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induced reactions have been studied in P ponderosa and P. sylves_ris. In P. po_derosa the nitrogen and tannin concentrations
decreased, but those of proteins, phenols, procyanidins and certain nutrients increased in P. sylvestris. Needle water content
tended to rise shortly after damage. After one or more years (Table 1) the nitrogen, minerals, and monoterpene concentrations
in P. contorta increased, but only that of P decreased. A similar trend for the nutrient and water contents in P. sylvestris
has also been reported. The most interesting aspect is the increase in diterpene acids and sugars, but there are only two
reports of such findings (Buratti el al. 1988, Lyytik_iinen 1994).
Artificial and natural defoliation induce either rapid or delayed chemical changes in pine foliage. The response to
insect-caused defoliation is usually stronger than that to artificial damage (Neuvonen et al. 1987, Harttey and Lawton 1991),
but opposing reactions have also been reported (Baldwin 1988). It would appear that pines are more insensitive to defoliation
than deciduous trees. The damage level and its timing are probably more important factors for the expression of the reactions
than the damage mode. Deciduous trees can respond to low foliage damage (e.g., tearing) (Haukioja and Neuvonen 1987),
but conifers need at least moderate damage (Wagner and Evans 1985, Leather et al. 1987, Lyytikfiinen 1992a). In some cases
the qualitative changes in late summer were different than those iraearly summer (e.g., Lyytik_iinen 1994). Ericson et al.
(t980a,b) observed that Scots pines defoliated late in the season suffered from lowered starch concentrations, and the trees
with the highest degree of damage lost their current year's shoots and new buds. The studies referred to in Table 1 have
mostly been concerned with responses to sawfly dalnage, induced reactions have unfortunately been investigated with very
few other pine insect species, even though there are many other harmful pests on pines in Europe, e.g., Panolisflammea
(Barbour 1987).
NEEDLE QUALITY AND THE PERFORMANCE OF DEFOLIATING INSECTS
Rapid Responses
Qualitative changes do occur in pines, but are these changes targeted at herbivores and are the species used sensitive
enough to respond to induced reactions? Rapid reactions and insect response have been detected in two pine species: Pinus
ponderosa and P sylvestris (Table 2). Artificial damage adversely affected the growth of Neodiprionfulviceps, which should
indicate negative reactions in P. ponderosa. The response of Neodiprion sertifer was mainly positive on P. sylvestris shortly
after insect-caused defoliation, but after artificial damage there were no differences between control and experimental trees.
Another sawfly species responded either negatively or not at all. Furthermore, the results with Scots pine were not parallel.
It would appear that Diprion pint was the least sensitive to qualitative changes in the needles. On the other hand, the low
defoliation level for the whole canopy may have an effect on the indifferent performance of different sawfly species (Niemel_i
et at. 1984). In general, slight defoliation caused no changes in insect performance. In some cases slight defoliation may
even have adverse effects on insect success compared to heavy defoliation (LyytikE.inen 1992a). There were some positive
responses in early summer and soon after damage, but mainly negative responses in late summer. The lack of impact on
sawfly survival indicates no effect on population dynamics. On the basis of these reports (Table 2), it would appear that
Scots pine possesses no effective, rapid resistance reactions against diprionid sawflies.
Delayed Responses
More studies have been carried out on delayed reactions in pines than on rapid reactions. In P. contorta the investiga-
tions deal only with experiments on seedlings (Table 3). The negative effect of treatment was obvious after defoliation by
Panolisflammea, reflecting the persistence of induced reactions harmful to insects. One possible explanation for this may be
that current-year needles were used in experiments and they obviously contain more secondary compounds than older needle
year-classes (Buratti et al. 1988, 1990). There was a trend for positive or neutral responses after previous early summer
defoliation. However, the responses were mostly negative following late summer damage in both pine species (Table 3). The
results for P flammea only are dissimilar due to the different experimental design. In general the results for P sylvestris were
negative or neutral. There were also experiments with N. sertifer and Gilpinia pallida in which there were no responses. As
in studies on rapid responses, delayed responses did not usually affect larval survival, thus indicatingnegligible effects on
population dynamics.
The importance of defoliation level was also obvious in the reports of delayed induced reactions in pines. The
direction of the response was not similar at different levels of defoliation (Lyytik_iinen 1993, 1994), or else the response in
sawfly performance only occurred after moderate damage (Lyytik_iinen 1992a). In some cases, the sawfly response was
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