Spider mites in the greenhouse.

11.08.2026.

Greenhouse note 41 · Diagnosis & crop care

Spider mites in the greenhouse.

Identification, prevention and control start with a close look at a leaf. Early warning is rarely obvious webbing. More often it is fine pale stippling, a stressed plant and a small population gaining ground in a sheltered, dry microclimate.

First inspection

Diagnose more than the damage.

Spider mites do not chew holes or leave one clearly bounded wound. They pierce leaf cells and draw out their contents. The first injury therefore appears as tiny pale or yellow dots: characteristic stippling. In a greenhouse, this can initially resemble dust, nutrient trouble, leaf ageing, sun stress or thrips damage. Rather than treating everything in haste, check three levels calmly: the whole plant, the leaf surface and signs of living activity.

Start with the whole plant. Spider mites often first appear as a local hotspot: at the warm end of a cucumber row, on a thirsty container, beside sun-warmed glazing or in older, hard-to-reach foliage. Note whether stippling follows veins, occurs on lower leaves, affects one side only or has reached neighbouring pots. Then turn over several leaves. Two-spotted spider mites commonly occupy undersides, where mites, eggs, cast skins and fine silk are most reliably found.

Use a hand lens where possible. Alternatively, hold white paper below a suspect leaf and tap it gently. Tiny moving dots on the paper are not a certain species identification, but they justify closer inspection. Check leaves at different heights and in different climate zones. An early, dependable diagnosis prevents treatment of an entire greenhouse because of one old damaged leaf when the active hotspot is still local.

Greenhouse design matters too. Paths, spacing and working height should allow you to lift leaves, clean under benches and reach dense growth without pushing neighbouring plants together. For an accessible, well-ventilated growing space, a Classic aluminium greenhouse provides a considered basis for plant care, climate management and regular pest checks.

01 · Plant appearance

Read the distribution.

02 · Leaf surface

Read the pattern.

03 · Living clues

Read the colony.

Plant appearance. Look for a tired grey-green cast, stippled lower leaves and isolated stressed plants as possible starting points. Mark the position instead of relying on memory.

Leaf surface. Pale punctures may merge into uneven mottling and later dull bronze patches. Compare upper and lower surfaces on several leaves, not only the most conspicuous one.

Living clues. Look for moving dots, rounded eggs, pale cast skins and finest threads along veins or in leaf axils. Only these clues show whether visible damage is still progressing.

Damage atlas

From stippling to bronzing.

The severity of visible damage helps set priorities, but does not reliably show the current population size. A heavily bronzed leaf may record an older infestation after mites have already declined. Conversely, fresh eggs and active colonies can sit on a plant that still looks surprisingly green. Always assess both the lasting damage and present activity on leaf undersides.

Spider mites are arachnids, not insects. Adults are tiny and often appear as moving dots without magnification. Depending on stage and colour, they may look greenish, yellowish or reddish. The two-spotted spider mite, Tetranychus urticae, is a common greenhouse pest; two dark dorsal spots are typical. Decisions should still rest on the actual evidence on the plant, not an assumed characteristic colour alone.

Fine stippling

Dense mottling

Bronzing

Silk appears

Leaf exhausted

Evidence, not worry

Spider mites and common lookalikes.

Stippling deserves attention, but does not belong only to spider mites. A sound diagnosis answers two questions: does the damage fit sap-feeding mites, and can the animals or direct traces of their activity be found? Check undersides, young and old growth, symptom distribution, dark droppings and surface texture. Also consider whether changes appeared after heat, irregular watering or a spray application.

Thrips can cause pale to silvery, often streaked scraping marks that resemble mite stippling. Small dark droppings and slender moving insects in flowers, tight folds or on the surface are useful clues. Lime deposits and spray residues sit on the leaf and can often be wiped away when damp; spider-mite injury is within the tissue and remains visible. Oedema is more likely to appear as corky, blistered or raised areas linked to an uneven water balance. Normal leaf ageing is usually more even: an older leaf declines in an orderly way rather than developing a spreading field of fresh punctures in a warm zone.

Water stress and physiological leaf changes also call for restraint. Soft, curled or irregular leaves alone do not prove spider mites. If moving mites, eggs, silk and fine underside stippling are absent, first check the root zone, irrigation and recent climate. Where damage is uncertain or advancing, seek qualified plant-protection advice rather than treating on suspicion.

