About this product
Whitefly are a different problem from most greenhouse pests. Any movement sends the adults up in a white cloud that resettles seconds later, so sprays hit leaves and rarely the insects. Sticky traps catch some and never enough. And a single female lays 100 to 200 eggs, which is how a few whitefly on the tomatoes becomes hundreds on everything inside a warm fortnight.
Encarsia formosa is a parasitic wasp about 0.6mm long, smaller than the pest it hunts, and it has been used in commercial tomato and cucumber houses since 1926. The female finds whitefly scales, the motionless nymph stages on leaf undersides, and lays a single egg inside each one. The larva consumes the nymph from within. After about ten days the scale turns visibly black, which is your confirmation that it worked. A new adult emerges from that black scale and starts again. One Encarsia parasitises 5 to 10 nymphs a day and lives around 30 days, so over a few weeks the balance on the plants shifts from mostly whitefly to mostly wasp.
They cannot sting, pose no risk to people, pets, plants or pollinators, and are effectively invisible. They stay on the plants where the whitefly are, and die back naturally once there is nothing left to lay eggs in.
Identify your whitefly before ordering, because species matters here. Encarsia is highly effective against glasshouse whitefly, Trialeurodes vaporariorum, and works considerably less well against tobacco whitefly, Bemisia tabaci. Look down at a resting adult: on glasshouse whitefly the wings close over the body in a heart shape. On tobacco whitefly they are longer and parallel, with a strip of body visible between them. If it is tobacco whitefly, get in touch rather than ordering, because you need a different control.
One more check. White threads sitting still on the tops of leaves are not whitefly, they are shed aphid skins or thrips larvae, and Encarsia will do nothing for either. Whitefly are almost always moving, and both body and wings are a ghostly white.
It suits greenhouse tomatoes and cucumbers, conservatory fuchsias, poinsettias, aubergines and peppers, polytunnels from late spring onwards, and overwintered plants brought inside. Live product, shipped refrigerated, and it cannot be returned.