Biology
The eggs are laid in punctures made by the female on the
underside of the leaves. Eggs incubation time is about 13
days. Females may laid up to 2,000 eggs. Larvae feed on the
leave petiole and move down to the crown of the thistle rosette.
They feed at the growing point of the thistle at the root-stem
junction. Damaged tissues turn black and the plant growth
will be distorted with multiple growing tips resulting. The
larval stage can last from 40-164 days. Much of the damage
from the weevil larvae is done in the spring of the year.
Pupae are formed in the soil near the rosette and lasts about
18 days. Adults emerge in late June to July. They feed on
the plant but spend most of the summer in hibernation. The
adults become active in late September and begin oviposition.
There is one generation per year.
History
This weevil is native to western, central, and southern Europe.
It was originally imported from Italy and first released in
Virginia in 1974. It is now established in several states
from Virginia to Washington. There is no record of establishment
in Texas
Status
Releases were made at three locations in Kerr County in 1987.
In 1994, 400 weevils were released near Harper, Gillespie
County. In 1995, 400 weevils were released near Hereford,
Deaf Smith County, and 250 at Jake Landers’ Ranch in
Menard County. No establishment has been documented from any
release in Texas.
Management
Sweep nets can be used to collect the adult weevils in July.
Adults can be shipped in cardboard cartons with leaves if
the containers are kept cool. Eggs can be collected by hand
in the early spring before the plants bolt. Eggs may be stored
at 4EC for up to three months.
Damage by this insect is seldom very effective by itself.
It does contribute to damage if used in conjunction with other
agents.
Weeds Affected
Most thistles in the subtribe Carduinae are affected including:
musk thistle, Carduus nutans complex; plumeless thistle,
Carduus acanthoides; Italian thistle, Carduus pycnocephalus;
Canada thistle, Cirsium arvense; bull thistle, Cirsium
vulgare; and Scotch thistle, Onopordum acanthium.
Literature
Rees, N. M., J. L. Littlefield, W. I. Bruckart, and A. Baudoin.
1995. Trichosirocalus horridus. In Biological Control
of Weeds in the West. Western Society of Weed Science.
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