What Is an Insect Light Trap?

An insect light trap in everyday terms

Many night-flying insects are drawn to light, especially ultraviolet (UV) light that people cannot see well. An insect light trap uses a UV lamp as bait, and insects that fly toward it end up on a sticky board behind the lamp or on a high-voltage wire grid.

Pest control pros often call these devices ILTs. In homes, the best-known type is the outdoor bug zapper. In restaurants, food plants and some homes, the more common type is a quieter fly light with a replaceable glue board.

In technical terms, a UV lamp in these traps usually gives off light in the 310–370 nanometer range, according to a study by USDA and Navy researchers ⁴. The U.S. EPA calls black-light traps a pest control device: a tool that works by physical means, not by a chemical ⁶.

NPIC lists bug zappers among the devices the EPA regulates, alongside glue boards ⁷.

How an insect light trap works

1. The lamp glows: One or more UV tubes give off light that many flying insects are drawn to ⁴.
2. Insects fly in: House flies and other light-seeking insects head toward the glow, especially when the room is otherwise dim. UC IPM notes these traps work indoors and should not compete with daylight ¹.
3. They are caught or killed: In a fly light, the insect lands on a glue board and stays there. In a zapper, it touches a charged grid and is killed.
4. The catch builds up: Glue boards fill and need changing; zapper trays need emptying.
5. Someone reads it: In food businesses, a technician counts and identifies the catch to spot changes, such as a sudden rise in one kind of fly.

Types of insect light traps

A trap sold with a chemical lure can be a pesticide product rather than a device in the EPA’s eyes ⁶.

Example: what a backyard zapper actually kills

This uses the real counts from a University of Delaware study, as reported by Iowa State Extension. Black-light zappers ran in suburban Newark, Delaware, for a whole summer ².

In this study, about one insect in 450 was a biting fly. The authors also estimated that 4 million zappers running 40 nights a summer would kill over 71 billion non-target insects a year ². For mosquitoes, our mosquito control service page covers what does work.

Where you will see light traps named

In a restaurant or food plant, a health inspector checks where fly lights hang. South Dakota’s food rule says devices that kill or trap flying insects may not be placed over a food preparation area, and dead insects may not fall onto exposed food or clean equipment ⁵.

On a commercial pest control service report, fly lights may be listed by number, with the catch and the date the glue board was changed. That record helps show whether flies are rising and where.

On a home product box, you will see the lamp wattage and, for a compliant device, an EPA Establishment Number. The EPA says that number does not mean the agency reviewed or approved the product ⁶.

In a pest control proposal for flies, a light trap may appear as one line under monitoring. Our guide to integrated pest management explains why it should sit next to sanitation and exclusion work.

What light traps do well

Why bug zappers fail against mosquitoes

Mosquitoes find people mostly by smell, not light. UF/IFAS entomologist Jonathan Day said mosquitoes are very sensitive to carbon dioxide and can detect it at levels as low as 50 parts per million. In a one-night test at the Vero Beach lab, one zapper killed about 10,000 insects; only eight were mosquitoes ³.

The Delaware study found the same pattern: only 0.22% of the catch were biting flies ². Iowa State’s summary concluded mosquito numbers stay the same with zappers running.

Even indoors, light traps have limits. Flies that come in through an open door or breed in garbage will keep coming. UC IPM says sticky boards and ribbons handle only a few flies in small spaces, and work poorly for heavy problems or outdoors ¹.

What drives the cost, and how long parts last

We do not list prices. The cost of owning a light trap comes from three parts: the unit, the replacement glue boards, and the UV lamps. In a business, the cost of a service plan also depends on how many units are placed and how often a technician changes boards and records catches.

UV lamps lose strength over time even while they still glow, and glue boards fill and gather dust. Replacement timing varies by maker, so follow the schedule in the product manual rather than a general rule.

For mosquitoes, money is better spent on removing standing water and, if needed, a professional plan. Our mosquito cost estimator shows what drives that price, and the main pest control cost guide covers other pests.

How to place an indoor fly light

These steps follow UC IPM and state food rules. A pro placing units in a business follows the same logic.

1. Name the fly first: House flies, fruit flies and drain flies breed in different places. See pest identification.
2. Keep it out of daylight: UC IPM says UV traps should not compete with daylight ¹. Avoid spots facing windows.
3. Keep it away from food prep: State food rules bar these devices over food preparation areas ⁵. UC IPM advises against zappers near food prep at all ¹.
4. Put it on the flies’ path: Between an entry point and the room you want to protect is a common choice.
5. Mount it at a sensible height: In one lab study, traps sat about 90 cm (about 3 feet) above the floor at a counter edge ⁴. Follow the maker’s height advice.
6. Log and change boards: Note the date and catch on each change.

Pair the trap with screens and tight doors. Our page on the door sweep covers one of the cheapest fixes for flies getting in at the bottom of a door.

