Rhinocyllus conicus is a native of southern and central Europe, North Africa, and western Asia. This thistle-head weevil was the first insect introduced into North America for the biological control of musk or nodding thistle. It was collected from the Rhine Valley in France and released in Canada in 1968. In 1969, it was introduced into California, Virginia, and Montana. Subsequently, it was relocated from Virginia to other states in the USA.
The adults are dark brown in color and about 10 to 15 mm long. In early spring, they are yellowish brown in color due to the yellow tufts of hair on their wings. As they age, they lose some of these hairs and turn brownish black.
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 weevil is quite host specific and feeds mainly on thistles belonging to the Carduus, Cirsium and Silybum group in the family Asteraceae. None of the economical plants belonging to the same family is attacked by these weevils.
R. conicus overwinters in the adult stage. It becomes active in mid to late April, depending on temperature. Each female lays about 100-200 eggs. Oviposition is well synchronized with musk thistle development. Eggs are laid on the bracts of developing buds and hatch in about 6 days.
Newly hatched larvae feed through the bracts into the buds. The larvae feed on the receptacle and prevent the production of viable seeds. They complete development in 4 to 6 weeks and turn into pupae in the thistle heads. The new adult emerges in 7-10 days. These new adults do not stay on the plants for long. They seek shelter and pass the summer in hiding, and then hibernate in winter. They re-emerge in spring to lay eggs before dying. There is one generation a year.
This thistle-head weevil is very effective in the control of musk or nodding thistle. The first success in biological control of musk thistle was documented in 1975 soon after the weevils were released in Virginia. Typical musk thistle stand reductions of 80-95% occurred in sites where the weevil became established. However, it only provides partial control of plumeless thistle because the egg laying period of the weevil only covers the development of the terminal thistle heads, but not the lateral heads.
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 can be used in conjunction with herbicides for thistle control. Larval development is indirectly affected by herbicides if the thistle dies before larval feeding is completed. Herbicides are best applied after the insect has completed development.
R. conicus is available commercially and is shipped in the adult stage (see the off-site publication, , page of the California Department of Pesticide Regulation website). Collections of the overwintered adults in early spring must be released without delay to allow them to lay their eggs before dying. 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. These weevils will hibernate and the survivors that successfully overwinter will lay eggs in the spring.
Kok, L. T. and Surles, W. W. 1975. Successful biocontrol of musk thistle by an introduced weevil, Rhinocyllus conicus. Environ. Entomol. 4:1025-1027.
Rowe, D. J. and Kok, L. T. 1985. Determination of larval instars, and comparison of field and diet-reared larval stages of Rhinocyllus conicus (Col.: Curculionidae). Virginia J. Sci, 36:277-280.
Smith, L. M., Ravlin, F. W., Kok, L. T., and Mays, W. T. 1984. Seasonal model of the interaction between Rhinocyllus conicus (Coleoptera: Curculionidae) and its weed host, Carduus thoermeri (Campanulatae: Asteraceae). Environ. Entomol. 13:1417-1426.
Surles, W. W. and Kok, L. T. 1977. Ovipositional preference and synchronization of Rhinocyllus conicus with Carduus nutans and C. acanthoides. Environ. Entomol. 6:222-224.
Surles, W. W., Kok, L. T., and Pienkowski, R. L. 1974. Rhinocyllus conicus establishment for biocontrol of thistles in Virginia. Weed Sci. 22:1-3.
Trumble, J. T. and Kok, L. T. 1980. Integration of a thistle-head weevil and herbicide for Carduus thistle control. Prot. Ecol. 2:57-64.