What they are
Two pests, opposite weather. They cover the whole greenhouse year between them.
Whiteflies are tiny sap-sucking hemipterans, 1 to 2 mm long, with mealy white wings. Two species dominate under glass. Trialeurodes vaporariorum, the greenhouse whitefly, runs the cool seasons. Bemisia tabaci, the silverleaf whitefly, takes over once temperatures climb past 25 Celsius. Both want humid, still air and soft new growth.
Spider mites are arachnids, not insects. Eight legs, 0.4 mm long, the colour of dust. Tetranychus urticae, the two-spotted spider mite, is the one you will meet. It thrives in dry, hot air below 50 percent humidity and explodes when a greenhouse runs warm and arid in late summer.
Run a damp greenhouse and you get whitefly. Let it dry out and you get mites. Most operations cycle through both inside one season.
How to spot them
Whiteflies give themselves away. Brush a tomato or cucumber leaf and a cloud lifts off the underside. They cluster on the youngest leaves at the top of the plant. Look under those leaves for pale scale-like nymphs glued to the veins.
Spider mites hide better. The first sign is stippling on the upper leaf surface, tiny pale dots where the mites have drained individual cells. Flip the leaf. Run a finger across the underside and look for grit that moves. Once you see webbing across the growing tip the population is already large.
Hang yellow sticky cards at canopy height, one per 10 square meters, and check them weekly. Whiteflies stick to them readily. Mites do not, but a sharp rise in whitefly counts is your earliest warning that conditions are shifting and pests are moving.
Why they win in greenhouses
No rain. No wind. No winter.
A field crop gets washed by storms, blown by gusts, and frozen flat in October. A greenhouse erases all three. Generation times collapse. Whitefly goes from egg to adult in three weeks at 21 Celsius and in 17 days at 27. Spider mite finishes a generation in 7 days at 30 Celsius. Both species reproduce parthenogenetically when they have to, so a single migrant can found a colony.
Then there is chemistry. Both pests have well-documented resistance to neonicotinoids (imidacloprid, thiamethoxam), pyrethroids, and several older miticides. Spider mite, in particular, can develop resistance to a new compound within five or six sprays. The spray treadmill is a losing game under glass.
Knock them down: physical first
Spider mites hate water on the leaf. A vigorous spray from a wand, directed at the underside of every leaf, drops the population by 80 to 90 percent and crushes eggs. Repeat every three days for two weeks. Raise the humidity above 60 percent at the same time and they will not bounce back the way they did.
Whiteflies are harder to wash off but easier to vacuum. A handheld vacuum run over the canopy at dawn, when they are sluggish, lifts the adults straight into the bag. Empty it into soapy water.
For both pests: prune out the worst leaves and bag them. Do not compost on site. The eggs survive.
Bring in the predators
This is where greenhouses pay back the investment. The closed environment that breeds pests also holds biocontrol agents in place. Two species do most of the work.
Encarsia formosa for whitefly. A 0.6 mm parasitoid wasp that lays its eggs inside whitefly nymphs. The parasitised nymphs turn black and you can see them with a hand lens. Sold commercially as pupae glued to small cards. Hang one card per 2 to 4 square meters at the first sign of whitefly. The wasp needs temperatures above 18 Celsius and a host population to find. Release weekly for at least four weeks.
Phytoseiulus persimilis for spider mites. A bright orange predatory mite, faster than its prey and obligate on Tetranychus. It will eat every spider mite in a hot spot then starve, which is the design feature, not a flaw. Release at a ratio of one predator per 10 to 20 prey mites. Distribute across the worst patches and let them work outward. Visible knockdown in 10 to 14 days.
Both predators die from broad-spectrum sprays. If you have used a pyrethroid or a neonicotinoid in the past two weeks, the predators will not establish. Stop spraying before you release.
When it goes wrong
Whitefly numbers spike after a wasp release. Normal for the first week. The adults you see now were already in the system. Watch the cards for parasitised nymphs over the next 14 days. If you see none, your temperature is too low or the cards were old.
Mite population crashes then comes back worse. Phytoseiulus ate itself out of food in one hotspot while a second patch grew unchecked. Scout the whole house, not the obvious damage, and release predators in multiple locations.
A neighbouring crop reinfects you weekly. Whitefly walks in on workers' clothing and on transplants. Hold new plants in a quarantine bench for two weeks before they enter the main house. Screen vents with 50-mesh insect netting.
Yellow leaves and sticky honeydew everywhere. Whitefly excretes honeydew and sooty mould grows on it. You are past the early-intervention window. Cut hard, release wasps heavily, and accept reduced yield this cycle.
The honest read on greenhouse pest control is that you are managing conditions, not eliminating insects. Keep humidity in the 60 to 70 percent band, vent for air movement, scout weekly, and release predators early. The chemistry-first approach burns through products and breeds resistance. The biology-first approach gets cheaper every year.