Biology Reference
In-Depth Information
gypsy moths were more sensitive to phenolic glycoside defenses than
were the native species (Hwang and Lindroth 1998; Osier et al. 2000).
Other studies also showed that the chemical variation of aspen tissues had
little influence on a nuclear polyhedrosis virus that is a gypsy moth
pathogen (Lindroth et al. 1999).
In a similar study, Havill and Raffa (1999) found that clones of vari-
ous
Populus
hybrids varied genetically in both constitutive and inducible
resistance to feeding by gypsy moth larvae. Growth of gypsy moth larvae
was 30 times faster on the least resistant clones than on those most resist-
ant, and their consumption of leaf biomass was 250 times greater on the
least resistant clones. Inducible resistance of some clones was very strong,
whereas other clones showed little or none. The potential for evolution-
ary response of
Populus
species to gypsy moth defoliation is thus clearly
appreciable over the long run.
Despite the massive impact of alien herbivorous vertebrates on woody
vegetation in locations such as New Zealand (Campbell 1990), Hawaii
(Stone 1985), and the Galapagos Islands (Schofield 1989), clear evidence
for evolutionary response by woody plants is still lacking. In New
Zealand, where tree fuchsia (
Fuchsia excorticata
) has nearly been eliminated
in some areas with introduced brushtail possums (
Tr ichosurus vulpecula
),
but not in other areas with this mammal, a genetic difference in palatabil-
ity has been postulated (Freeland and Winter 1975). Preliminary studies
by Sweetapple and Nugent (1999), however, could not confirm such a
difference.
Evolutionary Responses of Plants to Alien Diseases
Trees and shrubs typically show genetic variation in response to alien dis-
eases, even severe diseases such as chestnut blight (
Cryphonectria parasitica
)
and Dutch elm disease (
Ophiostoma ulmi
).Various species and varieties of
members of the genus
Cornus
also show markedly different patterns of
susceptibility to dogwood anthracnose (
Discula destructiva
) (Brown et al.
1996). For species such as the American elm (
Ulmus americana
), individu-
als showing some resistance have been used as a basis for artificial selec-
tion in an effort to create fully resistant varieties. The so-called “liberty
elm,” for example, is a highly resistant American elm developed in this
manner (Smalley et al. 1993).
In California, Monterey pines (
Pinus radiata
) are being attacked by
pitch canker (
Fusarium circinatum
), an alien fungal disease that appeared in
the state in the 1980s (Gordon et al. 2001). This disease first appeared in
