from breeding sites in trunks disperse to
the crowns of pines during mid- and late-summer. In order to become mature, young adults feed in pine shoots burrowing
intothe pith. The beetles attack mainly current year shoots of upper half of the canopy. The burrows may heal over only in
the thickest shoots, otherwise they turn brown, break, and fall down. Physiologically, Tomicus attacks in shoots have an
effect like the sudden cutting of ducts (Fig. 5).
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f:: ' • _ _: ,:,. . Figure 5. After about 10 years of repeated Tomicus piniperda damage the tops of
• _- _ i,,__. 5 Scots pine have become deformed as on the right. Under a permanent
__ ': '*'.... " " beetle pressure the spike-like tops die and the pines start looking flat-
_.w-_ "_'g_ ;:,:_ ,;. ,.(._ '. _:::':: i _: topped.
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The effects of removal of shoots on remaining parts of the tree can be divided into three groups: first, the environmental
effects, e.g., the changes in light conditions, water availability and release of nutrients from fallen shoots; second, the
structural reactions like leader shoot changes, formation of adventitious shoots, sapwood-heartwood relation, and the dying of
roots as the pipe model indicates; and third, the carbohydrate dynamics of the tree, including changes in assimilation
nutrient allocation, root activity and compensation processes. The last group of effects is the most difficult to include
model. These effects are discussed thoroughly by Swedish researchers (Fagerstrom et al. 1977, Ericsson et al. 1985,
Langstrom et al. 1990,Troeng and Langstrom 1991, Langstrom and Hellqvist 1991, 1992). In addition, the reactions
capacity,
in a
seem to
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be different according to age, growing site, and provenance of trees, v
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The distribution of damage classes of crown can be used to make rough estimates of growth losses in stands repeat-
edly attacked by pine shoot beetles (Kukkola et al. 1994). With a stand of model trees, the spatial pattern of beetle dispersal
and the distribution of growth losses of pines may be examined, b
The present model should be able to simulate the pruning pattern of Tomicus attack and the growth loss. The model
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is currently being used this way by letting individual Tomicus object instances to choose a shoot for foraging. The choices
attacking beetles depend on the shoots already allocate by the previously attacked beetles, and the amount the tree stands
above or below its neighbors.
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Defoliation by Insect Larvae
Scots pine is adapted to low nutrient resources and stores its major reserves in foliage. Figure 6 shows the dynamics e
that follow from a severe defoliation in this situation. A severe defoliation reduces the carbohydrates and nutrients overall in o
the tree, and also reduces the needle number. This leads to increased nutrient concentration in the remaining needles, as the s
fine root biomass is not immediately reduced and its nutrient uptake pumps more nutrients into the remaining needles. This a
increases the quality of the remaining needles for food of the defoliator. Increased nutrition in needles also leads to their d
increased size and increases production. The carbohydrate concentration in needles increases as well as the concentration of
the carbon-based protective metabolites (for a review, see Herms and Mattson 1992).
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