Are the locusts created by the Insect Plague spell affected by other spells like Fireball, Cloudkill, Abi-Dalzim’s Horrid Wilting or similar spells? By similar spells I mean spells that deal damage on an area greater or equal than the 20-foot-radius sphere of the Insect Plague spell.
The Insect Plague spell does not describe the locusts as a creature (as a Swarm of Insects, for example) nor gives HP or clues on how damage them. I strongly believe that if someone throws a Fireball inside this swarm some effect would arise, which will be different by the effect provided by an Ice Storm or by a Cloudkill: I find the wording of Insect Plague quite ambiguous.
This question is an offshoot of this one: Cloudkill plus Insect Plague – What do the players see?. The comments there give some insights about the presented issue, but I think the topic deserves a full answer.