Urophora cardui
Canada thistle gall fly
(Diptera: Tephritidae)
Left: Urophora Cardui adult fly
Right: Galls caused by Urophora Cardui larvae on Canada thistle stems.
Photos from: Dr. Alec McClay Alberta Research Council, [website] Crop and Plant Management Unit Biological Control of

[more photo's of adult, larva, and gall] ---Warning, slow download. 
[ gall ]

Origin: Central Europe, (From France east to near the Crimea, Sweden south to the Mediterranean) (5)

Life Cycle: Overwintering: Urophora Cardui overwinters in the third instar, mature larva, within a stem gall on Canada thistle.
Egg Stage: One to thirty eggs are laid in stems during the warm, growing season of the plant.
Larval Stage: First instar larva stay in the egg.  In the second instar eat into the stem and cause the formation of a gall (swelling).  The larva grow during this time.  When the insect goes to the maturation stage, the larva molt and achieve 98% of their body weight.  That is their third instar.  After overwintering the larva pupate and soon appear as a fly inside the gall.
Pupal Stage: The larva pupate in early spring for 24 to 35 days.  They are then reddish-brown in color and stay inside the gall.
Adult Stage: Possibly oxygen stimulates the appearance of the adult fly from the gall during gall deterioration.  The fly enters through a tunnel formed from the gall to the outside.  This happens during June to October.  Adult flies have black W-markings on their wings and are about the size of a house fly. (4-5)

Damage to Host: The destruction to the Canada thistle (Cirsium arvense) happens in the larval stage of the Urophora Cardui.  The area damaged is the stem of the Canada thistle. A gall is formed, size depends on number of larvae within the gall.  The gall is about the size of a large marble, and becomes a woody grey-brown after frost.  It is very hard except when wet.  The stem, flowers, and leaves above the gall are often stunted in growth or absent. (4-5)

Host Impact: The Urophora Cardui lays its eggs on the stem.  The eggs hatch and second instar larvae burrow into the stems.  That causes the gall to form and continue growing.  All the plants nutrients concentrate in the gall and do not go to other places of the plant.  The plant is then too weak to fight off other dangers and has a stunted growth.  The stem is usually too deformed to reproduce seeds.  Therefore, thistle reproduction is stopped or slowed, but Urophora Cardui does not normally kill the plant. (2-5)

Release Habitats: Urophora Cardui should be released where there are no herbicides or pesticides spread. Areas with severe winters and summers cause difficulties for the insect. The ground should be dense with Canada thistle and have a source of moisture for the adult flies such as a lake, river, ditch, or high humidity.  This insect prefers somewhat shaded areas.  Ground areas that will be affected by mowing, flooding, or overgrazing will not be a good release sites. (2-5)
Note: These flies have been released in the Whitehall area several times (in stream side habitat) with no evidence of establishment at this time- 3/2006. T. Breitenfeldt (3)

Collection, Transportation, and Release: Collection happens in fall, winter, or early spring.  Galls are snipped from the stems and are then stored in cardboard boxes or paper sacks to be refrigerated at 39 to 46 degrees Fahrenheit (4 to 8 degrees Celsius).  If the refrigerator is ventilated, the galls should be lightly misted with water every 2 to 4 weeks to maintain a balance of moisture.  Placing galls in cages or sleeve boxes will confine the adult flies coming out so they can be used for a mass release.  Misting galls a maximum of 5 times a day will help with the emergence.  Food for the adult flies is best provided by soaking a sponge in sugar water and setting it in the cage.  This form of nourishment allows the counting and sorting of flies.  Release from 50 to 100 (or more if you have them) flies in local areas with Canada thistle.  The thistles should be misted with water or the flies should be released in cages over the plants.
  Collection of adult flies can be done with a sweep net, but results are poor.  The best time to hunt for them is after a prolonged stormy period in the area of release. (4-5)

Redistribution: Galls can be collected from the infested areas in fall, winter, or early spring, stored, and then released as adult flies (see above).  Transportation should occur only with larvae in galls or adult flies caught with a sweep net. (5)

Purchasing: Contact your local county extension office or weed district. (3)

Remarks: This insect has been approved by the Animal and Plant Health Inspection service.  Study shows that a combined effort of insects, including the Urophora cardui, has reduced thistle by as much as 48%.  The insect is not a threat to non target species, and the return of grasses and forbs has been seen in previously infested areas. (1)

Useful Sites:
1.  http://res2.agr.gc.ca/lethbridge/weedbio/agents/aurocard_e.htm 
2.  http://www.angelfire.com/or2/SEORNoxiousWeeds/urophoracardui.htm  
3.  http://extension.ag.uidaho.edu/minidoka/Stem%20Gall%20Fly%20pictures.htm 
4.  http://www.blackwell-synergy.com/links/doi/10.1046%2Fj.1365-294X.1997.00236.x
5.  http://asuwlink.uwyo.edu/~caps/numbers.htm
6.  http://www.invasiveplants.net/biologicalcontrol/17CanadaThistle.html
7.  http://www.efn.org/~ipmpa/Noxcthis.html
8.  http://www.landcareresearch.co.nz/research/biosecurity/weeds/cal_thistle/CalifornianThistlegallfly.asp
9.  http://res2.agr.ca/lethbridge/weedbio/agents/aurocard_e.htm  
10.  http://www.eap.mcgill.ca/PCBCW_1.htm

 1) Rice, Ross and Roddy, Dan  7/22/99.  [website] Canada Thistle Control By Insects, <>  http://www.aqd.nps.gov/pubs/yr_rvw96/chapter7/thistle.htm.  7/22/99.