When you see buds drying up like that, they've been chewed through at the base. That means there's lots of caterpillars on there munching away as we speak. The best thing is any product that contains BT, or bacillus thuringiensis. You don't need canna specific product, Safer Caterpillar Killer from the garden center will work as well as any other. Important thing is speed. Run to the store, buy it and spray as soon as possible.