Creatures inside the insect plague, regardless of Armor Class, sustain 1 point of damage at the end of each round they remain within, due to the bites and stings of the insects. Invisibility is no protection. All creatures with 2 or fewer HD are driven from the cloud at their fastest possible speed in a random direction and flee until they are at least 100 feet away from the insects. Creatures with 3 to 5 HD flee as well, though a Will save negates this effect. Heavy smoke drives off insects within its bounds.
Fire also drives insects away.
For example, a wall of fire in a ring shape keeps a subsequently cast insect plague outside its confines, but a fireball spell simply clears insects from its blast area for 1 round. A single torch is ineffective against this vast horde of insects. The creature or object touched vanishes from sight, even from darkvision. If the recipient is a creature carrying gear, the gear vanishes, too. If the character casts the spell on someone else, neither the character nor the character's allies can see the subject, unless the character can normally see invisible things or employ magic to do so.