Beer traps work by drowning slugs but hardly ever catch snails as they are not really beer drinkers. The best way to prevent snails and slugs is to create barriers that they cannot cross over to get to your basil. Encourage natural enemies such as birds, toads, and salamanders, also chickens and ducks are efficient snail hunters. Good luck and don't have too much fun. Ladybugs are a great pleasure to see in your garden because they are so indispensable for fighting aphid problems.
Ladybugs cause no plant damage at all and are sold in garden centers everywhere.
They will eat large amounts of aphids and then fly onto another plant to eat more. Be aware that if you buy a box of ladybugs they may move on to your neighbors house when they can't find enough to eat at your house. It is a good idea to spray your plants with a diluted sugar-water solution before you release them to give them a drink and make them want to stay home. The Mexican Bean Beetle attacks most varieties of Bean plants eating away at the undersides of the leaves until they look similar to lace. They can be very destructive to bean plants and the pods.