Biology Reference
In-Depth Information
responses to grazing and trampling (see, e.g., Neuffer and Meyer-Wolff
1996).
Evolutionary Responses of Woody Plants
to Alien Herbivores
Several alien insects have had enormous impacts on many native North
American shrubs and trees. These include Japanese beetles ( Popilia japon-
ica ) as adults on many fruit trees and larvae of the gypsy moth ( Lymantria
dispar ) on many species, particularly oaks ( Quercus spp.) and poplars ( Pop-
ulus spp.). Differences in susceptibility to Japanese beetle exist among var-
ious taxa and cultivars of birches ( Betula spp.), elms ( Ulmus spp.), and var-
ious fruit trees (Potter and Held 2002). Several studies have shown that
some oaks and poplars vary genetically in vulnerability to gypsy moth
herbivory. As we noted at the beginning of the chapter, one of these
species is red oak (Byington et al. 1994). Red oak also tends to show local
adaptation to site conditions, with the patterns of adaptation conferring
resistance to leaf-feeding herbivores (Sork et al. 1993). In experimental
studies, seedlings were more resistant to herbivory when grown on the
same slope aspect as their parental trees than when transplanted to slopes
of differing aspect.
Trembling aspen ( Populus tremuloides ) is one of the poplars frequently
attacked by the gypsy moth. Aspen is also subject to outbreaks of native
tent caterpillars ( Malacosoma disstria ). Aspen clones show great variation in
the concentration of constitutive secondary compounds, particularly phe-
nolic glycosides and condensed tannins (Hemming and Lindroth 1995).
The degree of defoliation of different aspen clones during insect out-
breaks also varies greatly (Hwang and Lindroth 1997).
Experimental studies (Osier et al. 2000) have shown major differences
in survival and growth of gypsy moth caterpillars on different aspen
clones. Survival of second-instar caterpillars, for example, varied from
about 45 to 95% on 13 clones taken from Wisconsin and Colorado and
grown in a common garden.These differences were negatively correlated
with the concentrations of phenolic glycosides in the aspen foliage. The
aspen clones most resistant to gypsy moth herbivory, and containing the
highest concentrations of phenolic glycosides, showed the lowest growth
rates. These results suggested that major genetic variation in herbivore
resistance exists in aspen and that there is a considerable trade-off between
herbivore defense and growth. Comparing the results for gypsy moths
with those for native lepidopterans that feed on aspen suggested that
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