🐞✗ Tested and false“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
“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.
