Biology Reference
In-Depth Information
alien plants, usually invasive weeds, in a particular region compared to
their native area.These studies have shown that the species that first col-
onize alien plants tend to be feeding generalists.The rate of accumulation
of herbivorous arthropods varies greatly. For some time after plant intro-
duction, the diversity of herbivorous arthropods tends to be lower than
for the plant in its native region, especially if the plant lacks close relatives
in the new region. In general, alien herbaceous plants accumulate new
herbivores faster than woody plants. In a few cases, introduced herbaceous
species may host more herbivorous arthropods than close native relatives
(see, e.g., Frenzel et al. 2000).
The herbivores of thistles alien to North America have been a popu-
lar object of study, usually in relation to a search for potential biocontrol
species. The phytophagous insects associated with milk thistle (
Silybum
marianum
) and Italian thistle (
Carduus pycnocephalus
) in southern Califor-
nia are numerous but consist mostly of polyphagous species that feed on
the foliage of these plants (Goeden 1971, 1974). In Eurasia, however,
many insect species that feed internally, and thus are much more destruc-
tive to these plants, are known. Species of this type, of course, are the ones
most often sought for classical biological control. Similar observations
have been made for Russian thistle (
Salsola tragus
) and purple loosestrife
(
Lythrum salicaria
) in North America (Goeden and Ricker 1968; Diehl et
al. 1997).
For Italian thistle, Goeden (1974), for example, compared the insects
associated with the plant in southern California with those associated
with it in southern Europe.This species first appeared in California in the
early 1930s. Slightly more than 40 species of insects belonging to 7 orders
and 28 families were recorded on Italian thistle in southern California.
Only about 15 of these were reproducing on the plant. In southern
Europe, more than 80 species belonging to 8 orders and 31 families were
recorded. At least 33 of these were reproducing on the plant.Thus, in the
30-40 or so years that Italian thistle had been present in California, it had
accumulated about half the total species that were present in Europe.The
low representation of reproducing species in California supports the
hypothesis that many of the first colonizers are generalists.
Another thistle of European origin, Canada thistle (
Cirsium arvense
),
arrived in North America in the 1700s (Moore 1975). This species has
occupied North America much longer than the Italian thistle and is much
more widespread. In Europe, at least 86 species of insects are found on
Canada thistle, and in Canada, some 84 species are known to feed on this
species, many of them reproducing within plant organs rather than feed-
