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summer, but the other sawfly species can also eat the current foliage during rniddle or late summer. Needle year-classes may
also react differently after needle damage (Niemel_ and Tuomi 1993), or interactions among needle year-classes may occur
(Buratti et al. 11988). Thirdly, as earlier rnentioned, there could also be phenological changes in the same year-classes during
the summer (Bernard-Dagan 1988). The quality of mature needles is different m early season species compared to late
season species.
The existence of induced responses in P. contorta and R potzderosa may in part ofter one explanation for the population
dynamics of the species feeding on them (e.g., Barbour 1988). In Scots pine the weak induced reactions may be the
reason for the tack of cydicity in sawfly outbreaks (Niemela and Tuomi 1993). The lack of damage-induced reactions in the
needles of mature and pole-size Scots pine enables outbreaks of N. sertifer to occur for several years at the same site. The
explanations for the different phases in population dynamics must be due to factors (e.g., parasitoids, diseases) other than
qualitative changes in the foliage.
SUMMARY
In pine foliage, induced responses against defoliating insects have been detected in P#zus ponderosa, P. contorta and
P. syh,estris. Contents of nitrogen, minerals, and water tended to increase after damage. The damage level and timing are
important factors for the expression of these reactions. P. ponderosa showed evidence of rapid induced reactions, but not in
E sylvestris. Delayed induced reactions were obvious in P. cotztorta, but in P. sylvestris the responses were weaker and
expressed only under certain conditions. Foliage of different ages may respond differently, because young foliage acts early
on as a sink for metabolites. Furthermore, foliage quality can change according to season. The induced reactions detected in
certain pine species are linked to their seedlings and fast growth. The defensive strategy of pines is probably highly agespecific:
seedlings resemble fast-growing species. The explanation fbr the typically weak reactions in pines may be due to
physiological differences between them and deciduous trees. The weak induced resistance in Scots pine may explain lack of
cyclicity in pine sawfly outbreaks.
ACKNOWLEDGMENTS
I wish to thank Pekka Niemel_i _br his critical and helpful comments on the manuscript and John Derome for kindly
revising the English text. The work was financially supported by The Academy of Finland (project no. 209103 1).
LITERATURE CITED
BALDWIN, I.T. 1988. The alkaloidal response of wild tobacco to real and simulated herbivory. Oecologia 77: 378-381.
BALTENSWEILER, W., BEN, G., BOVEY, R and DELUCCHI, V. 1977. Dynamics of larch bud moth populations. Annu.
Rev. Entomol. 22: 79-100.
BARBOUR, D.A. 1987. Pine beauty moth population dynamics. General considerations and life-table work. In Leather,
S.R., Stoakley, J.T., and Evans, H.E, eds. Population biology and control of the pine beauty moth. For. Comm. Bull.
67: 7-13.
BARBOUR, D.A. 1988. The pine looper in Britain and Europe, p. 291-308. In Berryman, A.A., ed. Dynamics of Forest
Insect Populations: Patterns, Causes, and Management Strategies. Plenum Press, New York.
BERNARD-DAGAN, C. 1988. Seasonal variations in energy sources and biosynthesis of terpenes in maritime pine, p. 93-
115. In Mattson, W.J., Levieux, J., and Bernard-Dagan, C., eds. Mechanisms of Woody Plant Defenses Against
Insects: Search for Pattern. Springer-Verlag, New York.
BRYANT, J.R, CHAPIN, F.S.III, and KLEIN, DR. 1983. Carbon/nutrient balance of boreal plants in relation to vertebrate
herbivory. Oikos 40: 357-368.
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