According the linked article, late July or August in Northern Indiana. I would move that back about 2 weeks for your climate to early-to-mid July. I didn't notice them in my berry patch until I was removing canes in late winter this year. Cane borers impacted my trailing blackberries the worst and some damage to triple crown. The TC damage was more difficult to notice due less pronounced swelling from the galls since the canes were thicker. I finally noticed the TC damage early summer when the canes, with green fruit, simply wilted and died. I removed and disposed of the infected canes and treated the trailing blackberries with a systemic pesticide.

Here is a link that might be useful: Red-necked Cane Borer in Blackberries

JT. Thanks for your post. Last year I didn't notice any borer damage and as a result I picked a few gallons of blackberries. I had high hopes of maximum production this year but thanks to all the cane damage I lost about 80% of the crop. As of right now I have not seen any new damage. I am trying to grow organically so I have attempted to avoid sprays when possible. Does anyone know if the borers can be controlled with a systemic spray after the berries have been picked? If a systemic spray is used how long does it stay in the plant?