Biology Reference
In-Depth Information
attacking native thistles in Colorado, Nebraska, and South Dakota, includ-
ing the Platte thistle (
Cirsium canescens
), an untroublesome species
endemic to sand prairies of the upper Great Plains. Reproduction by this
thistle is limited by a number of native insects, so the addition of a seed
predator such as
R. conicus
presents a serious threat to its survival. A close
relative of the Platte thistle, pitcher's thistle (
C. pitcheri
), a sand-habitat
species of the Great Lakes region, is a federally listed endangered species,
so the potential exists for extinction of one or more distinctive native
plants due to the utilization of native thistles by
R. conicus
. Numerous
other native
Cirsium
thistles in the western United States have been
attacked by this weevil, with populations on these species increasing
through time (Louda and Arnett 2000; Pemberton 2000).
Another seed-head weevil,
Larinus planus
, has also attacked native
North American thistles (Louda and O'Brien 2001).This weevil, screened
for its potential as a biological control for Canada thistle (
Cirsium arvense
),
was found to attack several other European species of thistles of the genus
Cirsium
and was considered inappropriate for introduction to North
America. In the 1960s and early 1970s, however,
L. planus
was discovered
in the wild in NewYork State and Maryland. It was then screened further
for its ability to attack Canada thistle and native North American thistles
of the genus
Cirsium
. These tests seemed to show that development was
poor on native thistles, and
L. planus
was therefore distributed in Canada
and the United States. In 1999, Louda and O'Brien (2002) discovered that
it was attacking native wavyleaf thistle,
Cirsium undulatum
, in Colorado.
Infestations on this native thistle were, in fact, heavier than on Canada
thistle in the same area.
L. planus
has recently been found attacking sev-
eral native thistles in Oregon (Louda et al. 2003b).
A similar case involves the cinnabar moth (
Ty r ia jacobaeae
), introduced
for control of the tansy ragwort (
Senecio jacobeae
).Tansy ragwort, native to
Eurasia, is a rangeland weed that is highly toxic to livestock. It is a major
problem species in the Pacific Northwest and the Canadian Maritimes.
Larvae of the cinnabar moth feed on the foliage of tansy ragwort, giving
a moderate degree of control.The genus
Senecio
is represented by numer-
ous native species in North America, and the cinnabar moth has begun to
use some of these (Diehl and McEvoy 1990). In Oregon, for example, it
commonly feeds on
Senecio triangularis
, especially where this species grows
in close association with tansy ragwort. Growth is somewhat slower on
S.
triangularis
, and the pupae are lighter in weight than on tansy ragwort.
Still another case involves the moth
Cactoblastis cactorum
, which has
been introduced in various regions for control of invasive species of
