West-Central Forage Association

Canada Thistle Stem-Gall Flies



Gall Fly Adult.jpg

Canada Thistle Stem-Gall Flies

The Canada thistle stem-gall fly is 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.

Ordering for these agents occurs in October and November, and they are delivered in June of the following year (approximately).

For more information contact Jessica Watson, Conservation Agriculture and Extension Program Manager at +1.780.727.4424 or conservationag@westcentralforage.com

More information can also be found in the following documents:

Gall Fly Information