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· collected 2026-08-19 · by TOI Science Desk
A biological control programme launched in the United States more than five decades ago has become a cautionary example of how attempts to manage invasive species can produce unexpected ecological consequences.
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attempts → launch → consequences
According to a USDA Forest Service assessment of invasive species in the Southwest, the thistle seedhead weevil (Rhinocyllus conicus) was introduced to the US between 1969 and 1972 to help control musk thistle and other invasive thistles.
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weevil → accord → thistle
The insect was expected to weaken the plants by attacking their reproductive structures and reducing seed production.
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insect → expect → production
However, after becoming established and spreading beyond its original release areas, the weevil also began feeding on native thistles.
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weevil → become → thistles
Its impact eventually extended to numerous native species, including endangered native thistles.
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impact → extend → thistles
The case highlights the difficulty of ensuring that a biological control organism remains focused on its intended target once it has been released into a complex natural ecosystem.
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it → highlight → ecosystem
Why musk thistle was targetedMusk thistle (Carduus nutans) is a non-native flowering plant belonging to the sunflower family.
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thistle → flower → family
It can become a serious weed in rangelands, roadsides and disturbed areas because of its ability to spread rapidly and compete with existing vegetation.
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It → become → vegetation
It produces numerous seeds that can be dispersed by wind, water and animals and can also be transported unintentionally by people, vehicles and equipment.
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that → produce → people
Once established across large areas, musk thistle can be difficult to control.
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thistle → establish → areas
This made biological control an attractive option.
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control → make → ?
Instead of repeatedly removing the plants or treating them with herbicides, scientists hoped that a natural enemy could suppress their ability to reproduce.
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enemy → remove → ability
The weevil's intended jobThe thistle seedhead weevil, Rhinocyllus conicus, is native to Europe and was selected because it feeds on thistles.
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it → select → thistles
The insect's life cycle made it particularly useful for the intended purpose.
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it → make → purpose
Adult females lay eggs around developing thistle flower heads.
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females → lay → heads
After hatching, the larvae enter the flower heads and feed on plant tissue and developing seeds.
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larvae → hatch → seeds
This means the insect attacks the plant at an important point in its life cycle.
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insect → mean → cycle
If fewer seeds mature successfully, fewer new plants may be produced in the following generation.
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plants → mature → generation
The US releases began in 1969 and continued through 1972.
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releases → begin → 1972
The objective was to establish populations of the weevil that could naturally suppress invasive thistles over time.
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that → establish → time
Weevil (Representative Image, Canva)
The unexpected spreadThe strategy encountered a major problem after the weevil became established.
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weevil → encounter → problem
Rather than remaining associated only with the invasive thistles that scientists wanted to control, R. conicus began using native thistles as hosts.
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conicus → remain → hosts
As the insect spread from its release locations, it encountered members of the native “Cirsium” genus and attacked their flower heads.
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it → spread → heads
This was particularly concerning because native thistles have an ecological role of their own.
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thistles → have → own
They can provide resources for insects and other wildlife and form part of naturally occurring plant communities.
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They → provide → communities
The problem was therefore not simply that the weevil had found another food source.
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weevil → find → source
It was that an introduced biological control agent was placing additional pressure on native plants that had not been its intended targets.
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that → introduce → plants
Rare species faced another threatThe
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species → face → threatThe
The USDA Forest Service assessment specifically identifies the endangered Sacramento Mountain thistle (Cirsium vinaceum) in southern New Mexico as a native species attacked by the seedhead weevil.
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assessment → identify → weevil
If insects repeatedly consume developing seeds, the number of new plants that can establish may decline.
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that → consume → plants
Scientific research has also documented the weevil's use of native thistles.
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research → document → thistles
According to a study published in Environmental Entomology, researchers noted concerns over the weevil feeding on at least 25 native Cirsium species and found that it could complete its development on two native species under controlled conditions.
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it → accord → conditions
Why seed production mattersThe weevil's impact is closely connected to how thistles reproduce.
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thistles → connect → ?
Flower heads are where seeds develop, so an insect that feeds inside these structures can directly interfere with reproduction.
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that → develop → reproduction
Even if an individual plant survives, losing a substantial portion of its viable seeds can affect how successfully it reproduces.
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it → survive → seeds
Biological control does not necessarily need to kill the target plant to influence its population.
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control → need → population
Reducing reproduction over several generations can gradually weaken a population.
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Reducing → reduce → population
However, this same mechanism becomes a problem when the insect attacks a native species.
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insect → become → species
A rare thistle may already produce relatively few seeds or occupy only a small geographical range.
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thistle → produce → range
Additional seed losses could therefore add pressure to an already vulnerable population.
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losses → add → population
…and 17 more, not listed.
