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Objectives
Given the importance of a plant's herbivore loading, i.e., the number of herbivore species or non-zero elements in its
species vector, this study tested the hypothesis that artificial white x blue spruce FI hybrids will inherit the phytophages from
each of their parent tree species, with the consequence that the number of elements in the hybrid species vector will be the
sum of the unique (specialists) plus the shared or common (generalists) elements of both parents. Because hybrid spruces do
not occur naturally in Michigan, we only conducted tests in several test plantations where the trees are exposed to the natural
phytophages of white spruce but not those of the other parent, blue spruce, a western North American tree species. However,
one western spruce galling insect, Adelges cooleyi, is common in eastern North America on ornamental blue spruce. We also
tested whether total insect loading on hybrids, i.e., the total number of individuals (pooled over all phytophage species) found
per unit of foliage per plant, is equal to or greater than that found on each of the parent species.
METHODS
Hybrid spruces (P glauca x P. pungens) were sampled in 1991 and 1992 at two different locations in Michigan, along
with populations of the parent species in order to assess differences in species, and total insect loading. One sample site
occurred in south-central Michigan at the Michigan State University Kellogg Experimental Forest. The other occurred some
450 km to the north in Michigan's Upper Peninsula at the Michigan State University Dunbar Experimental Forest. Kellogg
(n = 40) and Dunbar (n = 24) sample trees were all F1 hybrids. Equivalent numbers of parent spruces were likewise sampled
from adjacent or nearby (< 1 kin) plantations.
At each site, trees were randomly selected, and examined twice per growing season (late May-early June, and late
June-mid July). Trees were first examined visually to score for adelgid galls (Adelges abietis, A. cooleyi, and Pineus similis).
Next they were sampled using standard beating methods whereby the apical half (45 cm length) of a midcrown branch on the
west side was held over a specially designed stainless steel collection pan, and the branch was gently tapped with a wooden
dowel for I0 seconds to encourage free feeding species to drop off into the pan. Such insects were gathered in vials and then
stored in a freezer for later identification and counting.
Data from both June and July samples (numbers of insect species, and total insect counts per tree) were analyzed
separately using a completely randomized ANOVA, after log (X + 0.1) transformation. Means for each tree species were
ranked and then separated using the SNK multiple range test.
RESULTS
Gall-forming Adelgid Specialists from Both Parent Spruces Successfully Attack Hybrids
At the Kellogg experimental tbrest in southern Michigan, there were significant populations of both Adelges abietis,
and A. cooleyi, largely specialists, respectively, on P. glauca, and P pungens and their near relatives (Furniss and Carolin
1977). We found that the ihybrids contained substantial numbers of both kinds of adelgids, whereas the parent spruces had
primarily their respective specialist adelgids (Table 1). One glauca individual had evidence of two poorly formed A. cooleyi
galls, but in general glauca in eastern North America are not susceptible to the eastern A. cooleyi populations (personal
observations) which live largely on ornamental blue and Engelmann spruces introduced from western North America.
Similarly, there was only minor evidence for A. abietis on pungens: one tree, with one small, poorly formed gall. Although
Table 1 shows very low populations ofA. cooleyi on pungens (one infested tree in a sample of 30-42), this was because the
adelgid population had drastically declined in the preceding 2 years (1989, 1990) in the sampled pungens plantation which
was about 1 kln from the hybrids. In 1988 and earlier there were much larger populations of A.cooleyi there: 76 of 100
sample trees were infested (Mattson, unpublished data). Pineus simitis, another galling adelgid that is largely a specialist of
glauca and near relatives, was not abundant enough to test hybrid susceptibility.
At the Dunbar site in northern Michigan, A. cooleyi and P. similis populations were negligible, so we could not
measure their colonization of the spruce hybrids. But, we did find there that A. abietis readily attacked hybrids, but not so
readily pungens, as shown in the following tabulation of the percentage of trees attacked: white (84), hybrids (83), blue (15).
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