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Flat-headed cane borers

Flat headed cane borers lay their eggs in raspberry primocanes or first year canes that sprout in spring. There are two species: the red-necked cane borer (Agrilus ruficollis), and the bronze cane borer (Agrilus rubicola). Although cane borers are common throughout Minnesota, they cause minor damage in home gardens.


Cane borer adults are small beetles that are 1/4 inch long and narrow. The red necked cane borer is all black except for a reddish thorax. The bronzed cane borer is the same size, but with a coppery color. The white, legless larvae live in the raspberry canes, and are up to 1/2 inch long and have flattened bodies. However, cane borer adults and larvae can be difficult to find in raspberry patches.

Swollen cane or gall with cane borer in late August.

Thaddeus McCamant, Central Lakes College


When larvae hatch, they burrow through the cane just below the bark, causing the cane to develop a distinct swelling or gall one to three feet above the ground. On primocanes the swellings can be seen starting in August. Damaged canes often occur in groups, with multiple canes in one part of the patch having galls and other parts of the patch with no galls.

red stems with sections that are slightly swollen

Raspberry canes with galls from cane borers in May. Typically the cane dies above the gall.

Thaddeus McCamant, Central Lakes College

The cane above the borer usually dies after the winter, while the cane below the swelling remains healthy. Damage from cane borers tends to be minor. In many cases, the lower part of the cane will overcompensate for the loss of the top of the cane, and the cane will still produce over 50% of a normal crop. In most raspberry patches, less than 10% of the canes are affected by flat-headed borers.

Important biology

Flat-headed cane borer adults emerge in June and July. They feed on raspberry leaves for a few days before laying single eggs on the bark of the primocanes. After hatching, the larvae bore small tunnels that spiral through the bark. The spiraling causes the primocane to swell. After spiraling through the sapwood, the larvae burrow into the pith and eat through the pith for several inches before they overwinter. After overwintering, the larvae pupate, turn into adults and emerge in late spring or early summer. There is one generation per year.


The best way to reduce the number of adults is to cut the canes five inches below the galls and burn or dispose of the canes during late winter pruning. The larvae are still in the canes at the end of winter, and removing the canes will prevent adults from emerging. Treating flat-headed cane borers with an insecticide is ineffective, because treatments cannot reach the larvae when they are under the bark. Killing adults before they lay their eggs is difficult and impractical.

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