HUGE ANT PROBLEM!!!

Hey guys, I know several of you have greenhouses. I have put one of my big begonia's in my dad's greenhouse. I went by this a.m. just to check on it and when I walked in noticed that my dad has a HUGE ANT INFESTATION!!!! I started digging around in the pots to see what was going on and it looks as if the ants are making themselves right at home in all of the pots. I mean they are everywhere, on the walls, shelves, floor, all you see is frantic ants just going to town. I asked my dad if he realized what a problem he had and he said he has been trying to get rid of them with 7 dust? Well, it isn't working. They aren't effecting the plants but everytime anyone steps foot in the greenhouse they get covered in ants. I was wondering if those of you who have a greenhouse or those who don't, know how he can treat this problem without causing harm to his plants. My dad grow's his tomatoe plants from seed and wouldn't want to do anything that would harm his little babies. These seed's have been in our family for years and buying replacement plants from Lowe's or HD is out of the question for him. Please, anyone have any suggestions? I will have plenty of home grown tomatoes to send. If you have never had a home grown Alabama tomatoe, you just don't know what you are missing!!! Thanks guys! ~ Kristi