Ordered by post and released, not attracted with planting. Here's when to use encarsia formosa, what it needs, and how to tell if it worked.
What does it look like?
Adults are tiny, about 0.6mm long, and rarely seen directly: the practical sign is the blackened scale, not the wasp. Females have a dark head and thorax with a yellow abdomen; males, which are rarely seen, are entirely dark.
How to use it
Availability: Available by mail order, supplied as infected black whitefly scales fixed to pieces of leaf or card. Introduce it before plants are heavily affected, since it can't give instant control once whitefly numbers are already high.
Conditions: Females lay 50 to 100 eggs singly in whitefly scale nymphs. This species is specific to glasshouse whitefly and isn't effective against other whitefly species. RHS's general guidance for glasshouse predators and parasitoids, this one included, calls for daytime temperatures of at least 21C; they can survive down to about 13C, but tend to die out over winter and often need reintroducing the following year.
How to tell it's working: Parasitised nymphs turn black, while healthy scales stay whitish-green: a built-in way to tell whether it's working. After release, adult wasps emerge within a few days, and the first blackened scales should appear after about three weeks.
What it helps control
Myths about Encarsia formosa
5 common claims about Encarsia formosa, checked: 1 busted1 disputed0 untested3 hold up
“Encarsia formosa controls whitefly effectively in the open garden, not just under glass.”
Encarsia formosa is sold and discussed purely as a glasshouse whitefly control, released onto infested plants under cover rather than in the open garden. It targets glasshouse whitefly specifically and isn't effective against other whitefly species, and general guidance for glasshouse predators and parasitoids calls for daytime temperatures of at least 21C, conditions an open Scottish border rarely holds for long. There's no guidance describing outdoor release as a working strategy.
Our read The absence of outdoor guidance isn't an oversight: the wasp is bred and supplied specifically as a protected-cropping product because it depends on stable warmth and shelter it won't find outside.
What to do: Release Encarsia formosa only in a greenhouse, polytunnel or conservatory with whitefly-infested plants, and don't expect it to help with whitefly on outdoor crops or ornamentals.
In Scotland: Scotland's cooler, more changeable outdoor conditions make the enclosed warmth of a greenhouse even more essential for this species than it would be further south.
2 sources
- RHS: Glasshouse whiteflyEncarsia formosa is an established method of control for glasshouse whitefly, introduced onto infested plants under glass.
- RHS: Biological control in the gardenGlasshouse predators and parasitoids, Encarsia formosa included, need daytime temperatures of at least 21C to breed effectively.
“A single release of Encarsia formosa is enough to keep whitefly under control long-term.”
A single release establishes a working population that parasitises whitefly scales over several weeks, but glasshouse predators and parasitoids of this kind can survive down to about 13C, yet tend to die out over an unheated winter and often need reintroducing the following year. So one release can carry a growing season, but it isn't a permanent fix if the greenhouse cools in winter and the wasp population collapses.
Our read Whether a single release is 'enough' depends on what timeframe a gardener means: enough for one summer's whitefly, generally yes; enough forever, no, since the population needs re-establishing after cold spells.
What to do: Treat each release as covering the current growing season, and plan to reintroduce Encarsia formosa the following spring if the greenhouse isn't heated enough to carry the wasps through winter.
2 sources
- RHS: Biological control in the gardenGlasshouse predators and parasitoids can survive down to about 13C but tend to die out over winter and often need reintroducing the following year.
- RHS: Glasshouse whiteflyAdult wasps emerge from a single introduction of infected scales within a few days, with the first blackened, parasitised scales appearing after about three weeks.
“Encarsia formosa needs daytime temperatures of at least 21C to work well.”
Guidance for glasshouse biological controls, which covers Encarsia formosa alongside other predators and parasitoids, specifies daytime temperatures of at least 21C for effective breeding, while noting the insects can survive down to roughly 13C without thriving. Below that range, egg-laying and development slow enough that the wasp struggles to keep pace with a whitefly population.
What to do: Check a min/max thermometer in the greenhouse before releasing Encarsia formosa, and hold off releasing until daytime temperatures are reliably reaching the low twenties, supplementing with heating if the crop justifies it.
In Scotland: An unheated Scottish greenhouse often won't hold 21C daytime temperatures until well into late spring, so early-season releases are more likely to underperform than the same release timed for a milder English greenhouse.
1 source
- RHS: Biological control in the gardenGlasshouse predators and parasitoids need daytime temperatures of at least 21C to breed effectively, though they can survive down to about 13C.
“Encarsia formosa should be released as soon as whitefly are spotted, not after the infestation has built up.”
Encarsia formosa should be introduced before plants are heavily affected, because the parasitoid cannot give instant control once whitefly numbers are already high. Its mode of action, laying eggs singly into individual whitefly scale nymphs, scales slowly compared with a whitefly population that's already exploded.
What to do: Start checking the undersides of leaves for whitefly scales regularly from early spring, and order Encarsia formosa for release at the first sign of them rather than waiting until an infestation is visible at a glance.
1 source
- RHS: Glasshouse whiteflyIt is important to introduce the parasitoid before plants are heavily affected, as it cannot give instant control.
“Spraying pesticides alongside an Encarsia formosa release defeats the purpose, because the sprays kill the wasps too.”
Predators and parasites used for biological control, Encarsia formosa among them, are very susceptible to most pesticides, so a general insecticide spray applied around the same time as a release can wipe the wasps out along with the whitefly. Some lower-impact products are compatible on a delay: natural pyrethrum can be used up to seven days before release, and fatty acid or plant oil sprays up to the day before introduction.
What to do: Stop using broad-spectrum insecticides in the greenhouse before releasing Encarsia formosa, and if a spray is needed nearby, stick to the recommended pre-release windows for pyrethrum or fatty acid products rather than spraying after the wasps are established.
1 source
- RHS: Biological control in the gardenPredators and parasites are very susceptible to most pesticides; natural pyrethrum can be used up to seven days before release and fatty acid or plant oil products up to the day before.
Frequently asked questions
Where do I get encarsia formosa?
Ordered by post from a biological control supplier, not bought off a shelf. See "How to use it" above for timing and conditions.
How do I know if encarsia formosa are actually helping?
You won't see encarsia formosa themselves working: they're too small to spot. Watch the pest population instead: fewer glasshouse whitefly over the following weeks is the real sign it's working.
Sources
2 sources, recorded with what each was used for
- RHS: For Glasshouse Whitefly: An established method of control for glasshouse whitefly is to introduce tiny parasitoid wasps, Encarsia formosa, which prey on the whitefly scale nymphs. Available by mail order. Parasitised nymphs turn black.
- RHS: RHS's dedicated biological control page: Encarsia formosa's identification (0.6mm, dark head and thorax with a yellow abdomen in females, entirely dark males), egg-laying rate, supply as infected scales on card, and timing of introduction before whitefly numbers get high.