Pests & Diseases
A field guide to the insects, mites and diseases that actually attack crops in Thailand — how to recognise them with your own eyes, how fast they multiply in Thai heat, when in the Thai year each one arrives, and what to do first. Written for growers who can see that something is wrong but do not yet know what to call it.
This guide is written for Thailand. The seasons used throughout are the Thai seasons — hot (March–May), rainy (May–October), cool (November–February) — not a four-season temperate year. The pest list is what is recorded in Thailand and Southeast Asia, not a European or North American list. Several widely repeated assumptions from foreign guides are wrong here, and each one is flagged where it matters.
This is part one of three. It covers diagnosis, scouting method, and the nine insects and mites you are most likely to meet. The other two parts:
On this page
How to use this guide
Start at the quick diagnosis table. Find the thing you can actually see — a silvery patch, a curled tip, a wilting plant — and it will point you at two or three suspects and tell you the first thing to check. Then read that organism's section in full before you treat anything.
Every section follows the same shape: what it is, how to recognise it with the naked eye or a hand lens, what it does to the plant, how fast it breeds at Thai temperatures, when to act, and then control in the order that actually works — cultural first, biological second, chemical last. That order is not a preference. It is how integrated pest management is defined, and for a farm working to GACP it is also the order that protects your residue results.
Three labels appear throughout this guide. VERIFIED means the figure comes from a named published source, listed on the sources page. INFERRED means we reasoned it out from verified data — usually by taking a measured biological threshold and reading it against Thai climate normals — and it has not been measured in Thailand. UNVERIFIED means we looked and could not confirm it; it is included only where knowing that nobody has measured it is itself useful. Do not treat an inferred figure as a measurement.
Quick diagnosis: what you see → what to suspect
Work down the left column until you find what is in front of you. This table narrows the field; it does not replace reading the section.
| What you can see | Suspect first | First thing to check |
|---|---|---|
| Silvery or bleached flecks and streaks on leaves or petals, with a metallic sheen at a low angle | Thrips. Not zinc deficiency. | Look inside the silvered patch with a 10× lens for tiny black varnish-like specks (frass). Frass present = thrips. Then tap a flower or growing tip over white paper and watch for fast-moving slivers. |
| Fine pale pinpricks on older, lower leaves; leaf feels dull and slightly rough; later fine webbing | Spider mites. Not magnesium deficiency. | Turn the leaf over, 10× lens. Moving specks, round eggs and webbing on the underside. Magnesium chlorosis is a continuous wash between veins with no texture change. |
| New growth at the top is small, hard, shiny, cupped downward, while the lower canopy looks fine; growth has stalled | Broad mite. Not calcium or boron deficiency. | You need 20–30× magnification, not 10×. Look on the underside of the youngest expanded leaf for transparent eggs covered in rows of white bumps. Nothing else on a plant looks like those eggs. |
| Cloud of tiny white insects lifts off when you shake the plant; leaves sticky; black sooty film on lower leaves | Whitefly | Turn over an upper leaf for adults and eggs, a lower leaf for the flat oval scales and pupae. Sticky leaves are honeydew — a nutrient problem never makes a leaf sticky. |
| White cottony or mealy lumps in leaf axils, under the crown, along stems or on the medium surface; ants present | Mealybug | Prod the white material. Mealybug wax moves and has a soft insect body underneath; fungal mycelium does not move. |
| Clusters of soft insects on growing tips; white cast skins on the leaf below; honeydew; ants running the stems | Aphids | The shed white skins below a colony are the earliest reliable sign. At 10×, look at the two tailpipes (cornicles): long and pale with black tips, or short and stubby. |
| Chewed holes, skeletonised patches, dark frass pellets in leaf axils | Caterpillars | Look for a fuzzy buff-coloured egg patch on a leaf underside, and search the medium surface and litter by day — the larger larvae hide there and feed at night. |
