Hang yellow sticky traps among greenhouse plants to act as an early warning system alerting you when flying pests are about including thrips, aphids, and particularly whitefly.
Whitefly are such a nuisance if they get established in the greenhouse. They enthusiastically suck the sap of tomato plants, reducing performance and harvests; and every time you go in the greenhouse you have to run the gauntlet of flies in your hair, eyes and nose.
This habit of fluttering up when disturbed can be used against them, though. Yellow sticky traps reflect light at a wavelength particularly attractive to them. As they get stuck fast, you'll be able to monitor how many there are in your greenhouse. As soon as you see numbers building, an infestation won't be far behind, so you'll know to stock up on insecticidal sprays or biological controls. The sticky traps can also help control the worst of the infestation: hold them over your plants as you gently agitate the branches, and whitefly shoot up straight onto the sticky surface.
Hang among your plants from spring onwards among the leaves but not touching them. Replace every week or two: you'll know you're on top of the problem when your traps stay clear.
