J.M. DiTomaso, The Regents of the University of California

J.M. DiTomaso, The Regents of the University of California

CANADA THISTLE

APPROVED BIOLOGICAL CONTROL AGENTS


Hadroplontus litura

Hadroplontus litura, or the Canada thistle stem weevil, is a biological control agent that attacks Canada thistle by larval feeding within the stem, opening the plant up for secondary damage from other organisms such as opportunistic pathogens. The adults also feed on the plant foliage in the early spring, although their effect on the plant is limited.

Adult weevils are 3-4 mm long with whitish hairs and a pronounced white T-shaped marking on their back. They overwinter as adults and can typically be found in the field from August until June. Females deposit eggs from March until mid-May, while the plants are still less than 5 cm tall. Each female lays an average of 120 eggs targeting smaller shoots for oviposition.

The larvae mine the tissues of the leaf toward the main vein. Older larvae mine the stem and root crown. Pupation occurs in the soil surrounding the plant and generally last two to three weeks.

For redistribution, adults can be collected in the early spring with forceps, fingers, an aspirator, or a modified vacuum. When releasing the insects at a new location, it is best to release the adults over young plant material similar in size to the plants where the weevil was originally collected.

Urophora cardui

Urophora cardui, or the Canada thistle gall fly, is a biological control agent that attacks Canada thistle by forming a gall structure which acts as a metabolic sink preventing the plant’s nutrients from being allocated to other areas and thereby reducing plant vigor and competitive ability.

Adult flies emerge from galls in late spring to early summer and deposit one to 30 eggs in the vegetative shoots during the plant’s growing season. Larvae spend the first growth stage within the egg. During the second stage larvae initiate gall development by tunneling into the stem. Larvae grow slowly while the gall is developing and then molt to the third growth stage and attain 98% of their body weight as the gall matures. Multiple larvae (3 to 10) can be found within large galls.

Pupation occurs within the gall in early spring. Galls resemble a small green crabapple generally in the middle of the stem and can vary in size, but they are generally marble to walnut-sized.

The fly does best in semi-shaded, moist, disturbed areas with scattered Canada thistle plants. Galls are collected in the fall and placed in cages to protect them from rodent and bird predation in the fall and winter.