Montana Field Guide

Thistle Crown Weevil (Trichosirocalus horridus) is an insect, specifically a type of beetle in the Bark Beetle Family, Curculionidae.

ADULTS: Small, brown, and round-shaped weevils with white mottling. They grow to about 4 mm long. Snouts are long. Thorax has obvious spines. Sources: Winston et al. 2012 and 2016

LARVAE: Creamy white with dark brown head capsules, and grow to 3 mm long. Sources: Winston et al. 2012 and 2016

EGGS: Opaque white and small. Sources: Winston et al. 2012 and 2016

Populations of Thistle Crown Weevil do best in open infestations or of the target weed and, on the edges, where plants do not grow as tall (Winston et al. 2012 and 2016). Thistle Crown Weevil insects do not establish well at high elevations or in marshy habitats (Winston et al. 2012 and 2016).

Populations have failed to establish on Scotch Thistle in the USA (Winston et al. 2016).

HOST PLANTS
Thistle Crown Weevil prefers Musk Thistle over Spiny Plumeless Thistle (Winston et al. 2016).

DAMAGE TO PLANTS [Adapted from Winston et al. 2016]
Thistle Crown Weevil predates on Bull, Canada, Italian, Musk, Spiny Plumeless, and Slenderflower Thistles. The weevil is most effective at controlling non-native thistles when occurring with other biological control agents.

Adult weevils feed in the spring with minimal impacts to the host plant.
Larvae feed on the root crown of the thistle causing the plant to weaken, altering its growth, and reducing seed production.

LIFE CYCLE [Adapted from Winston et al. 2012 and 2016]
Thistle Crown Weevil produces one generation per year. Adults overwinter in soil litter. Adults emerge in the spring to feed on basal leaves (rosettes) of the host plant. On young plants, adults deposit eggs on the lower side of the leaf along the midrib and other primary veins. Larvae hatch and move to the junction of the stem and root to feed on plant tissue. Larvae develop through three larval stages (instars). Pupation occurs in the soil during early summer. Adults emerge during the summer but are inactive until autumn. Adults superficially feed on the host plant’s foliage. for a short time before overwintering in protected places. Adults of the previous year overwinter and live through the next generation of adults born in autumn.

Populations may take three to five years to establish. Once established the weevil disperses successfully on its own.

BIOLOGICAL CONTROL [Adapted from Winston et al. 2012 and 2016; NRCS 2019)
A fair amount of preliminary work and an array of factors must be considered when developing a biocontrol plan for controlling thistles. Readers are encouraged to consult the "Useful Links" and cited literature in this profile.

Thistle Crown Weevil was intentionally released into Canada (1974) and the USA (1975) to control exotic thistles – specifically for Musk Thistle and Spiny Plumeless Thistle (Winston et al. 2012 and 2016). Thistle Crown Weevil naturally spread to feeding on Bull and Slenderflower Thistle species. Later is was re-distributed to Italian and Scotch Thistles in the USA and Bull, Scotch, and March Thistles in Canada. After establishment, it was discovered that Thistle Seedhead Weevil feed on native thistle species. Therefore, interstate transport is not permitted in the USA, and some states have prohibited its redistribution within their boundaries. Where Thistle Crown Weevil is approved, it is imperative to not release the weevil into sites where related or susceptible native thistles occur.

Melissa Maggio-Kassner is the coordinator for the Montana Biological Weed Control Project. She can be reached at (406) 258-4223 or mmaggio@missoulaeduplace.org

Useful Links:
Montana Invasive Species website
Montana Biological Weed Control Coordination Project
Field Guide for Biological Control of Weeds in Montana
Montana Department of Agriculture - Noxious Weeds
Montana Weed Control Association
Montana Weed Control Association - Weed District Contacts
Montana Fish, Wildlife, and Parks - Noxious Weeds
Montana State University Integrated Pest Management Extension
Weed Publications at Montana State University Extension - MontGuides

Thistle Crown Weevil is an introduced biocontrol insect which has been found to feed on and can damage native Cirsium thistles in Montana and elsewhere in the USA (Winston et al. 2012 and 2016).