Routine upkeep

• Change glue boards when they fill or get dusty; a full board stops catching.
• Replace UV lamps on the maker’s schedule, even if they still light up.
• Unplug the unit before cleaning or changing a board or lamp.
• Empty and clean zapper trays so dead insects do not attract other pests.
• Check that the unit still is not over food or clean dishes after any kitchen rearranging ⁵.

Risks, warning signs and when to call a pro

The main health risk is contamination. UC IPM warns that zappers can throw insect bodies several feet, onto food or surfaces ¹. That is why food rules keep these devices away from food prep ⁵.

Electrical safety matters too. These are plugged-in devices with lamps and, in zappers, a high-voltage grid. Follow the maker’s instructions, keep them dry and out of children’s reach.

Warning signs that a trap is not enough: flies keep coming every day, the board fills within days, or you see maggots or flies near drains or garbage. That means flies are breeding nearby. Call a pro to find the source. Our guide to choosing a pest control company lists the right questions.

Rules that apply

Federal (EPA, FIFRA): The EPA lists black-light traps as insect devices when they have no attractant. Devices need no EPA registration before sale but must be made in an EPA-registered establishment, carry an EPA Establishment Number, and not make false or misleading claims ⁶⁷.

Lures change the rules: The EPA says a device that also includes a pesticidal substance may be treated as a pesticide product ⁶. NPIC adds that states may add their own requirements for devices ⁷.

Food service (state): States adopt food codes for restaurants and stores. South Dakota’s rule, for example, says devices used to kill flying insects must have escape restraints, may not be placed over food preparation areas, and may not drop insects onto exposed food, clean equipment or utensils ⁵.

Licensing: A company that installs and services these units for pay works under state pest control licensing. See how licensing works.

Insect light trap vs other fly and mosquito tools

Common light trap myths

Myth A bug zapper will clear mosquitoes from my yard.

Reality In one study only 0.22% of insects killed were biting flies ². UF/IFAS called zappers useless for mosquitoes ³.

Myth The crackle means it is working on pests.

Reality Most insects killed are harmless or helpful, such as aquatic insects, predators and parasites ².

Myth Hanging a zapper over the kitchen counter is fine.

Reality Zappers can throw insect parts several feet ¹, and food rules ban these devices over food prep ⁵.

Myth Adding an octenol lure fixes zappers for mosquitoes.

Reality UF/IFAS said octenol adds little to mosquito control ³.

Myth A fly light near a sunny window catches more.

Reality UC IPM says UV traps should not compete with daylight ¹.

Where state rules and seasons differ

When a light trap is worth it

Indoors, a glue-board fly light is a reasonable helper for house flies, as long as it is placed away from windows and food prep, and you also deal with garbage and gaps around doors.

Outdoors, skip the bug zapper if your goal is fewer mosquito bites. Remove standing water, and consider a professional plan if bites stay heavy.

In a business that handles food, use glue-board units placed and serviced by a licensed company, often as part of a recurring service plan, and keep the catch records.

Related terms

House Fly

A house fly (Musca domestica) is a gray, non-biting fly under 3/8 inch long, marked by four dark stripes on its back, that breeds in garbage, manure and other moist, rotting matter and can carry germs from that waste onto food.

Fruit Fly

A fruit fly, in a home, is a tiny tan fly about 2 to 3 mm long, usually with bright red eyes, from the genus Drosophila, that breeds in overripe fruit and other fermenting residue and hovers around kitchens, trash and recycling.

Mosquito

A mosquito is a small, long-legged fly whose females bite people and animals for the blood they need to make eggs, and whose young grow only in standing water, from a pond to a bottle cap.

Pest Glue Board

A pest glue board is a flat card or tray coated with strong, slow-drying glue that catches insects, spiders or rodents when they walk across it, with no poison involved.

Pheromone Trap

A pheromone trap is an insect trap baited with a man-made copy of a scent that one species uses to find a mate, so it lures mostly males of that one pest onto glue or into a container.

Window Screen Mesh

Window screen mesh is the woven netting in a window or door screen, rated by its mesh count, the number of openings per inch in each direction, which sets how small an insect it can keep out.

Sanitation in Pest Management

Sanitation in pest management is the routine removal of the food, water and shelter that pests need, through cleaning, sealed storage, trash handling, moisture fixes and clutter control, so fewer pests can survive and other control methods work better.

Pest Monitoring

Pest monitoring is the regular, recorded checking of a place for pests, using traps, visual checks and logs, to learn which pests are present, where, how many, and whether their numbers are rising or falling.

Non-Chemical Pest Control

Non-chemical pest control is the set of methods that prevent or reduce pests without pesticides: blocking entry, removing food, water and shelter, trapping, using heat or cold, and letting natural enemies work.

Also referenced from: Pest Monitoring, Fruit Fly, Pest Glue Board, Pheromone Trap