Insects, or creepy-crawlies generally, can be quite scary. Especially en masse. Think of cockroaches, or locusts, or bees. It’s something about the bittiness of them, the impersonal nature of their advance, their unstoppability even when you kill some of them.
So an insect attack is quite a good subject or device in a horror story.
But better still is to think yourself, as an author, into the insect world itself.
I’m not a great one for reading horror stories; they elicit emotions in me that I prefer not to exercise. But I did read a story a long time ago that had someone waking up suspended by a butcher’s hook and lacking limbs. It gave me the heebee-geebees and still does.
And yet that’s more-or-less exactly what happens to grubs paralysed and imprisoned in clay ‘pots’ by certain wasps (potter wasps) in order to feed their young.
If you think about it, even the loss of individuality within a swarm (or flock or colony) is scary in itself. Imagine sacrificing yourself totally to the greater good of your hive, as a bee, to the extent that you deny your own reproductive capabilities and work your fingers to the bone only to be cast out when you’re past your best.
Come to think of it, this could be the inspiration for more than just a horror story …