Many harmful insects are attracted to the color yellow. It’s the color of plants under stress and millions of years of evolution have taught them to head directly to yellowing plants whose defenses are probably weakened.
You can find yellow sticky traps in most garden centers. They are simply pieces of yellow plastic or cardboard covered with a sticky, non-drying glue. Hang or clip one on or near a plant that is susceptible to infestations from flying insects (whiteflies, fungus gnats, aphids, thrips, leafminers, leafhoppers, moths, etc.)… then keep your eyes peeled. At the beginning of an infestation, you will quickly notice the presence of a lot of winged prisoners on the trap, a sign it’s time to think seriously about a pesticide treatment (insecticidal soap, neem, etc.).
And if you’re very lucky, the trap can even nip the infestation in the bud: if the very first whitefly arriving in your garden lands straight away on the sticky trap, it may be game over for whiteflies (or aphids or leafhoppers, etc.) for the year!