How do I use Black Flag Flying Insect Killer correctly?

Close all windows and doors so the room is sealed, remove or cover food and cover any aquariums, then shake the can and spray upward into the center of the room with a slow sweeping motion for the label's specified duration. Keep the room closed for roughly 15 minutes so the treatment has time to contact insects in the air, then ventilate thoroughly before anyone re-enters and sweep up any fallen insects. The single most important step is sealing the room first — an open or breezy space lets the fog disperse before it does its job.

Does Black Flag Flying Insect Killer actually work?

For fast knockdown of flying insects already in a sealed room, yes — tetramethrin and phenothrin-class pyrethroids act quickly, and a correctly closed-room application reliably drops flies, gnats, and mosquitoes within minutes. What it doesn't do is prevent the next wave. These actives degrade quickly and leave little meaningful residual once the room is ventilated, so the space is essentially unprotected again as soon as you air it out.

Why do flies or gnats keep coming back after I spray?

A space spray kills what's flying in the room at the moment of treatment — it does nothing about the breeding source or entry point letting new insects in afterward. If flies or gnats keep reappearing, the can isn't underperforming; something outside the treated moment — a torn screen, standing water, garbage, or a drain — is refilling the room faster than any spray can prevent. Finding and fixing that source is the actual fix, not another application.

Can I use this on ants or roaches instead of flying insects?

It's not built or labeled for that job. This is a space spray designed to be sprayed into the air of a closed room to knock down flying insects, not a crack-and-crevice contact aerosol aimed at surfaces and harborage where crawling insects like ants and roaches hide. Using a flying-insect space spray on a crawling-pest problem wastes the can and gets poor results — reach for a labeled ant-and-roach contact aerosol for that job instead.

What if the 'flies' I'm seeing are actually fruit flies or gnats from produce or a drain?

Check the source before reaching for this can. Small flies clustering around produce, a fruit bowl, or a kitchen drain are more likely fruit flies, fungus gnats, or drain flies — pests that respond much better to source removal and a targeted attractant trap than to a space spray, which only knocks down what's airborne at the moment you spray. If you're not certain which small fly you're dealing with, correct identification changes the fix entirely.

Is it safe to use around my pets?

Exit the room during treatment and keep pets and kids out until the space has been thoroughly ventilated and surfaces are dry — that's when airborne exposure risk is highest. Cover any aquariums and birdcages before spraying, since fish and birds are especially sensitive to airborne pyrethroid exposure, and cats in particular clear some pyrethroid compounds more slowly than dogs. For the full per-species pet-safety verdict on this product, see the companion review at /is-pest-control-safe-for-pets/flying-insect-killer-aerosol-by-black-flag.

Daniel Brady · NH-Licensed Pesticide Applicator · License #782664

How to Use Flying Insect Killer Aerosol by Black Flag

Situational

Tetramethrin verify current can — commonly ~0.2% in the equivalent formulad-Phenothrin (Sumithrin) verify current can — commonly ~0.2% in the equivalent formula

Black Flag Flying Insect Killer is a space-spray aerosol built for one job: knocking down flying insects that are already in the air in a room you can seal. Close the windows and doors, cover food and any aquariums, shake the can, and spray upward into the center of the room with a slow sweeping motion for the label's specified duration, then keep the space closed briefly before ventilating thoroughly and sweeping up the insects that fall. It works fast — flying insects typically drop within minutes in a properly closed room — but it leaves almost no meaningful residual once you air the space out, so it does nothing to stop the next fly, gnat, or mosquito that gets in. If the problem keeps coming back, that's not a sign to buy another can; it's a sign to find and fix the actual source — an open screen, standing water, garbage, or a drain — because this product was never built to solve that part of the problem.

At a glance

Efficacy: Situational
Best job: Quickly knocking down a room full of flying insects you can close up and treat as a sealed space
Format: Ready-to-use space-spray aerosol — sprayed upward into a closed room's air, not applied to a specific surface or crack
Technique: Easy technique

The honest verdict

Does this product actually work?

The verdict on this product

SituationalEasy technique

Situational

A genuine knockdown workhorse for flying insects in a room you can seal — with next to no residual, so it's a repeat-purchase relief valve, not a fix.

As a space spray, this does exactly one job well: seal the room, spray upward in a sweeping motion, and the flying insects in the air drop within minutes. What it doesn't do is protect the room afterward — tetramethrin and phenothrin-class pyrethroids are fast-acting and fast-degrading, so once you ventilate, the useful residual is essentially gone. If flies or gnats keep reappearing, the can isn't failing; an untreated breeding source or open entry point is refilling the room faster than any spray can prevent.

Best for

Fast knockdown of flies, gnats, and mosquitoes in a room you can close up and treat, then ventilate.

Skip it for

Preventing flying insects from coming back, or solving a recurring problem rooted in an open entry point, a drain, garbage, or standing water.

