Running a Swarm of Insects fight this week? Loreify records your session and writes the recap, including every monster, NPC and loot drop your party ran into.
Swarm of Insects 5e: Stats, Swarm Rules and the 2024 Version
A swarm of insects is a CR 1/2 Medium swarm of Tiny beasts. In the 2014 rules it has AC 12, 22 hit points and a bite that deals 10 piercing damage to a creature sharing its space. In the 2024 rules it has AC 11, 19 hit points, bites from 5 feet away for 6 poison damage, and climbs or flies (the DM picks). Like every beast swarm, it shares spaces, can't be healed, resists bludgeoning, piercing and slashing damage and ignores eight conditions. Weapons barely dent it. Area spells delete it.
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 insect. The swarm can't regain hit points or gain temporary hit points.
Actions
Bites.Melee Weapon Attack: +3 to hit, reach 0 ft., one target in the swarm's space. Hit: 10 (4d4) piercing damage, or 5 (2d4) piercing damage if the swarm has half of its hit points or fewer.
Stat block from the SRD 5.1 (2014 rules) by Wizards of the Coast, licensed under CC-BY-4.0. Check an encounter with this monster in the encounter calculator.
Reach 0 ft. is the line to notice: the swarm can only bite a target in its own space, so it has to crawl onto someone before it can do anything. Once it's there, 4d4 (average 10) is a lot of damage for CR 1/2. At 11 hit points or fewer, the bite falls to 2d4.
SRD 5.1 follows this block with "Variant: Insect Swarms": beetles gain a 5-foot burrow speed, centipedes leave anyone they drop to 0 hit points stable but poisoned and paralyzed for an hour, spiders gain Spider Climb, Web Sense and Web Walker, and wasps walk 5 feet and fly 30 with no climb speed. Three of them have their own pages: swarm of centipedes, swarm of spiders and swarm of wasps.
Spider Climb.If the swarm has a Climb Speed, the swarm can climb difficult surfaces, including along ceilings, without needing to make an ability check.
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 insect. The swarm can’t regain Hit Points or gain Temporary Hit Points.
Actions
Bites.Melee Attack Roll: +3, reach 5 ft. Hit: 6 (2d4 + 1) Poison damage, or 3 (1d4 + 1) Poison damage if the swarm is Bloodied.
Stat block from the SRD 5.2.1 (2024 rules) by Wizards of the Coast, licensed under CC-BY-4.0. Check an encounter with this monster in the encounter calculator.
The speed line reads "20 ft., Climb or Fly 20 ft. (GM's choice)", and that one line replaces the 2014 variants: give the swarm a Climb Speed (and with it Spider Climb) for spiders and centipedes, or a Fly Speed for wasps and flies.
2014 Swarm of Insects
2024 Swarm of Insects
CR (XP)
1/2 (100)
1/2 (100)
AC / HP
12 / 22
11 / 19
Speed
20 ft., climb 20 ft.
20 ft., climb or fly 20 ft. (DM's choice)
Blindsight
10 ft.
30 ft.
Bites
+3, reach 0 ft., 10 (4d4) piercing
+3, reach 5 ft., 6 (2d4 + 1) poison
Weakened bite
5 (2d4) at half HP or fewer
3 (1d4 + 1) while Bloodied
The 2024 swarm hits softer (6 instead of 10) but bites from the next square instead of crawling into a target's space. Its bite is Poison damage, so most undead and constructs ignore it and poison-resistant characters, such as 2024 dwarves, take half. And at 19 hit points and AC 11, one good area spell often finishes it.
How Swarm Rules Work
Every swarm of Tiny beasts in both SRDs (insects, rats, bats, ravens, snakes and fish) shares the Swarm trait and the same defenses:
•Shared space. The swarm can occupy another creature's space, and other creatures can occupy the swarm's. It also slips through any gap big enough for one Tiny insect, so a closed door with a gap under it doesn't stop it.
•No healing. It can't regain hit points or gain temporary hit points. Damage you deal stays dealt.
•Resistance to bludgeoning, piercing and slashing. The resistance in both editions has no "nonmagical" qualifier, so a +1 sword is still halved.
•Condition immunities. Charmed, frightened, grappled, paralyzed, petrified, prone, restrained and stunned. It is not immune to blinded, deafened, incapacitated, poisoned or unconscious.
•Difficult terrain. In 2014, another creature's space is difficult terrain, so stepping into the swarm costs double movement. In 2024 the same applies unless the creature is Tiny or your ally, and a Medium swarm is neither.
One 2024 wrinkle: the movement rules say that if you end a turn in a space with another creature, you have the Prone condition unless you're Tiny or larger than that creature. A Medium character inside a Medium swarm technically meets that test. DM ruling, not rules text: the Swarm trait exists so creatures can share a space with a swarm, so don't knock characters Prone for it.
How Swarms of Insects Fight
2014: get on top of someone. The swarm moves into a target's space (which costs it double, so 10 of its 20 feet to enter an adjacent space) and bites. +3 against AC 15 hits on a 12 or better, 45% of the time, for an average of 4.5 damage a round. Once it's in a character's space, it stays there. Moving away provokes an Opportunity Attack, and the swarm follows on its turn.
