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STAMINATE FLOWERING AND TREE PHENOLOGY AFFECT THE
PERFORMANCE OF THE SPRUCE BUDWORM
WILLIAM J. MATTSON, BRUCE A. BIRR, and ROBERT K. LAWRENCE
USDA Forest Service, North Central Forest Experiment Station
Pesticide Research Center, Michigan State University, E. Lansing, M148824, USA
INTRODUCTION
The important role of staminate flowers in the ecology and population dynamics of spruce budworm has been the
subject of speculation for more than 50 years (Blais 1952, Greenbank 1963). This suspicion has been fueled by the fact that
high populations of budworm have historically been associated with mature, flower-bearing trees (Mott 1963). Moreover,
overwintering budworm larvae typically spin their silk shelter in the old staminate flower bracts, and spring emerging 2nd
instars prefer to mine and feed in staminate flower buds rather than old needles. Budworms emerge 1 to 4 weeks before
vegetative buds have begun to expand and are suitable for attack, whereas flower buds, when present, are always more
advanced and thus better synchronized with the early, vernal budworms. Hence, second-stage larvae are invariably concentrated
in the host's expanding flower buds. On the other hand, whenever budworm emergence is retarded relative to tree
phenology, then larvae tend to go directly to expanding foliage rather than the staminate flower buds (Greenbank 1963).
Obviously, then, the importance of flower buds to budworms may be linked to the phenology of the hosts' vegetative buds
relative to the timing of spring emergence of the budworms.
Larvae that feed within staminate flower clusters may actually grow faster than foliage-feeding larvae because such
clusters are miniature greenhouses, trapping more radiant energy than comparable pure foliage environments. However,
there is little other experimental evidence to support the hypothesis that staminate flowers can enhance budworm performance
and thus contribute to outbreak development. For example, Jaynes and Speers (1949) and Blais (1952) reported that
there is no difference in fecundity of budworms having fed on pollen or new foliage.
Herms and Mattson (1992) hypothesized that plant reproduction can also indirectly affect herbivore performance
through its strong impact on the allocation of plant nutrients and photosynthates. When plants produce large crops of pollen
and/or seeds, the reproductive organs may preferentially receive scarce nutrients and energy that might otherwise have gone
to plant defenses or even new growth on which phytophages depend. Hence, staminate flowering could indirectly impact
budworms by reducing the concentrations of nutrients and perhaps even altering the levels of secondary metabolites in plant
foliage.
This study was undertaken to simulate and explore the consequences of abundant staminate flower production on the
performance of pre-outbreak spruce budworm populations. We hypothesized that budworm larval performance would
depend not just on the availability of staminate flower but also on the phenology of a tree's vegetative buds. We predicted
that the presence of staminate flowers would be most important on very late flushing trees, and least important on very early
flushing trees.
METHODS
In 1989 and 1991 we selected 20 half-sibling balsam fir, Abies balsamea, that were flowering at the Kellogg Experi-
mental Forest (Michigan State University) near Augusta, Michigan. Trees were planted in 1970 and about 6-8 m tall at the
time of the experiments. Before budbreak (April 21, 1989, and April 11, 1991), we selected per tree two flowering and two
nonflowering branches near midcrown, which were then enclosed with a fine-mesh, cloth sleeve cage wherein we placed
about 20 ready to emerge second instar budworm larvae still in their silken hibernacula on a gauze patch. Exactly 1 week
Mattson, W.J., Niemel_i, P.,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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