Read the label first

What it is — and what it's really for

The label is the law. Here's the honest read on the format, the one job it does well, and the job people wrongly buy it for.

Format

Ready-to-use space-spray aerosol — sprayed upward into a closed room's air, not applied to a specific surface or crack

Indoor / outdoor

both

Residual

Retail claims of extended residual overstate the practical reality for a space spray — tetramethrin and phenothrin-type pyrethroids knock down fast and degrade quickly, leaving little meaningful residual once the room is aired out

Target pests

Flies, Gnats, Mosquitoes, Small flying moths, Other flying insects, indoors and out

Best job

Quickly knocking down a room full of flying insects you can close up and treat as a sealed space

Active ingredient

Tetramethrin (verify current can — commonly ~0.2% in the equivalent formula), d-Phenothrin (Sumithrin) (verify current can — commonly ~0.2% in the equivalent formula)

Not for (honest)

Preventing the next wave of flying insects or fixing a crawling-insect infestation — it has essentially no useful residual and does nothing about the breeding source

The technician's method

The technique walkthrough

Where DIY succeeds or fails is in the placement, patience, and the small pro moves the label never mentions. Follow the label; these are the technique details on top of it.

Easy technique~5-15 min to apply; knockdown within minutes in a closed roomTools:The can (shake before use), Something to cover food and aquariums, A broom or vacuum for fallen insects afterward

The entire technique is sealing the room first — a space spray only works if the fog stays put long enough to actually contact flying insects.

Close the room
Shut windows and doors so the treated space is as sealed as possible — an open or breezy room lets the spray disperse before it contacts flying insects.
Common mistake
Spraying in a breezy or open room, or expecting it to prevent future flies. It's knockdown, not prevention.
Remove or cover food and sensitive items
Remove or cover food and food utensils, and cover aquariums — fish are highly sensitive to airborne pyrethroid exposure.
Shake and spray upward in a sweeping motion
Shake the can, hold it upright, and spray upward into the center of the room with a slow sweeping motion for the label's specified duration.
Pro move
Seal the room first — a space spray only works if the fog stays put long enough to contact flying insects.
Keep the room closed briefly
Leave the space closed for roughly 15 minutes per label so the treatment has time to contact flying insects in the air.
Ventilate and clean up
Ventilate the room thoroughly before re-entry, then sweep up and discard any fallen insects.
Caution
Keep kids and pets out during and shortly after treatment; don't spray toward people, pets, or open food.

Habits that decide it

Do & don't

Close the room before spraying
Cover food and aquariums first
Spray upward in a sweeping motion per label timing
Ventilate thoroughly before re-entry

Don't

Don't expect residual prevention once the room is ventilated
Don't use it for a crawling-insect infestation
Don't spray in an open, breezy, or occupied room
Don't skip finding and fixing the actual source (screens, trash, standing water)

The honesty moat

Where it works vs. where it fails

No product fixes everything. This is the honest boundary — the situations it genuinely handles, and the ones it simply won't.

Where it works

A room full of flies, gnats, or mosquitoes needs fast knockdown right now
The space can genuinely be closed and sealed during treatment
You need quick relief before an event or activity in the room
The flying-insect presence is a one-off or occasional nuisance, not a recurring source problem

Where it fails

The room can't be sealed — open windows, doors, or airflow scatter the fog before it works
Flying insects keep reappearing from an untreated breeding source (drains, garbage, standing water, houseplants)
The actual problem is a crawling-insect infestation, not flying insects in the air
It's being used as a substitute for fixing screens or entry points

Don't throw money away

Mistakes that waste the product

Spraying in an open room

The fog disperses before it can contact flying insects, wasting the can and achieving little knockdown.

Fix

Seal windows and doors before spraying, and keep the room closed for the label's stated duration.

Expecting lasting residual

Little meaningful residual exists after ventilation, so relying on it for ongoing protection leaves the room unprotected within hours.

Fix

Address the source — screens, drains, trash, standing water — for lasting control, not repeat spraying.

Ignoring the breeding site or entry point

Flies and gnats keep returning because the source that's producing or admitting them was never fixed.

Fix

Identify and fix screens, garbage handling, standing water, or drains alongside any spraying.

Using it on crawling pests

This is a flying-insect space spray, not a crack-and-crevice contact aerosol — it's not built or labeled for ants, roaches, or other crawling pests.

Fix

Use a labeled crack-and-crevice contact aerosol for crawling insects instead.

Calibrated expectations

What to expect — and when

This is a fast-in, fast-out knockdown tool — judge it on the minutes it takes to clear a sealed room, not on whether flying insects stay away.

