View or print this publication - Northern Research Station - USDA ...

From the point of view of trees, the evolutionary response in terms of defense seems to be consistent with a concept
k_own from deciduous species like oak or beech, as masting or mast seeding (Silvertown 1981). tn other words, in some
years there will be high beech-mast or acorn production, but no seeds in the years in between. In the latter years, pest
populations will decrease markedly. The strategy has probably evolved against seed predators but it could also work against
_omedefoliators.
The second line of defense is simply the variability of bud burst (Mitcherlich and Wellenstein 1942). Large variation
in bud burst between provenances exist, but also large variation occurs within provenances. However, variability as a defense
will mean that some larvae always will survive.
The third line of defense against flush-feeders is a chemical one. Apparently, rather few spruce herbivores are
deterred from feeding on newly burst needles to such an extent that they prefer old needles (Schopf 1986).
In contrast, in pine it seems essential that flush feeders avoid plant defenses and here male flowers are of vitae
importance. In pine, male flowers are a more predictable resource than in spruce. Larvae that hatch after the flowering
period will die and hence the risk of early hatch is less because male flowers are most often present. However, the strategy
might fail; they might hatch on a tree without flowers, or worse in a year without flowers in the whole stand.
In the context of the phenological window it is actually necessary to consider the choices made by the female moth.
Where does she deposit her eggs? Does she in fact spread the risk so that at least some of her larvae will survive? But at the
same _ime,does she abandon an optimal strategy of maximizing short term fitness? Essential to all these considerations are
_hedevelopment time of buds and eggs, both factors being related to spring temperatures, although probably not in the same
fashioa. I hypothesize that the position of the eggs is a key factor in this synchronization simply because the temperature
varies so much between microsites.
1#mantria monacha females deposit their egg clusters mainly on the tree trunk under bark scales. Egg numbers and
egg hatch differ consideraNy between north- and south-facing sides of the trunk (Raae 1979), and it might be that this is the
mechanism the females employ when they spread the risk. This could be the way whereby females ensure that some of their
progeny in this variable environment will hatch when the phenological window is open. Another bet-hedging mechanism
couid be that after laying their first batch and becoming lighter, females fly to another tree and deposit the rest of their eggs.
If the first tree is in the middle of a dense stand and the next is at the southern edge, the temperature regime is different and
accordingly the bets change.
litis interesting to observe that many flush feeders have wingless females or have reduced flight capability due to
their heavy loads of eggs. In L monacha, females are winged but poor fliers and it is likely that the females lay their eggs
close to if not directly on the tree where they pupated, thereby increasing the likelihood of synchronization between the egg
i_atchand bud burst.
In conclusion, avoiding defenses of new needles is absolutely necessary for pine defoliators but of minor importance
_o spruce de_bliators. Bet hedging might prove to be the evolutionary response by the defoliators in order to find the host's
phenological window.
LITERATURE CITED
DEN BOER, RJ. 1968. Spreading of risk and stabilization of animal numbers. Acta Biotheoretica 18:165-194.
FEEENY, P 1970. Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillars.
Ecology 51' 565-582.
KEDA, T., MATSUMURA, E, and BENJAMIN, D.M. 1977. Chemical basis for feeding adaptation of pine sawflies,
,_e_:dlpr_on rug(frorls and Neodiprion swainei. Science 197: 497-498.
JENSEN, T.S. 1988. Variability of Norway spruce needles; performance of spruce sawflies (Gilpinia hercyniae). Oecologia
77:313-32(t.