I am 100% certain it's physically impossible on insane. At some point the defending ants will be busy with a crab or DCH while ladybirds get near the aphids. Currently the ranged ants (your damage source) will keep attacking their first target before changing to the ladybirds. By the time their original target is killed, the ladybirds have already flown away after eating.
But maybe try it on hard, I didn't say the achievement is just for insane difficulty!
I tried this on medium and failed only because I was not focussing on defending the aphids, instead having half my army hunting on the lower ground at night. (I think with my full army defending it would have been doable to keep the ladybirds off the aphids at night.)
Alright, then the achievement is right for easy difficulty.
Why easy difficulty? Medium difficulty should be very doable, I think?
Then the achievement is good to go without a specified difficulty so you can do it in any difficulty if you are concern you can do it, even in insane if somebody could.