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Spear, variegated and nodding thistle suppression with the thistle receptacle weevil | LC0148 |
Keith Turnbull Research Institute, Frankston
December, 1998 |  |
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Common name
Thistle receptacle weevil
Scientific names
Rhinocyllus conicus (Froelich)
Family Curculionidae, weevils
Background
Two strains of thistle receptacle weevil have been released in Victoria. One strain from western France is adapted to the life cycle of spear thistle, Cirsium vulgare, while another strain from southern France is adapted to the life cycle of variegated thistle, Silybum marianum. A third strain, adapted to nodding thistle, Carduus nutans, has been released in New South Wales by CSIRO. Each strain does most damage to its host species of thistle. The weevil has been tested to ensure that it presents no danger to native plants or plants of economic importance.
Description
Adults - dark brown, speckled with ginger-brown when newly emerged; yellow-green when sexually mature. Cylindrical, 4 to 6 mm long with a pronounced snout. Visible on the leaves and flower heads of the thistles, especially on warm days (Figs. 1 and 2).
Eggs - spherical, 1 mm in diameter, covered by a distinctive brown cap of frass and chewed plant matter.
Larvae - fat, white, legless, up to 5 mm long, C-shaped; found inside the flower bud (Fig. 3).
Pupae - 6 mm by 3 mm, in hard black cells inside the flower receptacle.
Life cycle
There is only one generation per year. The adults emerge from their winter dormancy in spring and start feeding on thistle leaves and flowering stems. When sexually mature they develop a light yellow-green colour.
Females lay eggs on the underside of young thistle flower heads, often near the stalk, between mid spring and late summer. Eggs are laid singly and covered with a brown capsule of chewed plant material and frass (droppings), which prevents them drying out. Many eggs can be deposited on a single flower head. Each female is capable of laying over 100 eggs.
Figure 1. Adult thistle receptacle weevil.
Figure 2. Adult thistle receptacle weevil on an unopened flower head of spear thistle.
Larvae hatch from the eggs within 7 days and burrow into the developing bud until they reach the receptacle, the fleshy part of the flower that supports the developing seeds. Larvae feed continually for 3 to 4 weeks, then prepare a chamber in the receptacle in which to pupate.
Pupal development lasts up to 2 weeks. In mid to late summer the next generation of adults emerge, and seek overwintering sites. A proportion of the adult weevils remain in the flower heads. The remainder seek shelter elsewhere, possibly in the ground litter for the period of winter dormancy.
Figure 3. Larva of the receptacle weevil in a spear thistle flower head. In this example larval feeding has destroyed the developing seeds, leaving only sterile pappus.
Impact
Adult feeding causes only minor damage to bracts around the flower head. However larval feeding in the flower buds restricts the passage of nutrients to the seeds and can completely prevent their development. A large population of weevils will reduce seed production and in turn slow down the spread and the density of thistle infestations. This process will take many years because there is likely to be a large bank of seeds already in the soil. These seeds will continue to germinate.
The weevil survives hot, dry summers particularly well, making it suited to lower rainfall regions of Victoria. The spear thistle strain has established at numerous sites around Victoria.
Releases
Weevils are released at 'nursery sites', where populations are allowed to build up for re-distribution to other sites. Nursery sites are selected by NRE staff in consultation with landholders and local Landcare groups.
Integrated control
The spear thistle gall fly, Urophora stylata, has also been released to aid in the biological control of spear thistle in Victoria. Both the gall fly and the weevil have a direct effect on thistle seed production. It is expected that the complementary effects of these agents will increase pressure on the plants making them less competitive.
Nodding thistle is a State Prohibited Weed in Victoria and the small patches which occur are actively eradicated, but it is an important weed in New South Wales where the rosette weevil, Trichosirocallus horridus, and the nodding thistle gall fly, Urophora solstitialis, have been released.
Biological control cannot totally eradicate a weed but can reduce the spread and density of infestations. In some cases, control is achieved to the level where the weed is no longer of concern and no other control is necessary. More commonly other methods are still required to achieve the desired level of control; however these need not be applied so frequently. For this reason, biological control should not be considered the complete answer to a spear thistle problem. It is a technique that should be used in conjunction with other control measures in an integrated management plan.
Further information
For further information on the management of thistles refer to Landcare Notes in the Pest Plant series on Spear thistle and Variegated thistle and in the State Prohibited Weeds series for Nodding thistle. Additional information on the biological control of spear thistle is available in the Landcare Note Spear thistle suppression with the spear thistle gall fly.
For further information on the biological control of nodding, spear and variegated thistles contact:
Keith Turnbull Research Institute,
PO Box 48, Frankston, Victoria, 3199.
Tel (03) 9785 0111
Fax (03) 9785 2007
Acknowledgments
Prepared by El Bruzzese, Peter Stevens, Brad Roberts, Ian Faithfull and Nicole Freeman, November 1996. Revised December 1998.
Funding for the thistle biological control program has been provided by NRE’s Catchment Management and Sustainable Agriculture Division, The Woolmark Company and Meat and Livestock Australia. The biological control of thistles program is supported by the Cooperative Research Centre for Weed Management Systems.
This note replaces note number BC0014
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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