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

Galls, leaf miners, or leaf folds were counted on 50 shoots per plant for hybrids and S. eriocephala and on 300 shoots
for S. sericea in 1991 only. Data are expressed as number of herbivores per 300 shoots. Because leaf folds of Phyllocolpa
r_igritaand Phyllocolpa sp. nov. were not distinguished on S. sericea in 1991, we combined them for analyses in all years.
All other species densities are considered separately. Species densities were considered statistically independent among
plants within taxa for all years (Fritz et al. 1994), and species-wide significance tests rather than table-wide significance tests
were applied. ANOVAs were performed for each year separately. We first performed a multivariate ANOVA, using loge
transformed densities of alt herbivore species, and then we performed univariate ANOVAs for each species separately.
Orthoganol contrasts were performed to test for the fit of each herbivore species to the hypotheses. We performed
only two orthoganol contrasts to test the significance of herbivore densities on willow taxa (species) to avoid overparameterization
of the analysis, since only three taxonomic groups were present (SAS Institute 1985). To test the Additive hypothesis,
we tested the significance of the hybrid-midparent contrast (Contrast A) using sequential Bonferroni analysis with specieswide
significance at P < 0.05/2 = 0.025 (Rice 1989). If this test was significant, we rejected the Additive hypothesis and then
tested the contrast between herbivore density on hybrids and density on the numerically closest parent (Contrast B) (P < 0.05/
1 = 0.05). This contrast tested if there was a significant departure from the Dominance hypothesis. If the Dominance
hypothesis was rejected, we inspected the means to determine if the Hybrid Susceptibility hypothesis (density on hybrids
exceeded that of the highest parent) or the Hybrid Resistance hypothesis (density on hybrids was lower than the lowest
parent) was supported, or if the density of hybrids was intermediate between that of one parent and the midparent value. This
last possibility would support a partial Dominance hypothesis.
RESULTS
Multivariate ANOVAs showed highly significant contrasts between herbivore densities on hybrids and the mean
herbivore densities of parent species for all three years (Table 1). This is a rejection of the Additive hypothesis overall.
Further multivariate contrasts could not be made since the test of the Dominance hypothesis requires comparison to the
numerically nearest parent, which would not be the same for all herbivores. In 1991, five herbivore species had significant
hybrid vs. parents contrasts (Contrast A) at P < 0.025, and two herbivore species had marginally significant hybrid vs. parents
contrasts at P < 0.05 (Table 1). In 1992, four species had significant hybrid vs. parents contrasts (Table 1). Three species that
had significant contrasts in 1991 did not have significant contrasts in 1992. In 1993, five species had significant hybrid vs.
parent contrasts (Table 1). Overall, then, there appear to be differences among the years in the species of herbivores that had
significant departures from the Additive hypothesis.
To illustrate the year-to-year differences in more detail and to draw conclusions about which hypotheses mentioned
above are supported by each species, we present the separate results for each year for each species, which included all
censused plants.
Both contrasts for Phyllonoo, cter salicifoliella were significant in 1991 and 1993 (Table 1), and the means support
for the Hybrid Susceptibility hypothesis (Table 2). In I992, neither contrast was significant, which supported the Additive
hypothesis. The other leaf miner Phyllocnistis sp. also showed differences between years. In 1991 and 1993, the first
contrast was not significant, supporting the Additive hypothesis (Tables 1 & 2). Fritz et al. (1994) concluded that this species
supported the Hybrid Susceptibility hypothesis in 1991, because of the marginally significant contrast. In 1992 the Dominance
hypothesis was supported with densities on S. eriocephala and hybrids being equal and lower than on S. sericea (Table
2).
Among species of the leaf galling guild, Phyllocolpa termMatis showed consistent results in each year (Tables I &
2). In all cases the Hybrid Susceptibility hypothesis was supported. Phyllocolpa spp. supported the Additive hypothesis in
1991 and 1993 (Tables 1 & 2). In each year, including 1992, densities on hybrids were intermediate between the two parental
species. In 1992, the first contrast was significant and the second contrast was not significant, supporting the Dominance
hypothesis (Tables t & 2). Pontania sp. supported the Additive hypothesis in 1991 and 1992, but the Dominance hypothesis
was supported in 1993 (Tables 1 & 2). Iteomyia salicifolia supported the Hybrid Susceptibility hypothesis in 1991 and 1993
(Tables 1 & 2). In each year, including 1992, densities on hybrids were greater than on either parental species. In 1992, the
first contrast was not significant at P < 0.025 (although it was marginally significant at P < 0.05), thereby supporting the
Additive hypothesis, even though the mean density on hybrids was more than twice as high as on S. eriocephala (Table 2).
The last leaf galling species, A. tetanothrix, supported the Dominance hypothesis in 1991 and 1993 (Tables 1 & 2). In both of
135