How do I use biological controls in a greenhouse?
Which predator or parasite goes against which greenhouse pest, the temperature and timing each one needs, and the single rule that decides whether the release works.
- Difficulty
- intermediate
- Time
- 30 min
- Read
- 4 min
Short answer
Introduce early, at the first sign of a pest, while numbers are still low — predators multiply more slowly than pests and cannot rescue a heavy infestation. Match the agent to the pest, check the temperature window, stop spraying weeks beforehand, and release the day the delivery arrives.
A greenhouse is the ideal place for biological control because it is enclosed, warm and free of the weather that disperses released organisms outdoors. It is also the place where whitefly, red spider mite and vine weevil become unmanageable fastest. The chart in the diagram sets out the standard agents against their targets and conditions.
Step by step
- Identify the pest correctly before ordering anything.Each agent attacks one pest or a narrow group. Whitefly, red spider mite, thrips, aphid, mealybug, vine weevil larva and sciarid fly larva all have different controls, and the wrong one does nothing at all.
- Order early, at the first sighting.This is the rule that matters most. Predators reproduce more slowly than their prey, so a release into a low population establishes control while a release into an established infestation is overwhelmed. Check plants weekly and act on the first few insects.
- Match the agent to the pest.Encarsia formosa, a parasitic wasp, for glasshouse whitefly. Phytoseiulus persimilis, a predatory mite, for two-spotted red spider mite. Amblyseius species for thrips and young spider mite. Aphidius wasps and Aphidoletes midge for aphids. Cryptolaemus, a ladybird, for mealybug. Steinernema and Heterorhabditis nematodes for sciarid fly and vine weevil larvae.
- Check the temperature window.Every agent has a minimum temperature below which it does nothing. Encarsia needs sustained warmth and long days, so late spring at the earliest. Phytoseiulus needs warmth and humidity. Nematodes need moist compost above a stated soil temperature. Releasing too early wastes the order.
- Stop spraying well before release.Most agents are killed by residues from insecticides, including some products marketed as organic. Check the withholding period stated by the supplier — it is often several weeks — and stop spraying that far in advance.
- Release the day they arrive.These are living organisms with a short shelf life. Order for a specific delivery date when you will be at home, open the pack immediately, and distribute according to the instructions. Do not store them.
- Distribute properly through the house.Cards, sachets or carrier material placed among the plants at the rate stated, concentrated where you have seen the pest but spread across the whole house. Nematodes go on as a drench into moist compost, watered in.
- Repeat as the supplier specifies.Most programmes need two or three introductions a few weeks apart to establish, not a single release. Budget for the whole programme rather than one pack.
- Support them with cultural conditions.Damping down raises humidity, which favours Phytoseiulus and disfavours red spider mite. Yellow sticky traps monitor whitefly numbers, but remove them during a release or they catch your predators too.
Tips
- Yellow sticky traps are for monitoring, not control. Use them to detect the first whitefly, then take them down when you introduce Encarsia.
- Biological control does not eradicate a pest — it establishes a balance at a low level. Seeing a few whitefly after a successful release is normal and necessary, because the predators need prey to persist.
- Nematodes for vine weevil are applied in late summer against the young larvae, and again in spring. They are the only effective domestic control for vine weevil in containers.
Common mistakes
- Ordering after the pest has taken hold — It is the commonest reason a release appears not to work. Predators cannot out-multiply an established infestation, and money spent late is largely wasted.
- Spraying an insecticide first and releasing afterwards — Residues persist on foliage and kill the agents on contact. If you have sprayed, wait out the full withholding period the supplier states.
- Releasing too early in the season — Below its temperature threshold an agent does not reproduce or hunt. Check the stated minimum against your greenhouse's actual temperatures, using a max-min thermometer.
- Storing the pack for a convenient weekend — They are perishable living organisms and viability falls quickly. Order for a date you will be there.
Questions people ask
Do biological controls work in an unheated greenhouse?
In summer, yes. In spring and autumn the temperature often sits below the threshold for the warmth-loving agents such as Encarsia, which is why the recommended introduction dates start in late spring.
Will the predators escape or become a problem?
No. The agents sold for greenhouse use are specialists that depend on their prey, and populations collapse when the pest is controlled. They are not a garden pest in their own right.