by L. T. Kok, Department of Entomology, Virginia Institute of Technology and State University, Blacksburg, VA 24061-0319
Trichosirocalus horridus is a native of south and central Europe that feeds on the rosettes of Carduus thistles. This rosette weevil was the second insect introduced into North America for Carduus thistle control. It was imported from Italy into the USA in 1970-1972 for quarantine testing. Host specificity testing revealed that it would not be a threat to nontarget plants and it was approved for field release in the USA in 1974. It first became established in Virginia and has since been relocated to many other states.
The adult is dark brown to black in color and about 10 mm long. Newly emerged adults are reddish. They change to dark brown to black with time.
Nodding (musk) thistle and plumeless thistles are introduced Eurasian noxious weeds in pastures, rangelands, croplands, and along state highways in many parts of the USA. Their success is largely due to their prolific seed production, seed longevity, competitive ability of the plants, and the lack of natural enemies.
This rosette weevil is host specific and feeds mainly on thistles belonging to genera Carduus, and Cirsium in the subtribe Carduinae of the Asteraceae family. It does not attack economical plants belonging to the Asteraceae.
T. horridus overwinters in the adult, egg and larval stages. Adults emerge in late winter and early spring to lay eggs on young thistle rosettes. Each female lays an average of 500-800 eggs. Eggs are laid within the mid-rib of the underside of the leaves in clusters of 2 or 3, and hatch in about 10-12 days. Newly hatched larvae feed within the mid-rib towards the center growth point of the thistle rosette. The larvae reach the growth point in 7 days and continue feeding on the young tissues, causing the center point to become dark in color. They complete development in 6-8 wks. The full grown larva leaves the plant to pupate in the soil in pupation chambers. The new adult emerges in 12-20 days. The new generation of adults appears in May or June and goes into hiding until the fall, when they emerge to feed. Some may start laying eggs until the first frost before hibernation. There is usually one generation per year.
This thistle rosette weevil is very effective in the control of musk as well as plumeless thistle. The first success in biological control of musk thistle was documented in 1986. Biological control of musk thistle occurs after 5-6 years but it takes longer (10-12 years) to control plumeless thistles. Typical thistle stand reductions of up 95% were obtained at sites where the weevil became established. It also combines well with the thistle-head weevil as they attack different stages of the plant growth.
This weevil is susceptible to the commonly used insecticides and thus should not be exposed to insecticidal sprays. It is not directly affected by commonly used herbicides and thus can be used in conjunction with herbicides for thistle control. This can be carried out as long as the insect has completed development before the thistles die from herbicide action.
T. horridus is available commercially and is shipped in the adult stage. Collections of the overwintered adults in early spring must be released without delay to allow them to lay their eggs before they die. Adults of the new generation that are collected in the summer will not lay eggs until the following spring. Thus they can be released anytime after collection as they will hibernate and the surviving weevils that successfully overwinter will lay eggs in the spring.
Kok, L. T. 1975. Host specificity studies on Ceuthorhynchidius horridus (Panzer) (Coleoptera: Curculionidae) for the control of musk and plumeless thistle. Weed Research 15:21-25.
Kok, L. T. 1986. Impact of Trichosirocalus horridus (Coleoptera: Curculionidae) on Carduus thistles in pastures. Crop Protection 5:214-217.
Kok, L. T. 1992. Biological control of musk and plumeless thistles. Virginia Coop. Ext. Pub. 444-019:1-8.
Kok, L. T., and Mays, W. T. 1989. Comparison of the seasonal occurrence of Trichosirocalus horridus (Coleoptera: Curculionidae) in Virginia between 1981-83 and 1979. J. Entomol. Sci. 24:465-471.
Kok, L. T. and Mays, W. T. 1991. Successful biological control of plumeless thistle, Carduus acanthoides L. [Campanulatae: Asteraceae (=Compositae)], by Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae) in Virginia. Biological Control 1:197-202.
Trumble, J. T. and Kok, L. T. 1982. Integrated pest management techniques in thistle suppression in pastures of North America. Weed Research 22:345-359.