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Spear thistle suppression with the spear thistle gall fly | LC0147 |
Keith Turnbull Research Institute, Frankston
November, 1998 |  |
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Common name
Spear thistle gall fly
Scientific Names
Urophora stylata (Fabricius)
Family Tephritidae, fruit flies
Background
In Victoria spear thistle, Cirsium vulgare, is a Regionally Controlled Weed in the Glenelg, Corangamite, Port Phillip East, North East, West Gippsland and East Gippsland Catchment and Land Protection Regions. The spear thistle gall fly is common in Europe and parts of western Asia where the weed originated. The gall fly strain imported to Australia is native to western France. The fly has been tested to ensure that it is specific to spear thistle and presents no danger to native plants or plants of economic importance.
Description
Adults - 4 to 8 mm long; mainly black with a pale yellow-green head and bright red eyes. The wings are distinctively patterned with two interrupted darker bans. Females (Figure 1) have an elongate, tapered ovipositor at the tip of the abdomen. Males (Figure 2) lack the ovipositor and have a rounded abdominal tip.
Figure 1. Female spear thistle gall fly.
Figure 2. Male spear thistle gall fly.
Eggs - white, elongate and curved, approximately 0.7 mm by 0.2 mm.
Larvae - legless maggots; pale yellow, approximately 4.0 by 2.5 mm when fully developed.
Pupae - brown, cylindrical, fat and mummy-like, 4.0 by 1.5 mm.
Life cycle
Female flies lay eggs in small batches into unopened flower buds. The distinctive shapes of the spear thistle floret and spiny bracts stimulate egg laying. Larvae hatch from the eggs in 5 to 8 days and eat into the florets, burrowing down into the developing tissues at the base of the flower head (the receptacle).
When a larva has fed on a developing seed, an abnormal localised swelling starts to form around it. This tissue becomes a gall. Usually there are several larvae within the same flower head and tissue forms around each one separately. Eventually the tissues merge to form a hard, multi-chambered, woody gall. The larvae feed on the soft tissue lining the inside of the gall and mature by mid summer.
An early partial generation of adults may emerge to lay eggs into flower heads which develop late in the season, but the majority of maggots hibernate inside the gall over winter, and pupate when temperatures rise and day length increases in late spring.
Flies mostly emerge from the galls in December. Males emerge before females and establish a territory around selected flower heads, which they defend against other males. Mating commences one or two days after the females emerge and occurs frequently.
Impact
The larvae feed on the developing seed tissues of spear thistle. The plant reacts by forming a woody gall within the flowering head. Gall formation diverts energy from the developing seed into growth of gall tissue. Many seeds are locked up within the gall and cannot disperse. A large population of flies reduces seed production and in turn slows down the spread of the thistle and decrease the density of infestations.
Figure 3. Female spear thistle gall fly.
Figure 4. Woody gall formed in the flower head of spear thistle around gall fly larvae.
Releases
Adult gall flies are released into field cages at ‘nursery sites’ or free released. Populations at nursery sites are allowed to build up until sufficient numbers are available to be harvested for distribution to other sites. A proportion of the galled heads are harvested during autumn and stored over winter. The cage is moved to a new site the following season and the harvested heads are placed inside to establish a new infestation. Emerging flies are allowed to disperse from the initial nursery site.
Local Department of Natural Resources and Environment staff and KTRI officers select the nursery sites in consultation with landholders and landcare groups. The gall fly does not tolerate prolonged periods of extreme heat and is therefore better suited to areas with cooler climates. It has been successfully established at a number of sites in Victoria.
Integrated control
The thistle receptacle weevil, Rhinocyllus conicus, has also been widely released in Victoria to aid in the biological control of spear thistle. Both species have a direct effect on seed production. The crown-boring weevil, Trichosirocalus horridus, is also being released in Victoria, in cooperation with CSIRO. It is expected that the complimentary effects of these agents will increase pressure on the plants, making them less competitive.
Biological control cannot totally eradicate a weed, it can only 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, although they do not need to be applied so frequently. Biological control is not the complete answer to a spear thistle problem, but can be used in conjunction with other control measures in an integrated management plant.
Further information
For further information on the management of spear thistles please refer to the Landcare Note Spear thistle. For further information on the biological control of spear thistle refer to the Biological Control Landcare Note Spear, variegated and nodding thistle suppression with the thistle receptacle weevil, or contact:
Keith Turnbull Research Institute,
PO Box 48, Frankston, Victoria, 3199.
Tel (03) 9785 0111
Fax (03) 9785 2007
Acknowledgments
Prepared by E. Bruzzese, P. Stevens, B.Roberts, I.Faithfull and N. Freeman, 1996. Revised November 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 BC0013.
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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