Sunflowers question

keeping out bugs so they don't destroy the sunflowers?????

I usually use Sevin in my deer food plots usually about 3-4 plots about an acre each so the Sevin isn't to terribly expensive. However, I have a field under a highline that I want to plant sunflowers in the is 15 acres so sevin financially is out of the question. Once I disc the field what would you use to put in the dirt to keep the insects out? Would you spray or use a granule? How long do you think the insecticide would last?

Thanks everyone.