Trichosirocalus horridus (Panzer)
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.
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
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.
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.
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.
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.