Spear Thistle (Cirsium vulgare)

Spear thistles are found in most areas of the Meander Valley Region, although not considered a priority weed or a weed of concern, very heavy investations occur in pastoral areas in some seasons, depending on rainfall distribution, grazing pressure and soil fertility status.

Heavy thistle infestations in pasture can virtually exclude grazing stock. Under such conditions pasture grasses may become rank, predisposing the area to damage by insect pests. The large rosette also suppresses pasture growth and competes for moisture and nutrients.

Thistle spines in the fleece of sheep grazing infested areas cause difficulties at shearing.

Spear thistle reproduces from seed with a pappus or 'parachute' of long hairs which is capable of carrying it over very long distances. However, the pappus seperates readily from the seed, allowing most to fall within a few feet of the plant and leaving the 'thistle down' to float away empty. The majority of seeds are dispersed by machinery and vehicles, internally and externally by livestock, by run-off after heavy rains and very frequently from feeding out contaminated fodder to livestock.

Seed production is prolific, most seed heads producing about 100 seeds. Up to 300 seeds per head have been recorded and large plants have been known to produce a total of some 4000 seeds.

Spear thistle is a declared Secondary Weed under the Tasmanian Noxious Weeds Act of 1964.

Rosette Weevil (Trichosirocalus horridus)

This winter-active weevil, which produces many hundreds of eggs per female, has great potential as a control agent for spear thistle.

Damage to the plant is caused by larvae feeding on the rosette. The eggs are laid in pockets within the midrib on the underside of the rosette leaf. Once hatched the larvae mine down the midrib into the crown of the rosette to feed, damaging the central growing point of the plant. Black discharge from this area of the rosette is the first sign that the plant is heavily infested with Trichosirocalus horridus larvae.

Fully fed larvae leave the plant to pupate in the soil. In spring newly emerged adults feed extensively for several weeks before seeking shelter in the ground litter. They remain in a state of rest for the majority of the summer.

Field trials, conducted in New South Wales, to date have shown a reduction of 50% in rosette diameter and 80% reduction in seed set.

Enough rosette weevils (150) for one release site, were obtained from the CSIRO and released within the Meander Valley Region in June, 1998. Continual monitoring of this release site will be undertaken to assertain establisment of this rosette weevil. Once the weevil has established and reproduced in sufficient numbers, collection will be undertaken for release in other areas.