Rhinocyllus conicus
Encyclopedia
Rhinocyllus conicus is a species of true weevil
. It is best known as a controversial agent of biological pest control
which has been used against noxious thistle
s in the genera Carduus
, Cirsium
, Onopordum
, and Silybum.
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
s of the thistle flower head
. She covers the eggs with masticated plant tissue to protect them from predators. When the white larva
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
and chewed plant tissue on the walls of its chamber, producing a rigid protective shell in which it will pupa
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
, are controlled well by this weevil and its seed head destroying larvae. Some thistles are able to reproduce vegetatively
, 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
, bull
, milk
, and other thistles. However, the weevil will also readily attack native thistles of genus Cirsium, in some cases contributing to population decline. 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.
Curculionidae
Curculionidae is the family of the "true" weevils . It was formerly recognized in 1998 as the largest of any animal family, with over 40,000 species described worldwide at that time...
. It is best known as a controversial agent of biological pest control
Biological pest control
Biological control of pests in agriculture is a method of controlling pests that relies on predation, parasitism, herbivory, or other natural mechanisms...
which has been used against noxious thistle
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 often occur all over the plant – on surfaces such as those of the stem and flat parts of leaves. These are an adaptation that protects the...
s in the genera Carduus
Carduus
Carduus is a genus of about 90 species of thistles in the family Asteraceae, native to Europe, Asia and Africa. Carduus is Latin for a thistle....
, Cirsium
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 accurately known as Plume thistles. These differ from other thistle genera in having feathered hairs to their achenes...
, Onopordum
Onopordum
Onopordum L. is a genus of about 40 species of thistles belonging to the family Asteraceae, native to Europe , northern Africa, the Canary Islands, the Caucasus, and southwest and central Asia...
, and Silybum.
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
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 often different from foliage leaves. They may be smaller, larger, or of a different color, shape, or texture...
s of the thistle flower head
Head (botany)
The capitulum is considered the most derived form of inflorescence. Flower heads found outside Asteraceae show lesser degrees of specialization....
. She covers the eggs with masticated plant tissue to protect them from predators. When the white larva
Larva
A larva 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...
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
Feces
Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...
and chewed plant tissue on the walls of its chamber, producing a rigid protective shell in which it will pupa
Pupa
A pupa is the life stage of some insects undergoing transformation. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, going through four life stages; embryo, larva, pupa and imago...
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
Musk thistle
The Musk thistle or Nodding thistle is a member of the sunflower family Asteraceae. It is a biennial herb with showy red-purple flowers and sharply spiny stems and leaves...
, are controlled well by this weevil and its seed head destroying larvae. Some thistles are able to reproduce vegetatively
Vegetative reproduction
Vegetative reproduction is a form of asexual reproduction in plants. It is a process by which new individuals arise without production of seeds or spores...
, 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
Carduus acanthoides
Carduus acanthoides is a species of thistle known as the spiny plumeless thistle and welted thistle. This plant is native to Europe but is well-known in many other parts of the world, including parts of North and South America, New Zealand, and Australia, as a noxious weed. This thistle may exceed...
, Italian
Carduus pycnocephalus
Carduus pycnocephalus is a species of thistle native to the Mediterranean region of Europe and Asia which has become a noxious weed in Australia, New Zealand, South Africa, South America, and North America, especially in California, where it is a C-listed weed by...
, bull
Cirsium vulgare
Cirsium vulgare is a species of the genus Cirsium, native throughout most of Europe , western Asia , and northwestern Africa...
, milk
Milk thistle
The milk thistle is a thistle of the genus Silybum Adans., a flowering plant of the daisy family . They are native to the Mediterranean regions of Europe, North Africa and the Middle East...
, and other thistles. However, the weevil will also readily attack native thistles of genus Cirsium, in some cases contributing to population decline. 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.