I'd swear that some of the enemy mages you encounter actually have Insect Plague on a spell trigger.
More likely, it's simply in the pre-programmed script for that area: they immediately get off the spell before anything else happens. In that case, I suggest ranged weapons. If there are several enemy mages, don't bother focusing on just one: keep 'em all from getting off any spells. Or go back to your last save, and do a group invisibility spell. What isn't visible can't be attacked, and if you can't get to the enemy mages before they dispel your invisiblity, at least you'll have wasted one of their spells in the meantime.