When I was at Hadlow college, many moons ago, - one of the trips that we did was to a commercial tomato growing greenhouse. The size and scale of the operation was amazing, but the thing I remember with a smile on my face even now is one of my fellow students, (a young man of rather large build), who was wearing a yellow T-shirt. This was an unfortunate colour to wear as every greenfly, whitefly, thrip and hoverfly in the place took an interest in him, and he spent the entire visit in a moving cloud.
When I spotted greenfly on the Sage plants I was potting up yesterday (grown from seed, so i'm feeling slightly smug, and don't want the greenfly spoiling my feeling of smugness!) I cracked open a new pack of sticky strips.
I've found them very useful in my greenhouse, and use them either hanging from the roof struts, or, with a wire holder, sticking into the plant pots.- I find it best not to hang them in the centre of the greenhouse, simply because they are very painful if you walk into them!
Within a week, they'll be covered in dead flies, and hopefully my plants will be a lot less bothered by aphids and other flying pests.