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.

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.


Important Program Update

After a careful review of our biological control agent for Canada thistle program, WCFA has made the decision to discontinue this program for the time being. This means WCFA will no longer be importing and delivering Canada thistle stem-mining weevils, or stem-gall flies for the foreseeable future.

We appreciate the many of you who have supported this program in the past, and we apologize for any inconvenience this decision may cause. Please see below for resources related to the insects should you wish to pursue obtaining some yourself.

Stem-Gall Fly Suppliers

Import Information