Alice Dupont Mandevilla

How do I spray, the tropical plant, Alice Dupont Mandevilla with pesticides?

I bought a Alice Dupont Mandevilla last year…by season end it was absolutely beautiful. I was told by a fellow gardener to spray it with a pesticide before bringing it in. I did. As the days went by, the leaves dropped off – totally. I kept it at the east window -a couple months ago green little leaves began to appear, a new vine about an inch grew, wilted and turned brown and the same thing just happened all over again. Is it done for? Thanks!