Rhinocyllus

Germar, E.F., 1817

thistle head weevil

Species Guides

Rhinocyllus conicus(thistle head weevil)

Rhinocyllus is a small of true weevils (Curculionidae) comprising four described . The genus is notable for its specialized association with thistles in the subtribe Carduinae. The most widely known species, R. conicus, was introduced to North America in the 1960s as a agent for thistles including musk thistle (Carduus nutans), but subsequently revealed significant non-target impacts on native North American thistles, including threatened and . The genus is sister to Bangasternus.

Rhinocyllus by no rights reserved, uploaded by Jesse Rorabaugh. Used under a CC0 license.

Rhinocyllus conicus by (c) RAP, some rights reserved (CC BY), uploaded by RAP. Used under a CC-BY license.

Rhinocyllus conicus by (c) skitterbug, some rights reserved (CC BY), uploaded by skitterbug. Used under a CC-BY license.

Pronunciation

How to pronounce Rhinocyllus: /rɪnoʊˈsɪləs/

These audio files are automatically generated. While they are not always 100% accurate, they are a good starting point.

Identification

Rhinocyllus weevils are distinguished from similar thistle-associated weevils by their relatively short rostrum (snout) compared to related such as Larinus. The genus can be differentiated from its sister group Bangasternus by plant associations and subtle morphological features of the rostrum and antennal insertion. R. conicus specifically is recognized by its small size, dark coloration, and association with developing thistle flower .

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Habitat

Associated with thistle plants (subtribe Carduinae) in agricultural fields, meadows, rangelands, and disturbed areas where thistles occur. are found on leaves and developing flower of host plants.

Distribution

Native to Europe, North Africa, and western Asia. R. conicus was introduced to North America (Canada 1968; USA 1969) and has established throughout much of the United States and Canada. Other occur in their native Palearctic ranges.

Host Associations

Carduus nutans - primary ( musk thistle)target of biocontrol introductions
Carduus acanthoides - spiny plumeless thistle
Carduus thoermeri -
Carduus macrocephalus -
Cirsium - ()includes native North American , some threatened or endangered
Cirsium vinaceum - non-target Sacramento Mountains thistle, federally threatened and endangered
Silybum - ()
Onopordum - ()

Life Cycle

Females deposit on bracts and leaves of developing thistle flower . Larvae hatch and bore into flower heads, consuming reproductive tissues and preventing seed formation. Larval development occurs primarily within primary flowers, with fewer attacks on secondary and later flowers. The weevil overwinters successfully in temperate climates. Each female may lay 100–200 eggs during her one-year lifespan.

Behavior

weevils feed on thistle leaves, causing defoliation. Mating occurs on plants. Females exhibit host recognition mediated by floral volatile organic compounds (VOCs), with electrophysiological response to seven active chemical compounds in floral scent blends. In laboratory tests, females showed indifferent behavioral response when offered VOCs from musk thistle versus threatened native thistle (Cirsium vinaceum), but prefer musk thistle VOCs to purified air and display longer searching time to native thistle VOCs.

Ecological Role

agent for thistles; can reduce invasive thistle by 90% or more within 10 years of introduction. However, significant non-target effects on native thistle populations, including federally listed threatened and , have been documented. Subject to attack by native hymenopteran in introduced range.

Human Relevance

R. conicus was widely introduced for of noxious thistles in North America, beginning in 1968–1969. USDA cancelled all permits for interstate movement in 2000 due to documented non-target impacts on native thistles. The represents a prominent case study in risk assessment and specificity testing limitations. Cannot be recalled once released.

Similar Taxa

Larinus planussimilar thistle-associated weevil introduced for biocontrol; distinguished by longer rostrum and different larval feeding habits (bud vs. flower )
Bangasternussister ; distinguished by morphological features and associations