Larinus planus

The adult weevil is mostly dark brown or black, but has red-brown antennae. It is about 5-10 millimeters long and oval shaped. The wing covers have light colored mottled hairs on them and appear pock-marked. The nose is long and narrow, with a pronounced curvature.[5][6]

The eggs are black and appear the size of a "pin-prick." Once hatched, the larva has a segmented white body with a brown head, and is "C" shaped.[1]

This weevil is indigenous to southwest England, as well as Central and Eastern Europe.[5][1]

It was accidentally introduced to Canada and the United States in the 1960s. The first published record is from Maryland in 1971.[7] It is now widespread throughout North America.

Adults emerge in the spring, and begin feeding on thistle buds. They require buds ranging in size from 5-7mm. The insects mate when the temperature is at least 22 °C (72 °F), which is usually about 14-16 days after they emerge. The female deposits a single egg in each floral bud, and then seals it. After hatching, the larva feeds on the floral bud it is hatched in. In doing so, it destroys about 95% of the seed contents. Another brood of adults emerges in late Autumn. These adults spend the winter in plant litter, to emerge in the spring.[1]

Larinus planus prefers dryer ranges, but can also adapt to wetter locations. It does well in sunny areas with well drained soils.[1]

In Canada, it has been used as an agent of biological pest control against the noxious weed Canada thistle (Cirsium arvense). It was subsequently evaluated in the United States as a biocontrol agent and distributed across a range of the Great Plains in the western United States.[8][9]

In 2000, it was found feeding on Cirsium undulatum var. tracyi, a sparsely distributed thistle native to western Colorado and eastern Utah. Subsequent research determined that the weevil reduced seed production of the native thistle.[9] It has also been found to have an adverse effect on the native and threatened Cirsium pitcheri.[10]