2024: crawl along walls, then drop. With the climb option, Spider Climb lets the swarm move along walls and ceilings and drop onto the party from above. With the fly option, it hovers over pits and water. Either way, its 5-foot reach means it can attack from a spot the party can't easily reach. Expect about 2.7 damage a round against AC 15.
Go for the light. Insects don't plan. Run the swarm as drawn to light and warmth, so it crawls onto whoever carries the torch. That gives players a tell and a choice: drop the light or wear the swarm.
Blindsight in the dark. 10 feet in 2014, 30 feet in 2024. Darkness, fog and invisibility don't hide anyone inside that radius. Beyond it the swarm relies on ordinary senses and a passive Perception of 8, so a quiet party can often slip past.
Running a Swarm of Insects Encounter
A swarm is worth 100 XP. For four characters:
Party
Swarms
2024 difficulty
2014 DMG guideline
Four level 1
2
Low
Hard
Four level 1
3
Moderate
Deadly
Four level 2
4
Low
Deadly
Four level 2
6
Moderate
Deadly
Four level 3
6
Low
Hard
The 2014 Dungeon Master's Guide guidelines (the math in our encounter calculator) double the XP of three to six creatures, so they rate a few swarms much harder than the 2024 budget does. In play, area damage decides it. A level 1 party of weapon users does half damage and grinds for many rounds; a party with Burning Hands and Thunderwave clears two swarms in two or three.
The best use of a swarm is as a hazard next to a real monster. A swarm crawling over the cleric while a giant spider or an ettercap attacks from the webs ties up a character the party needs.
How Players Beat a Swarm of Insects
•Area damage.Burning Hands (3d6, average 10.5) takes a 2014 swarm to about half its 22 hit points on a failed save, and one more hit puts it in weak-bite range. Fireball (8d6, average 28) almost always kills either version on a failed save. Shatter and Thunderwave target its Constitution (+0 in 2014, +2 in 2024).
•Target Wisdom. Wisdom 7 (−2) is its worst save among the ones spells usually call for. Against DC 13, Spirit Guardians (3d8) gets full damage 70% of the time and halves its speed in the area.
•Poison damage works. Swarms have no poison resistance. Cloudkill at 5d8 (average 22.5) kills a 2014 swarm on a little over half of failed saves and a 2024 swarm on most of them.
•Don't bother with control spells.Entangle, web and grapples all try to restrain or grapple, and the swarm is immune to both.
•Weapons still chip. Resisted damage is half, not zero: a 9-damage hit takes off 4.
Swarm of Insects Encounter Ideas
The grain store. A village granary has been sealed for a month because something is eating the harvest. Inside are three swarms of grain beetles (the SRD 5.1 beetle variant: add a 5-foot burrow speed) and, under the floor, a giant centipede nest that drove them up. A sealed room with one door and a lot of dry grain is a terrible place to cast Burning Hands. Let the party realize that before or after they do it.
The flooded crypt. Carrion flies thicken the air of a half-flooded crypt. Use two 2024 swarms with the fly option: they hover over the water where the party can't stand, and blindsight lets them hunt in the dark while the party carries one torch. The real threat is the ghoul feeding in the next room, which the flies' buzzing covers.
The beekeeper's grudge. A hermit beekeeper treats her hives like children. When the party's horses kick over a hive, two angry swarms come out, and the beekeeper sides with the bees. Talking her down stops the swarms (she knows the smoke trick). Fighting her makes it three enemies.
Frequently Asked Questions
What CR is a swarm of insects in 5e?
CR 1/2 (100 XP) in both editions. The 2014 version has AC 12 and 22 hit points. The 2024 version has AC 11 and 19 hit points.
Can a swarm of insects attack from outside your space?
Not in 2014: its bite has a reach of 0 feet and targets a creature in the swarm's space. In 2024, yes: the bite has a reach of 5 feet, so the swarm can attack from an adjacent space.
Are swarms resistant to magical weapons in 5e?
Yes. A swarm's resistance to bludgeoning, piercing and slashing damage has no "nonmagical" condition in either edition, so magic weapons are also halved. Fire, cold, lightning, thunder, radiant, poison and other damage types aren't resisted.
Can you grapple or restrain a swarm?
No. Swarms are immune to the grappled and restrained conditions, along with charmed, frightened, paralyzed, petrified, prone and stunned. Spells like Entangle and Web don't stop them.
What happened to the swarm of spiders and swarm of wasps in 2024?
SRD 5.1 prints them as variants of the swarm of insects. The 2024 SRD drops the variants and gives its single Swarm of Insects a Climb or Fly Speed of 20 feet chosen by the DM: Climb for spiders or centipedes, Fly for wasps.
Stat blocks: System Reference Document 5.1 (2014) and 5.2.1 (2024), Wizards of the Coast, CC-BY-4.0. Tactics, encounter math and ideas are Loreify's own.
This page includes material from the System Reference Document 5.1 and 5.2.1 by Wizards of the Coast LLC, available under the Creative Commons Attribution 4.0 International License. Loreify is not affiliated with Wizards of the Coast.