Canada thistle bud weevil

Canada thistle bud weevil is a common, exotic, small, true weevil. It is native to Europe and western Asia. It is believed to have been accidentally introduced into North America before 1968. It now occurs across most of southern Canada and the northern United States, but it is absent from the Great Plains.

Canada thistle bud weevil was first reported in Maryland in 1971. Since it was already established in the United States, it was considered adventive, and no permit was required to redistribute it. It was intentionally redistributed as a biocontrol agent in several states in an attempt to control invasive Canada thistle. Unfortunately, it also attacks native thistles, including field thistle and swamp thistle. In Wisconsin, it has caused a moderate reduction in population growth of pasture thistle, and it is considered a serious pest of this already imperiled plant species.

The larvae live in the flower heads of thistles and feed on the developing seeds. Adults are active from May to August wherever their host species are found. In the spring, they feed on tender developing leaves and shoots.

Adults are 3⁄16″ to ⅜″ (5.0 to 9.5 mm) in length. The body is broadly oval, dark brown or black, and covered with short grayish hairs. Recently emerged adults are covered with yellow wax.

The head is small, wider than long, and weakly convex. The surface is finely wrinkled. The mouthparts are greatly elongated into a snout-like projection of the head (rostrum). The rostrum is parallel sided when viewed from above, slightly curved when viewed from the side. It is longer on the female than on the male. The antennae are short and bent (elbowed), and they have 11 segments. They are attached to the sides of the rostrum beyond the middle. The first segment is very long, and it is gradually thickened toward the end. The last three segments are expanded, forming a compact, elongated-oval club.

The plate covering the first segment of the thorax (pronotum) is oval, wider than long, broadest at the rear, and narrowed to the front. The sides are evenly convex. The surface is densely wrinkled. The rear margin is strongly convex on each side, forming a long, triangular, rearward-pointing extension in the middle. The plate between the wing bases (scutellum) is short and wide.

The wing covers (elytra) are broadest behind the shoulders, parallel sided on the front half, weakly convex on the rear half, and broadly rounded at the tip. There are numerous scattered patches of grayish-white hairs, making the elytra appear pock marked. Each elytron has several narrow longitudinal grooves. The grooves are shallow, but they have a row of deep pits (punctate). The spaces between the grooves have crosswise wrinkles.

The legs are entirely dark brown. The last part of each leg (tarsus), corresponding to the foot, has five segments, but the fourth segment is small and hidden between the lobes of the third segment, making the tarsus appear to have just four segments. There is a pair of claws at the end of each tarsus.

Total length: 3⁄16″ to ⅜″ (5.0 to 9.5 mm)

One generation per year: May to August

Overwintering adults become active in late May. After mating, the female seeks a flower bud between 3⁄16″ to ¼″ (5 to 7 mm) in diameter, chews a hole into it, inserts a single egg, and seals it with fecal matter. When the egg hatches, the larvae feeds, completes its development, and pupates while still in the bud. Adults of the new generation emerge in August and September. They overwinter on the ground in leaf litter.

Developing thistle seeds

Leaves and young shoots of the host plant

Sources

22, 29, 30, 82, 83.

Common

Until recently, this species was treated as Larinus planus, and Larinus carlinae was treated as a synonym. Following a review of the subfamily Lixinae (Gültekin, 2006), the species were separated. The name Larinus planus was applied to the well-known Eurasian species previously called Larinus rusticanus, and the name Larinus carlinae was applied to the well-known Eurasian species that was also introduced into North America. Since the name Larinus planus is still in use for another species, it is not treated as a synonym for Larinus carlinae. However, all historical references to Larinus planus in North America should be treated as Larinus carlinae.

Not all sources have accepted the separation. Some sources continue to use the name Larinus planus and treat Larinus carlinae as a synonym.

Larinus carlinae
Jon Moore

Larinus carlinae couple (2)
DocBebitte

Wollige distelsnuitkever Larinus planus, op akkerdistel (Woolly thistle weevil Larinus planus, on field thistle)
Roy Kleukers

Alfredo Colon
8/11/2022

Location: Albany, NY

Alfredo Colon
8/2/2022

Location: Albany, NY