''Rhinocyllus conicus'' is a species of

true weevil True most commonly refers to truth, the state of being in congruence with fact or reality. True may also refer to: Places * True, West Virginia, an unincorporated community in the United States * True, Wisconsin, a town in the United States * T ...

. It is best known as a controversial agent of

biological pest control Biological control or biocontrol is a method of controlling pests, such as insects, mites, weeds, and plant diseases, using other organisms. It relies on predation, parasitism, herbivory, or other natural mechanisms, but typically also invo ...

which has been used against noxious

thistle Thistle is the common name of a group of flowering plants characterised by leaves with sharp prickles on the margins, mostly in the family Asteraceae. Prickles can also occur all over the planton the stem and on the flat parts of the leaves. ...

s in the genera ''

Carduus ''Carduus'' is a genus of flowering plants in the family Asteraceae, and the tribe Cardueae, one of two genera considered to be true thistles, the other being ''Cirsium''. Plants of the genus are known commonly as plumeless thistles.

'', ''

Cirsium ''Cirsium'' is a genus of perennial and biennial flowering plants in the Asteraceae, one of several genera known commonly as thistles. They are more precisely known as plume thistles. These differ from other thistle genera ('' Carduus'', ''Silyb ...

'', ''

Onopordum ''Onopordum'', or cottonthistle, is a genus of plants in the tribe Cardueae within the family Asteraceae. They are native to southern Europe, northern Africa, the Canary Islands, the Caucasus, and southwest and central Asia. They grow on disturb ...

'', and ''

Silybum ''Silybum'' (milk thistle) is a genus of two species of thistles in the family Asteraceae. The plants are native to the Mediterranean regions of Europe, North Africa, and the Middle East. One species has been introduced elsewhere, including in No ...

''. The adult weevil is black and covered in a thin black and yellowish mottled coat of hairs. It is a short-snouted beetle up to 6 millimeters in total body length. The female lays over 100 eggs on or near the

bract In botany, a bract is a modified or specialized leaf, especially one associated with a reproductive structure such as a flower, inflorescence axis or cone scale. Bracts are usually different from foliage leaves. They may be smaller, larger, or of ...

s of the thistle

flower head A pseudanthium (Greek for "false flower"; ) is an inflorescence that resembles a flower. The word is sometimes used for other structures that are neither a true flower nor a true inflorescence. Examples of pseudanthia include flower heads, compos ...

. She covers the eggs with masticated plant tissue to protect them from predators. When the white

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

emerges from its egg it burrows into the flower head and feeds on the flower parts and developing seeds. As it grows it deposits

frass Frass refers loosely to the more or less solid excreta of insects, and to certain other related matter. Definition and etymology ''Frass'' is an informal term and accordingly it is variously used and variously defined. It is derived from the ...

and chewed plant tissue on the walls of its chamber, producing a rigid protective shell in which it will

pupa A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their ...

te. Pupation takes up to two weeks and when the weevil emerges as an adult it remains inside the chamber for a few more weeks before tunneling out of the plant. Damage to the plant occurs mainly from larval destruction of the flower head, which prevents seed production. Some larvae tunnel through the upper stem instead of chambering in a flower head; this can also be destructive to the plant. Adults do some damage as well when they feed on the foliage. Thistles which reproduce only via seed, such as

musk thistle ''Carduus nutans'', with the common names musk thistle, nodding thistle, and nodding plumeless thistle, is a biennial plant in the daisy and sunflower family Asteraceae. It is native to regions of Eurasia. Description ''Carduus nutans'' is usua ...

, are controlled well by this weevil and its seed head destroying larvae. Some thistles are able to

reproduce vegetatively Vegetative reproduction (also known as vegetative propagation, vegetative multiplication or cloning) is any form of asexual reproduction occurring in plants in which a new plant grows from a fragment or cutting of the parent plant or speci ...

, and while they are impacted as well, they can sometimes survive. This weevil is native to Eurasia and North Africa. It was first introduced to the United States for thistle biocontrol in 1969, and it is now widely established in that country. The weevil was found to be very effective in reducing the spread of invasive thistles, particularly musk thistle, but also welted,

Italian Italian(s) may refer to: * Anything of, from, or related to the people of Italy over the centuries ** Italians, an ethnic group or simply a citizen of the Italian Republic or Italian Kingdom ** Italian language, a Romance language *** Regional Ita ...

bull A bull is an intact (i.e., not castrated) adult male of the species ''Bos taurus'' (cattle). More muscular and aggressive than the females of the same species (i.e., cows), bulls have long been an important symbol in many religions, includin ...

milk Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digestion, digest solid food. Immune factors and immune ...

, and other thistles. However, the weevil will also readily attack native thistles of genus ''Cirsium'', in some cases contributing to population decline.Rose, K. E., S. M. Louda, and M. Rees. 2005. Demographic and evolutionary impacts of native and invasive insect herbivores: a case study with Platte thistle, ''Cirsium canescens.'' Ecology 86: 453-465. It is for this reason that this weevil is no longer recommended for distribution as a biocontrol agent. Further releases of the weevil are prohibited in many areas.

References

* Coombs, E. M., et al., Eds. (2004). ''Biological Control of Invasive Plants in the United States''. Corvallis: Oregon State University Press, 365.

External links

TAMU Biocontrol Profile

Lixinae Insects used for control of invasive plants Biological pest control beetles Beetles described in 1792 {{Curculionidae-stub