00WHAT THE GARDENERS SAY

A card that
counts the flies.

A bright greenhouse bench of young potted plants with a plain yellow sticky card hanging on a cane among them — AI-generated image, not a photograph of a real person or place.

A sticky trap is mostly an early-warning system.

Hung among houseplants or greenhouse crops, a yellow or blue sticky card shows which small flying pests are about and how many. The RHS treats them as a way to monitor pests, with a few cautions about what else they catch.

MONITORmore than controlUpdated 18 October 2026

Match a pest

01Principles

Yellow for whitefly and gnats
Blue for thrips under glass
Indoors, not out
See the problem early

02THE SHORT ANSWER

Pick the colour,
then read the card.

Different pests prefer different colours.
The RHS uses cards to watch numbers, not to wipe pests out.

YELLOW

Whitefly and fungus gnats

The RHS suggests yellow sticky sheets among plants to trap adult glasshouse whitefly, mainly to monitor activity rather than control it, and yellow traps near pots for adult fungus gnats.

Match a pest

BLUE

Thrips under glass

For thrips, the RHS suggests blue sticky traps in glasshouses, and advises against using them outdoors, where they catch large numbers of other invertebrates.

See the research

BYCATCH

They catch friends too

Sticky traps often catch non-target insects, including parasitoid wasps that prey on whitefly, so the RHS says to weigh their use with biological controls.

Use them well

03MATCH A PEST

What is flying
round your plants?

Pick the pest you think you have to see what the RHS says about sticky traps for it. The bar shows how useful a card is as a first step.

Based on RHS advice pages on fungus gnats, glasshouse whitefly and thrips. General gardening information.

Pest matcher

Choose a pestTiny dark flies over damp compostClouds of tiny white flies when touchedSilvery, mottled leaves under glassPests on plants in the open garden

Tiny dark flies over damp compostYellow card

Likely fungus gnats: dark greyish-brown flies, mostly 3 to 4 mm long. Yellow sticky traps near the plants can reduce adults; drier compost and fresh houseplant compost help with the larvae.

Clouds of tiny white flies when touchedYellow card

Likely glasshouse whitefly, about 1.5 mm long, flying up when a plant is disturbed. Yellow sticky sheets help monitor activity rather than control it.

Silvery, mottled leaves under glassBlue card

Thrips are narrow insects up to 2 mm long. Blue sticky traps in glasshouses can trap them and help monitor and reduce numbers.

Pests on plants in the open gardenSkip the card

The RHS does not recommend blue sticky traps outdoors because they capture large numbers of non-target invertebrates.

Sources: RHS, Fungus gnats or sciarid flies; RHS, Glasshouse whitefly; RHS, Thrips.

04TWO FINDINGS WORTH KNOWING

Colour matters,
and so does the glue.

Two recent studies on why insects land on a card,
and how small details change the catch.

58%

Drop in thrips' attraction to yellow with a hazy glue

In a wind tunnel, western flower thrips preferred yellow over blue with a clear glue. With a whitish, hazier glue, attraction to yellow fell 58% and attraction to blue rose 65%.

GLUE · SCI REP 2021

3 receptor types

Insect colour receptors used to design better yellow

Whitefly and thrips have UV, blue and green-sensitive receptors. Swansea University researchers found yellow cards tuned to that vision improved capture of both pests on a single card.

VISION · SWANSEA 2025

In greenhouses, the 2021 study found similar effects: with the hazier glue, attraction to yellow fell by 35% and attraction to blue rose by 32%.

05FOUR WAYS TO USE A CARD

Hang it,
then read it.

A card works best as a regular check on what is about. These habits follow RHS advice.

Hang it at plant height
Place cards above or among the plants, where adult flies move, rather than far across the room.
Check it regularly
Count new catches to see whether numbers are rising, so you can act while they are still small.
Fix the cause too
For fungus gnats, let compost dry between waterings and repot into fresh houseplant compost; cards only catch the adults.
Mind the helpers
If you use biological controls such as parasitoid wasps, think twice: sticky cards catch them too.

A plain blue sticky card hanging among flowering plants under glass, lit by soft daylight — AI-generated image, not a photograph of a real person or place.

06NOT FOR MICE

Glue traps for rodents
are banned in England.

A houseplant in a pot on a sunny windowsill with a small plain yellow sticky card pushed into the compost — AI-generated image, not a photograph of a real person or place.

Insect cards are one thing; glue boards for rodents are another. Under the Glue Traps (Offences) Act 2022, it has been an offence since 31 July 2024 to set a glue trap in England to catch a rodent, or in a way that risks catching one.

The only exception is a licence for pest controllers, granted to preserve public health or safety where there is no other satisfactory solution. Anyone who finds a set glue trap that puts rodents at risk must, without reasonable excuse, make it safe.

07Questions

Questions people ask

Do yellow sticky traps get rid of whitefly?

The RHS says yellow sticky sheets help monitor whitefly activity rather than give control. They catch adults, while nymphs keep feeding under the leaves.

Do sticky traps work for fungus gnats?

The RHS says yellow sticky traps near plants can reduce adult numbers. The larvae live in damp compost, so letting compost dry and using fresh houseplant compost matters too.

Why blue traps for thrips?

Thrips are often targeted with blue cards, and the RHS suggests blue traps in glasshouses. Research shows thrips also land on yellow, and that even the glue's haze can shift which colour they prefer.

Can I use sticky traps outdoors?

The RHS does not recommend blue sticky traps out of doors, because they capture large numbers of non-target invertebrates.

Are rodent glue traps legal?

Not in England for the public. Since 31 July 2024 it has been an offence under the Glue Traps (Offences) Act 2022 to set one to catch rodents, unless done by a pest controller under a licence.

08Sources

A card that
counts the flies.