I recently bought 3 fig rooted starters from ebay.
Initially they looked fine, but later on, I noticed ants on the leaves.
On close examination, I noted that the ants were feeding on the
"stuff" secreted by then still young scale crawlers.
First I sprayed with Neem oil.
Nothing happened; them ants were still enjoying themselves.
Then I sprayed with insecticidal soap (from Gardens Alive).
Within a few minutes, even the ants dropped dead.
(One can brew own soap-spay from some mild dish washing detergent.)
Insects do not have lungs like us (or gills like fish) to breathe;
instead they have tiny holes along their body through which oxygen comes in.
(Thankfully, nature keeps them ugly/monster-featured looking-bugs very small - think of that horror-ant-movie.)
Apparently, the soapy film (think soap-bubbles) blocks the O2, and then suffocates them to death.