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How to identify it
- Adults are 4 to 5 mm long, mottled brown, with a large bent snout. They can be found all over knapweed plants in spring and summer but most often around the flower heads.[1]
- The two species look so alike that they cannot be separated by eye, and many releases have probably contained both. As a rule the lesser knapweed flower weevil is the one on diffuse knapweed and at hotter, drier sites; the blunt knapweed flower weevil favors spotted and meadow knapweed and cooler, moister sites.[1]
- Larvae are white, C-shaped grubs with a brown head, about 8 mm long, hidden inside the seed heads; the white 6 mm pupae turn brown before the adult emerges. A knapweed head with a round, open chamber of chewed seeds and fluff inside it has already produced a weevil.[1]
- Eggs are elongate and yellow, laid in clusters among the fluffy pappus hairs inside the flower head, where they are hard to see.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Bangasternus fausti | Another introduced weevil whose larvae feed in knapweed seed heads; the larvae are hard to tell apart, but adult B. fausti are smaller with a much shorter, blunter snout.[1] |
| Knapweed root weevil (Cyphocleonus achates) | A much bigger weevil, 13 to 15 mm, mottled gray-brown and white with a short thick snout, whose larvae live in the roots rather than the flower heads.[1] |
Where it lives
Both are native to Europe and the Middle East. The lesser knapweed flower weevil was released on diffuse and spotted knapweed in central and western states from 1991 and the blunt knapweed flower weevil from 1992, with later moves to squarrose and meadow knapweed. In the United States the lesser knapweed flower weevil is established on diffuse, spotted, squarrose and meadow knapweed and the blunt on spotted, diffuse and meadow knapweed; in Canada both occur in British Columbia and the lesser species also in Alberta. In Minnesota the two are collected from established sites and released at new ones.[1][2]
Biology and life cycle
Adults overwinter in litter at the base of the plants and come out through the summer, mating continuously and feeding on knapweed leaves, outer stem tissue and flowers before laying eggs. A female lays 28 to 130 eggs, up to five per flower head.[1]
Larvae hatch in about three days, first eating the pappus hairs and then the seeds and the base of the flower head, feeding through the whole flowering season and passing through three stages in about four weeks. How many larvae fit in a head depends on the head's size and the knapweed species.[1]
Each larva pupates in a chamber built from chewed seeds and pappus hairs inside the head. The new adult chews its way out, leaving the chamber open, feeds on foliage and florets, and moves down to overwinter at the plant base. There is one generation a year.[1]
The lesser knapweed flower weevil wants hot, dry sites with dense knapweed, little competing vegetation and well-drained coarse soil; compacted ground, especially where livestock graze during bolting, does not suit it, and it reportedly does poorly where it rains year-round. The blunt knapweed flower weevil takes cooler, moister sites, often near water, but dense competing vegetation may discourage it. Rodents eat large numbers of both weevils at some sites.[1]
Damage and signs
- Larval feeding destroys much of the seed in each attacked head, slowing the spread of knapweed. Adult defoliation can be severe enough to stunt and even kill plants.[1][2]
- In western Montana, where five seedhead agents became established between 1973 and 1992, seed-head insects cut spotted knapweed seed production per head by about 84 percent in a 2006 experiment, and at two long-term sites the reproductive potential (seeds per square meter) was 96 to 99 percent lower in 2004-05 than in 1974-75. Seedhead flies alone had reduced seeds per head by roughly a third to a half; adding the Larinus weevils pushed the reduction to 84 to 90 percent.[3]
- In Arkansas the lesser knapweed flower weevil was found in about 36 percent of spotted knapweed heads over three years and reduced seed in a head more effectively than the seedhead fly Urophora quadrifasciata; heads holding both agents produced no less seed than heads with the weevil alone.[4]
- The lesser knapweed flower weevil causes widespread declines in diffuse knapweed density across its range, and on squarrose knapweed in California, with Bangasternus fausti, it cut seed production 99 percent so that the weed is no longer of economic concern there. On spotted and meadow knapweed its populations build more slowly and the effect is variable; the blunt knapweed flower weevil has moderate impact on spotted knapweed and, with the other seedhead agents, cuts seed production substantially but often not enough to reduce plant density. Knapweed density may not fall until the seed bank drops below a critical level.[1][3]
- No nontarget effects have been reported.[1]
Health risk
These are plant-feeding weevils and weed biological control agents; none of these sources describes any risk to people.[1][2]
Control
Prevention
Match the weevil to the site: hot, dry, well-drained ground with dense knapweed for the lesser knapweed flower weevil, cooler and moister ground for the blunt knapweed flower weevil. Avoid releasing where soil is compacted or where livestock trample the stand while it bolts.[1]
Biological control
The weevils are the control. Minnesota's program pairs seedhead weevils with the knapweed root weevil, collecting both from established sites and releasing them at new ones; many sites are considered controlled, but large ones have taken up to a decade.[2]
Common questions
- How do I know the seedhead weevils are present?
- Look for the small mottled brown, long-snouted adults around the flower heads in summer, or open some old heads: a round pupal chamber of chewed seed and fluff, or an emergence hole, means a weevil developed there.[1]
- Which species do I have?
- Probably both. The two cannot be told apart by eye and most releases carried a mix. On diffuse knapweed at a hot, dry site it is more likely the lesser knapweed flower weevil; on spotted or meadow knapweed at a cool, moist site, the blunt one.[1]
- The weevils are everywhere but the knapweed is still thick. Why?
- Seedhead agents cut seed production, not standing plants, and the Montana work shows density may not drop until the seed bank is exhausted. Pairing them with the root weevil, which kills established plants, is how Minnesota and other programs use them.[3][2][1]
Sources
- North American Invasive Species Management Association (PDF hosted on Bugwood), “Knapweed Biocontrol Agents: History and Ecology in North America. In: Winston, ed., Biological Control of Weeds in North America, NAISMA-BCW-2023-34-KNAPWEEDS-A (2023).” bugwoodcloud.org/resource/files/27451.pdf (accessed 2026-11-10)
- Minnesota Department of Agriculture and Minnesota Department of Natural Resources, “Biological Control of Invasive Plants in Minnesota (Chandler, Skinner and Van Riper).” files.dnr.state.mn.us/natural_resources/invasives/biocontrolofplants.pdf (accessed 2026-11-10)
- Environmental Entomology (Entomological Society of America), “Story, Smith, Corn and White (Montana Agricultural Experiment Station). Influence of seed head-attacking biological control agents on spotted knapweed reproductive potential in western Montana over a 30-year period. Environmental Entomology 37: 510 (2008), doi:10.1603/0046-225X(2008)37[510:IOSHBC]2.0.CO;2.” pubmed.ncbi.nlm.nih.gov/18419924/ (accessed 2026-11-10)
- Environmental Entomology (Entomological Society of America), “Minteer (USDA-ARS), Kring and Wiedenmann (University of Arkansas). Larinus minutus (Coleoptera: Curculionidae) and Urophora quadrifasciata (Diptera: Tephritidae), evidence for interaction and impact on spotted knapweed in Arkansas. Environmental Entomology (2016), doi:10.1093/ee/nvw011.” doi.org/10.1093/ee/nvw011 (accessed 2026-11-10)
- iNaturalist, “Larinus minutus taxon page (order and family).” www.inaturalist.org/taxa/455901 (accessed 2026-11-10)