West-Central Forage Association

Canada Thistle Stem-Gall Fly

Gall Fly Adult.jpg


Canada Thistle Stem-Gall Fly

The Canada thistle stem-gall fly is a biological control agent for use in controlling Canada thistle. In 2017 WCFA began facilitating the distribution of these agents to producers throughout the province, bringing them in from Montana.

The stem-gall fly attacks the stem of the thistle plant, boring in and causing the plant to form gall tissue. Females lay their eggs on the apical meristem (tip) of developing shoots in the early summer, and larvae burrow into the shoots. Larval feeding triggers gall formation, which stresses the plant. The gall becomes a nutrient sink, directing nutrients away from the plant’s normal metabolic & reproductive functions, lowering normal plant function and reproduction. Abnormally developed flower heads frequently occur above the gall, resulting in fewer flowers and lowered seed production. Galls vary in size, depending on the number of larvae present within. Galls may range in size from small (marble) to large (walnut/plum), containing anywhere from three or four larvae to upwards of 25 larvae. The flies overwinter in the gall as mature larvae and emerge as adults in the spring (around June) as the gall tissue deteriorates.

Stem-gall flies are typically delivered in mid-June each year, with orders needing to be placed ahead of time. We are not accepting orders for flies at this time. 

For more information contact Jessica Watson, Conservation Agriculture & Extension Program Coordinator at +1.780.621.8670 or conservationag@westcentralforage.com

More information can also be found in the following documents:

Gall Fly Information