Why a small find becomes a greenhouse problem

Heat, drought stress and delay increase pressure.

Spider-mite development and reproduction are strongly shaped by conditions. Warmth can accelerate population growth, while drought-stressed plants are less resilient. Dusty leaves, crowded pots, inaccessible undersides and late discovery add to the effect. There is no universal development time: temperature, host plant, mite species and microclimate interact. In a home greenhouse, a consistent inspection routine is therefore more dependable than a standard number of days.

Zone oneWarm leaf surfaces

Zone twoDry root zone

Zone threeCrowded growth

Zone fourLate discovery

Warm leaf surfaces: Leaves beside glazing, sun-facing containers and still air in upper foliage can be much warmer than a thermometer near the door suggests.

Dry root zone: Repeated water shortage increases loss of useful leaf area and reduces a plant’s ability to tolerate an initially limited infestation.

Crowded growth: Touching plants create sheltered crossings. Overcrowding also makes underside checks and even mechanical or approved treatment harder.

Late discovery: Eggs, immature stages and adult mites can occur together. A measure reaching only exposed mites may be followed by renewed activity without follow-up checks.

Prevention therefore does not mean forcing permanently high humidity. More important are lower avoidable plant stress, timely ventilation and shading, clean accessible crops, and a checking rhythm that finds small colonies before dense foliage forms a sheltered, webbed microclimate.

Prevention through climate and access

A greenhouse you can inspect.

Inspect new plants carefully before they join the established crop, and keep them separate initially where possible. This is especially worthwhile for herbs, vegetable seedlings, annual flowering plants and overwintered containers, which can carry a light infestation without obvious damage. Keep a short note of plant name, position, date, symptoms, living finds and actions taken. Such records reveal recurring heat zones and vulnerable crops more quickly and reliably than memory.

Water for steady growth rather than crisis management. Spider mites do not disappear because a dried root ball is then kept permanently wet; wet foliage is no substitute for sensible irrigation. Check pot weight or substrate moisture and water each crop according to need, as evenly as possible. A well-planned growing space such as the Classic aluminium greenhouse creates clear positions and reliable working routes for regular care.

Ventilate and shade with actual leaf temperature in mind. Roof vents, open doors and suitable shading can ease extreme heat pockets, but do not replace observation. In New Zealand, intense sun, wind and abrupt weather changes can occur close together. A resilient growing space needs more than an attractive shell; it also needs room to reach every plant. The Classic aluminium greenhouse offers planning options; prioritise clear paths, practical working heights and adequate spacing between plants.

Keep floors, bench undersides, pot rims and glazing clean. Dust can favour mite problems, while abandoned plant material makes inspection harder. Remove badly damaged leaves if this will not unnecessarily defoliate the plant. Bag infested material hygienically and take it out of the greenhouse. Do not leave a heavily infested plant or cuttings on an open compost heap immediately beside the structure, where mites may return to the crop.

New plants: keep separate first, inspect undersides and check again before moving them in.
Plant spacing: Avoid continuously interlocking leaves that act as bridges between neighbouring pots.
Watering: Prevent repeated drought stress without correcting it with permanently waterlogged substrate.
Heat: Assess stress around the leaves on bright afternoons, not only from the morning forecast.
Inspection rhythm: Check at least weekly, and much more often in heat, after new purchases or during active infestation.

Treatment is a question of reach

Reach undersides, then inspect again.

A targeted rinse with water can mechanically reduce mites and webbing, especially during an early local infestation on manageable plants. Work gently and avoid direct midday heat. For plant-protection products, use only products authorised or permitted for the exact crop, site and user category in New Zealand. Many measures work only through direct contact. What does not reach the protected colony beneath a leaf cannot reduce it adequately. The aim is careful coverage, not a stronger dose or hurried spray cloud.

Incomplete approach

Top side only

Methodical approach

Access before application

Excessive approach

More is not better

Top side only. Upper leaf surfaces are wetted while the colony below stays largely sheltered. The plant looks treated, yet active mites remain in their preferred location.

