When ants show up in great numbers like this on the leaves of a plant, it usually indicates that you have aphids on your plants. The ants feed on the sticky excretions of the aphids and engage in what is known as "farming" to protect the aphids as a source of food. It is the aphids feeding that causes the curling and bunching of leaves while the ants do their best to protect them from natural predators. They will actually transport the aphids to concentrate them on the juiciest, freshest part of a plant where they will produce an efficient and plentiful food supply for the ants.

There are a couple of things you can do to manage the problem. If your basil plants are in containers rather than a larger bed, you can use double sided sticky tape around the container to stop ants as they go up and down to return to their nest at night. (There are super sticky products sold specifically for pest control in the garden.) Remove the tape when the ants are gone to reduce the accidental capture of beneficial insects.

Alternatively, standard ant bait traps may help. Aphids alone will probably be manageable once the ants are gone. The heavily infested areas of the plants can be pinched off, and new aphids will fall away with a spray from a hose.

Courtesy of NYBG Plant Information Service