Often what they get is the empty shells as birds eat the seeds and discard the shell. Fortunately, most insects are not a problem. Occasionally ants enjoy the nectar from the flower or are common on the stalks. They are no real threat. Sunflowers seldom have disease problems.
Sunflowers are an annual.
They can withstand a mild frost. However, cover your seedlings if a hard spring freeze is anticipated. Here's nine pages of info.