Access before application. Move pots apart, support dense foliage carefully and work section by section according to product or beneficial-insect directions. Inspect again instead of assuming one pass reached every life stage.

More is not better. Overdosing, treating heat-stressed plants or using a product unsuitable for the crop can damage leaves and harm beneficials. Follow the label, protective directions, temperature guidance, withholding periods and all restrictions.

For a small isolated find, rinsing, removing badly damaged leaves and close follow-up may be enough. For a larger infestation, combine isolation, hygiene, stress reduction and an appropriate control strategy. Predatory mites such as Phytoseiulus persimilis are used in greenhouses against spider mites. Species, quantity and release method depend on infestation level, temperature, crop and products already used. Ask the supplier for advice and check compatibility with earlier or planned plant-protection measures. Broad-spectrum products can harm beneficials and disrupt biological control.

New Zealand requirements: Use products only as allowed by their New Zealand approval, label and directions for the relevant crop, pest, place and user. Product labels set application, protective measures, withholding periods and restrictions. A product described as natural or organic is not automatically harmless or exempt from controls. Check current EPA information and the product label before use.

Recovery follows observation

The next leaf matters, not the old scar.

A stippled or bronzed leaf will not become fully green again. Success is shown by less living activity, no fresh punctures, no increase in webbing and clean new growth that remains clear on repeated inspection. Leave a few cosmetically damaged but still functional leaves on the plant initially as comparisons. Removing every marked leaf at once can make it harder later to distinguish old injury from fresh feeding.

Day 0Confirm and contain.Mark affected plants, isolate where possible, inspect neighbours and note living mites, eggs or webbing.

First follow-upLook beneath the visible action.Reinspect undersides, leaf bases and dense plant centres. Check whether rinsing or treatment actually reached the original colony zones.

Second follow-upSeparate old damage from fresh stippling.Compare marked leaves with new growth. Fresh punctures, moving mites, new eggs or silk mean activity continues.

AfterwardsReturn to the normal inspection rhythm.Keep records, correct recurring heat or water stress and maintain access around plants so a new hotspot is noticed early.

A well-planned structure supports this routine. The Classic aluminium greenhouse can be arranged as an orderly growing space rather than a tightly packed storage area. Keep an inspection path clear, place vulnerable plants within reach and respond to local conditions with ventilation and shading. For accessible, care-friendly planning, explore the options of the Classic aluminium greenhouse.

Frequently asked questions

Clear answers for careful inspections.

Can spider mites be present without webbing?

Yes. Fine silk is often visible only once the population is larger. Early colonies may show only pale stippling, rounded eggs, cast skins and tiny moving animals on leaf undersides. Webbing is a strong additional clue, but not a requirement for a reliable diagnosis.

Must an infested greenhouse plant be discarded?

Not automatically. A valuable plant with a light infestation can often be isolated, rinsed carefully or treated appropriately, then monitored closely. However, a plant that is badly damaged, very dense and practically impossible to inspect fully can become a continuing source for the whole crop. Hygienic removal may then be the sensible choice.

Does higher humidity eliminate spider mites completely?

No. A stable, crop-appropriate water balance reduces drought stress and is preferable to a hot, extremely dry microclimate. Humidity alone is not a control programme. Do not keep the greenhouse continually damp or leaves persistently wet, as this can encourage other problems such as fungal disease.

Can I release predatory mites after spraying?

Possibly, but compatibility depends on the product used and how long it remains active. Some products affect beneficials more strongly or longer than expected. Check the label and approval, then ask the beneficial-insect supplier about waiting time, suitable predatory-mite species, release method and necessary climate conditions.

How often should I inspect during an infestation?

Inspect more often than in the normal weekly routine. The right interval depends on crop, temperature, infestation level and the chosen measure. The essential point is to revisit the same leaf undersides in time, before overlooked eggs or protected colonies create surprising new activity.

Are spider mites actually spiders?

Like spiders and ticks, they belong to the arachnids, but they are tiny plant-feeding mites. Their silk forms fine threads and, in heavy infestations, dense webbing on plants; it is not a large radial spider web.

Sources and further reading

A short, dependable reading list.

These sources support the diagnostic and integrated approach in this guide. For individual crops and applications, the current New Zealand product label, EPA controls and qualified advice always take priority.