[RPG] Can the spirits in the spell Spirit Guardians see in the fog or dark

Since this question was asked, Jeremy Crawford has changed his mind on how this works.

In the April Rules Answers column for Sage Advice, Crawford has this to say:

Does moonbeam [or Spirit Guardians] deal damage when you cast it? What about when its effect moves onto a creature? The answer to both questions is no.

He goes on to elaborate:

Reading the description of any of those spells, you might wonder whether a creature is considered to be entering the spell’s area of effect if the area is created on the creature’s space. And if the area of effect can be moved—as the beam of moonbeam can—does moving it into a creature’s space count as the creature entering the area? Our design intent for such spells is this: a creature enters the area of effect when the creature passes into it. Creating the area of effect on the creature or moving it onto the creature doesn’t count. If the creature is still in the area at the start of its turn, it is subjected to the area’s effect.

This is the list of spells the article applies to:

• blade barrier
• cloudkill
• cloud of daggers
• Evard’s black tentacles
• forbiddance
• moonbeam
• sleet storm
• spirit guardians

Contrast with his original tweet, and follow-up.

The insects from insect plague are a spell effect, and the spell clearly does not intend them to be treated as monsters (spells that do create items with stats clearly explain what they are and what the impact is of doing damage to them). Therefore, they are not stopped by damage to the swarm of any kind. In this regard they are similar to the Spirit Guardians and you could equally ask whether the insects kill the spirits - after all Insect Plague is a higher-level spell.

If it helps with scene description, you might consider that the spell continuously generates the swarm in its area for the duration, so that effects that could destroy small physical creatures are so fleeting as to not have any impact on the spell.

A way to use damage to stop Insect Plague is to break concentration of the caster.