An insect light trap is only a fly killer to the person who bought it. To a food safety auditor, and to anyone running an integrated pest management programme properly, it is a monitoring device. The glueboard is a weekly census of what is flying inside your building, and the numbers on it, read over time, tell you whether your doors seal, whether your drains are clean, and whether there is a moth breeding in the flour store. Most facilities mount the units, empty them, and throw the boards away. That is the waste of a good instrument. Positioned and read properly, ILTs are the monitoring backbone of our fly control programmes.
This article covers three things: mounting the traps so they catch and do not endanger product, keeping the lamps and boards on a schedule that actually works, and reading the catch by species so the board earns its keep. It sits alongside the bait station mapping guide, because the light traps belong on the same numbered site map.
Key takeaways
- Place traps to intercept insects as they enter, back from and to the side of doors, never above or facing exposed product.
- Keep the unit's glow out of sight from outside the building, or you will draw insects in rather than catch those already inside.
- Change UV lamps at least annually, before spring, because output falls off long before the lamp dies.
- Change glueboards on a defined schedule (typically monthly, more often in peak) and never let a board saturate.
- Identify and trend the catch by species: houseflies, fruit flies and stored-product moths each point to a different failure.
Where to mount the traps
The single most common placement error is treating the trap as an attractant to hang in the middle of the room. A well-placed unit intercepts insects on the routes they already take, mainly as they come in through doors. Work from these rules:
- Not over exposed product. Never mount a unit directly above or facing open lines, filling heads, exposed product or open packaging. A shattering lamp or a dislodged insect fragment over product is a contamination event. Keep units at least 1.5 to 2 metres clear of exposed product zones.
- Out of the external sightline. Position units back from and to the side of external doors so the UV glow cannot be seen from outside. A trap visible through an open door is a beacon pulling insects into the building rather than catching those that slipped in. Site the first interception unit inside the entrance, off to the side, not on the far wall opposite the door.
- Correct mounting height. For units targeting houseflies and similar strong fliers, mount at roughly 1.8 to 2.2 metres, at the height flies naturally cruise. Traps aimed at low-flying species and those set in dispatch and intake areas may sit lower. Wall-mounted encapsulating glueboard units are preferred in production because they contain the insect and the fragments.
- Away from competing light. Bright windows, skylights and other lit fixtures compete with the trap. A unit fighting daylight from a rooflight catches a fraction of what it should.
Every unit is numbered and marked on the pest control site map in its true position, exactly as the rodent stations are, so the map reconciles to the floor in both directions.
Lamp and glueboard replacement schedules
Ultraviolet output is invisible to the eye and it decays steadily. A lamp that still looks bright can be emitting a fraction of the attractant wavelength it did when new, so a "working" lamp is not necessarily an effective one. Replace UV lamps or tubes at least once a year, and time the change for early spring, before the South African summer fly season builds. In high-pressure sites a shorter interval is justified.
Glueboards are the consumable that does the catching, and a saturated board catches nothing while looking like it is working. Set a defined interval, typically monthly, and tighten it for high-catch units and during peak season. Change any board that is filling before it saturates. Use shatter-resistant (coated or sleeved) lamps everywhere in food areas so that a lamp failure cannot scatter glass.
Every lamp and board change is a dated record. On ASC food and beverage contracts the change is logged per numbered unit in myASConline, so the auditor can see at a glance that unit ILT-04 had its lamp replaced in September and its board changed monthly since.
| Item | Typical interval | Note |
|---|---|---|
| UV lamp / tube | Annually, before spring | Output degrades before the lamp fails visibly. |
| Glueboard | Monthly, more in peak | Change before saturation, not on a fixed date only. |
| Catch count and ID | Every service | Recorded per unit and trended. |
Identifying the catch
The board is a record. Before it is discarded, the catch is counted and the main species identified, because the identity of what you caught is what makes the number meaningful. The species that matter most in food premises fall into three groups.
Houseflies and filth flies
The common housefly Musca domestica, is the classic strong-flying intruder. Blowflies (Calliphoridae) fall in the same category. A cluster of houseflies on the unit nearest a particular door is an entry and hygiene signal: a door left open, a failed self-closer, a bin standing outside, or organic residue near that entrance. These are insects flying in, so the fix is usually exclusion and sanitation at the source rather than more killing power.
Fruit flies and small flies
Fruit flies Drosophila species, are weak fliers, so a high catch of them means a breeding source is close by and inside. They breed in fermenting and decaying organic matter: overripe fruit, spilled product, the biofilm in a floor drain, a mop bucket, the residue under a conveyor. Drain flies and phorid flies tell a similar story. A spike in small flies is almost always a sanitation problem in a specific, findable location, and it will not respond to a new lamp.
Stored-product moths
The Mediterranean flour moth Ephestia kuehniella, and the Indianmeal moth, Plodia interpunctella, are the ones that make a quality manager sit up. Catching these adult moths on internal light traps points not to insects flying in but to a breeding population in your raw materials or in a dead storage spot. They are the flying stage of the same problem covered in the stored-product insects guide, and a rising moth catch should trigger an inspection of flour, grain, dried fruit and any long-standing stock.
What the counts reveal when you trend them
A single board is a snapshot. The value comes from trending: plotting the count per unit and per species over weeks and months. Trends answer the questions auditors and quality managers actually care about.
- A steady seasonal housefly rise across all units in summer is normal and expected. A rise on one unit only points to a local failure at that location.
- A fruit fly spike after a wash-down change or a drain problem shows the sanitation regime slipped, and the trend confirms when the corrective action brought it back down.
- Any internal stored-product moth catch above your baseline is an early warning of infestation, often weeks before you would see damage in the product.
This is why BRCGS, FSSC 22000 and retailer audits expect flying insect activity to be trended and fed back into the hazard analysis, not just tallied. A trap that is emptied without its catch being read is a monitoring device switched off.
Keeping it auditable
Numbered units on the site map, a defined lamp and board schedule with dated changes, catch counts identified by species, and trend graphs with action levels: that is a complete flying-insect programme. Template catch logs and trend sheets are on our free checklists and forms page, and the species above are pictured in the pest species guide. On ASC contracts every count and change is captured digitally per unit, so the trend and the audit pack are one click away.
Frequently asked questions
Where should insect light traps be placed in a food factory?
To intercept insects as they enter: back from and to the side of external doors so the glow is not visible from outside, never above or facing exposed product, and generally at 1.8 to 2.2 metres for housefly units. Keep them clear of exposed product and competing light, and mark each on the site map.
How often should ILT lamps and glueboards be changed?
Replace UV lamps at least annually, before spring, because output degrades before the lamp fails. Change glueboards on a defined schedule, typically monthly and more in peak, always before saturation. Use shatter-resistant lamps in food areas and record every change.
What can catch counts tell you about a facility?
Trended by species they diagnose problems: houseflies near a door mean a proofing or hygiene failure; fruit flies mean a nearby breeding source such as a drain; internal stored-product moths mean an infestation in raw materials. The trend shows where the problem is and whether the fix worked.
