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THE IMPACT OF FLOWERING ON THE SUITABILITY OF BALSAM FIR
FOR SPRUCE BUDWORM VARIES WITH LARVAL FEEDING BEHAVIOR
1_.BAUCE and N. CARISEY
D6partement des Sciences Foresti_res, Facult6 de Foresterie et de G6omatique
Universit6 Laval, Ste-Foy (Qu6bec), Canada G 1K 7P4
INTRODUCTION
The production of staminate flowers by coniferous trees is known to affect the fitness of insect defoliators. For
instance, spruce budworm, Choristoneurafumiferana Clem., larvae on flowering balsam fir, Abies balsamea (L.) Miller, had
3 to 7 days shorter development times than those on nonflowering trees (Jaynes and Speers 1949, Blais 1952, Greenbank
1963). Based on this effect, Blais (1952) hypothesized that flowering of balsam fir could trigger spruce budworm outbreaks.
The impact of flowering on budworm survival and fecundity varies greatly from one study to another. Blais (1952)
found no significant effect of flowering on spruce budworm survival, while Mattson et al. (1991) detected a 25% increase in
larval survival when insects were reared on balsam fir branches bearing staminate flowers compared to those on nonflowering
branches. Moreover, balsam fir flowering had no effect on the fecundity of spruce budworm (Blais 1952), whereas Pinus
banksiana Lamb. flowering had a negative effect on the fecundity of the jack pine budworm, Choristoneura pinus. Although
there is no agreement on whether flowering positively or negatively affects budworm mortality and fecundity, most authors
believe that the impact of flowering results from either the high nutritive value of the pollen and/or the presence of staminate
flower clusters that may provide a more favorable micro-habitat in term of heat and protection from natural enemies (Lejeune
and Black 1950, Wellington 1950, Blais 1952, Greenbank 1963, Mattson etal. 1991).
In regard to budworm dynamics, the production of staminate flowers can be followed, the next year, by either an
increase (Graham 1935, Blais 1952, Volney 1988) or a decrease (Batzer and Jennings 1980, Bauce 1986) in budworm
population density. In order to explain such opposite trends, we hypothesize and test here that the impact of flowering on the
suitability of balsam fir for budworm larvae varies according to the feeding behavior of the larvae, which is affected by the
density of budworm population (Blais 1952).
METHODS
Field Rearing Experiment
The impact of flowering on spruce budworm growth, development, survival, and fecundity was determined by caging
second instar larvae in the mid- and lower-crowns of flowering and nonflowering balsam fir trees. In 1992, a total of 10
flowering and 10 nonflowering dominant balsam fir trees were randomly selected in a 60 yr-old balsam fir stand located in
compartment 20 of the For_t Montmorency (47° 19' N, 79° 09' W), an experimental forest of Universit6 Laval. The stand fits
Grandtner's (1966) description of the balsam fir-white birch association and was classified as site 1 quality with good
drainage (B61anger et al. 1983), 15%slope, deep uncompacted glacial till, and 65% crown cover.
A severe drought in 1991 triggered an intense production of staminate flowers the following spring. Trees located
near streams, or not fully exposed to sunlight did not produce staminate flowers. On each sample trees, two 90 cm long
branches, facing north-northwest, were selected at the mid and lower third sections of the crown. Staminate flowers were
present only in the midcrown of flowering trees. Reproductive buds burst betbre vegetative buds and the pollen was dispersed
during a period of 3 to 4 days. Each branch was enclosed with a fine-mesh cloth sleeve cage which served as an
Mattson, W.J., NiemelS., R, and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle. USDA
For. Serv. Gen. Tech. Rep. NC-183, N.C. For. Exp. Sta., St. Paul, MN 55108.
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