Minutes
Flying insects in a closed, treated room drop quickly — this is the product's genuine strength.
After ventilation
Protection is essentially gone once the space is aired out; there's little meaningful residual left behind.
Days afterward
New flying insects appear if the source or entry point isn't addressed — this isn't the product failing, it's the source refilling the room.

When results plateau

There's no real 'plateau' with a space spray — either the room stays sealed and treated (impractical to maintain) or the source keeps producing new insects faster than any can of spray can prevent. Recurring activity is a source problem, not a reason to spray more often.

Know the ceiling

When to stop DIY & call a pro

A space spray solves the moment, not the pattern. These are the honest signals a recurring flying-insect problem has outgrown a can:

Recurring indoor fly or gnat activity traced to a hidden source
A drain, organic buildup in a void, or a structural gap can keep producing insects faster than repeated spraying can knock them down.
The 'flying insects' are actually a different small fly with a different fix
Fruit flies, fungus gnats, and drain flies look similar but come from different sources — misidentifying the pest means treating the wrong problem entirely.
The activity is actually a nesting stinging insect, not a nuisance fly
Wasps, hornets, and yellowjackets need a fundamentally different, higher-risk approach than a space spray for flies and gnats.
A structural moisture problem is feeding the source
Persistent moisture behind a wall or under a fixture can sustain a breeding population indefinitely until the moisture itself is fixed.

When a recurring flying-insect problem keeps coming back despite closed-room treatment, Anchor's licensed NH technicians find and treat the actual source — drain, void, or structural issue — with targeted professional methods. Free inspection, same-day service across southern and central New Hampshire.

Flying insects keep coming back?

Get a Free inspection (603) 785-0118

Before you treat

Is it safe for pets & kids?

The short version lives here; the full per-species verdict lives on this product's pet-safety page.

This is a pyrethroid space spray — exit the room during treatment and keep kids and pets out until it's ventilated and surfaces are dry. Cover food and aquariums before spraying; fish are especially sensitive to airborne exposure.

Re-entry: Keep children and pets out during application and until the room is thoroughly ventilated.

If there's an exposure, call

ASPCA Animal Poison Control: (888) 426-4435 Pet Poison Helpline: (855) 764-7661 Northern New England Poison Center: (1-800) 222-1222

See the full per-species pet-safety verdict for this product

New Hampshire context

Flying-insect pressure in New Hampshire homes

Cluster flies and other occasional-invader flying insects push into southern and central New Hampshire homes at fall ingress, and mosquitoes and gnats spike with warm-weather moisture. A closed-room space spray is a reasonable quick fix for a room that's suddenly full of flying insects, but recurring activity through a season usually points to an entry gap or a breeding source outside the room, not a reason for repeated spraying. Anchor Pest Services identifies and treats the actual source across Manchester, Nashua, Concord, and the surrounding towns.

Service area

Southern New Hampshire

Anchor Pest Services · NH license #782664 · NEPMA member · family-owned since 2017.

Common questions

Frequently asked

Sources & further reading(4)

[1]
Black Flag — Labels and SDS
Directing to the current printed active-ingredient panel and EPA registration by SKU (Spectrum reformulates by SKU) — verify before use
manufacturerverified 2026-07-14
[2]
Black Flag Flying Insect Killer3 (Clean Fresh Scent) — Safety Data Sheet
Space-spray aerosol format and closed-room application directions (facts only)
manufacturerverified 2026-07-14
[3]
NPIC — d-Phenothrin (Sumithrin) Fact Sheet
d-Phenothrin/phenothrin-class pyrethroid behavior — fast knockdown, rapid environmental degradation
npicverified 2026-07-14
[4]
UNH Extension — Fungus Gnats Fact Sheet
Distinguishing fungus gnats from other small flying insects — misID changes the correct fix
extensionverified 2026-07-14

Written & reviewed by

Verified

Daniel Brady

NH-Licensed Pesticide Applicator · License #782664 (NHDAMF, RSA 430)

Daniel Brady is the New Hampshire–licensed applicator behind Anchor Pest Services, a family-owned pest control company serving the Manchester area since 2017.

Verify license

Last updated July 2026

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Professional Pest Control

When a Space Spray Isn't a Fix

A knockdown aerosol clears the room today, but it can't stop the next wave from an unaddressed source. Anchor Pest Services identifies and treats the actual source with targeted, licensed methods across southern and central New Hampshire. Family-owned since 2017, NH license #782664.

Get a Free inspection (603) 785-0118

Source identification, not just knockdown spraying
NH-licensed applicators (#782664)
Free inspection — same-day service across southern NH

NH License #782664 · No obligation · We answer in person

Keep Exploring

How to Use Flying Insect Killer Aerosol by Black Flag

Does this product actually work?

What it is — and what it's really for

The technique walkthrough

Close the room

Remove or cover food and sensitive items

Shake and spray upward in a sweeping motion

Keep the room closed briefly

Ventilate and clean up