Thank you for your inquiry. Scale is rarely found on snapdragons. Instead, the snapdragons most likely have rust, a common fungus disease for these summer flowers late in the season, self-sown seedlings, or for snaps that are in their second year. To be effective, sprays must be applied at the very first sign of the disease. Your snaps are likely long past that point.

The preferred management at this time can rely on removing infected leaves as soon as you see them.  When the plant is looking truly scraggly, remove it and all debris from the garden, then put it in the trash. Do not compost infested material.

To grow snaps again next season, place them in a different site, one with full sun all day and every day. Then, too, supply sufficient water to avoid plant stress such as wilt.