Help! Need Challenge Rating for Spell Casting Swarm

So I'm working on this monster, and I'm not happy with the CR on the swarm. I can't find anything with a good mechanical parallel. I want to compare it to a level 11 wizard, since it has a 6th level spell and is fragile, but I certainly didn't intend it to be as dangerous as the other CR 6 creatures I'm looking at, thoughts? 

Thanks!


Arcandidae are locust-like bugs with near sentience. Individuals have some cold affinity, but in swarms they have magical, instinctive dances that allow the swarm to cast powerful frost magics.



Arcandidae (Arcane Locust)

Tiny beast, chaotic neutral

Armor Class: 13

Hit Points: 2 (1d4)

Speed: 10 ft., fly 40 ft.

Strength: 2 (-4)

Dexterity: 14 (+2)

Constitution: 10 (+0)

Intelligence: 6 (-2)

Wisdom: 8 (-1)

Charisma: 2 (-4)

Damage Immunities: cold

Senses: blindsight 10 ft., passive Perception 9

Languages: –

Challenge: 0 (10 XP)



Actions

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 1 piercing damage and 2 (1d4) cold damage.



Swarm of Arcandidae (Arcane Locusts)

Medium swarm of Tiny beast, chaotic neutral

Armor Class: 13

Hit Points: 56 (16d8-16)

Speed: 10 ft., fly 40 ft.

Strength: 13 (+1)

Dexterity: 14 (+2)

Constitution: 10 (+0)

Intelligence: 6 (-2)

Wisdom: 14 (+2)

Charisma: 2 (-4)

Damage Immunities: cold

Damage Resistances: bludgeoning, piercing, slashing

Condition Immunities: charmed, frightened, grappled, paralyzed, petrified, prone, restrained, stunned

Senses: blindsight 10 ft., passive Perception 12

Languages: –

Challenge: 6 (2,300 XP)



Swarm. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny Arcandidae. The swarm can’t regain hit points or gain temporary hit points.

Innate Spellcasting. The swarm’s innate spell spellcasting ability is Wisdom (spell save DC 13). The swarm can innately cast the following spells, requiring no components:

At will: Ray of Frost

1/day each: Fire Shield (chill only), Cone of Cold, Freezing Sphere (self only)

Spell damage is half if the swarm has half of its hit points or fewer.



Actions

Bites. Melee Weapon Attack: +4 to hit, reach 0 ft., one target in the swarm’s space. Hit: 14 (4d6) piercing damage and 28 (8d6) cold damage, or 7 (2d6) piercing damage and 14 (4d6) cold damage if the swarm has half of its hit points or fewer.