Larinus planus

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Larinus planus

Larinus planus is a small true weevil (Curculionidae). It is oval-shaped and dark brown to black, about 5–10 mm long, with red-brown antennae. The wing covers have light, mottled hairs and look pock-marked. The nose is long and curved. Eggs are black and very small; the larvae are white with a brown head and are C-shaped.

Native to southwest England and Central/Eastern Europe, it arrived in Canada and the United States in the 1960s, with the first U.S. record in Maryland in 1971. It is now widespread in North America. It is also known as Larinus carlinae.

What it does:
- Feeds on floral buds, mainly of thistles. Adults eat buds; larvae are the most destructive, feeding inside the buds and destroying about 95% of the seed contents.
- Eggs are laid one per floral bud and sealed inside. The larva hatches, stays in the bud, and feeds there.

Life cycle:
- Adults emerge in spring and begin feeding on thistle buds (5–7 mm in size).
- Mating happens when temperatures reach at least 22 °C (72 °F), usually about 14–16 days after emergence.
- Each bud receives one egg; the larva develops inside the bud.
- A second brood appears in late autumn. These adults spend winter in plant litter and reappear in spring.

Habitat and impact:
- Prefers dryer areas but can adapt to wetter places; sunny, well-drained soils are favorable.
- In Canada, used as a biological control agent against Canada thistle (Cirsium arvense).
- In the United States, distributed across the Great Plains. In 2000, it was found feeding on Cirsium undulatum var. tracyi, reducing seed production of this native thistle; it can also negatively affect the native and threatened Cirsium pitcheri.