Ants love the messes that are associated with outdoor meals, so there's always an anthill near every picnic area. If you want to keep them from crawling on your seat and into your food, you can make some mini ant moats with pie tins.
This video from the HouseholdHacker YouTube channel is filled with great ideas for keeping ants away, including a simple method for keeping them off your picnic table. Get four pie tins and rest each of the table's legs in one. Then fill the tins with water so ants won't be able to climb up the table and ruin your meal. Even if they do manage to climb over the lip of the pie tins, they will end up taking a bath.
7 Genius Ways to Get Rid of ANTS! [YouTube]