| Small dark flies running and hopping on the surface of the growing medium, not flying in the canopy | Fungus gnats | Lay a yellow sticky card flat on the medium. Then put a slice of raw potato on the surface and lift it after 48 hours to count translucent, black-headed larvae underneath. |
| Plant is stunted and yellowing, roots look intact, EC and pH are correct, and feeding changes nothing | Root aphid, fungus gnat larvae, or early root rot | Wash the root ball in a bucket and look at it. This is a diagnosis you cannot make from the leaves. |
| Plant wilts in the afternoon and recovers at night, then one day does not recover | Pythium or Phytophthora root rot | Pull a plant. Brown, soft roots whose outer layer slips off leaving a thread = oomycete. White firm roots = it is a water, EC or VPD problem, not a disease. |
| Yellowing and wilting on one side of a leaf or one side of the plant, lower leaves first | Fusarium wilt | Cut the stem lengthwise near the base. A brown stain in the vascular ring is the answer. The roots may look reasonably normal — that is the trap. |
| White talc-like circular patches on the upper leaf surface that rub off on your finger | Powdery mildew | Confirm it rubs off. Then check whether it is on the upper surface (common mildews) or whether the upper surface is only blotchy yellow and the powder is on the underside — that is a different mildew and it is regularly mistaken for magnesium deficiency. |
| Soft brown rot with grey-brown fuzz, usually starting in a spent flower, a wound, or dense congested growth | Grey mould | Grey fuzz with an advancing edge into living tissue = Botrytis. Wet, slimy and foul-smelling with no fuzz = bacterial soft rot, a completely different problem. |
| Round or angular spots with a hard defined edge, sometimes with concentric rings or a yellow halo, on lower leaves | Fungal leaf spot — in Thailand most often anthracnose or target spot | Put a 10–20× lens on the centre of a spot and look for fruiting bodies — for anthracnose, a pinkish-orange spore mass in humid weather. New spots after every wet spell confirms it. |
| Small spots that look water-soaked and greasy when held up to the light, later black and angular, stopping at the veins | Bacterial leaf spot | The translucent water-soaked stage is the bacterial signature. Then stop all handling, pruning and harvesting while the crop is wet — that is the main way it travels. |
| Wet, structureless mush that collapses within a day or two and smells foul | Bacterial soft rot | Find the wound it started at. Remove and bag the plant in dry conditions and disinfect the tool you last used on it. |
| Mosaic, ringspots or bright mottling plus distorted, cupped or strapped leaves, on scattered individual plants | Virus. Not magnesium or iron deficiency. | Count how many plants. A nutrient problem hits every plant on the same tank at once; a virus hits scattered individuals and spreads. Deficiency changes colour only — virus changes shape and colour together. |
| Uniform yellowing or discoloration across every plant on the same feed line, no insects, no spots, no distortion | Nutrient disorder, not a pest | Go to the Nutrient Deficiency Guide, and read how to tell the two apart. |
The single most useful habit: before deciding anything, note whether the problem is clustered or uniform. Pests and diseases arrive at a point and spread outward — hotspots near doors, vents, the edge of a block, one bench. Nutrition arrives through the irrigation and hits everything on that line at the same time. This one observation settles more diagnoses than any lens.
Scouting: how to look, how often, with what
Equipment
A 10× loupe is enough for spider mites, aphids and whitefly. It is not enough for broad mite VERIFIED, which needs a 20× hand lens at minimum and is better with a microscope — and broad mite is one of the most damaging and most misdiagnosed problems in protected crops in this region. Buy a 30× lens with a built-in light, or an inexpensive USB microscope. Thrips can be seen at 10× but identifying the species needs 40× on preserved specimens VERIFIED, which is a laboratory job, not a field one.
Sticky cards
Run both blue and yellow cards. A Thai field study in orchid gardens tested eight colours against melon thrips and found blue caught the most, then aqua, then yellow, then white, then green, then fuchsia — red and black caught none VERIFIED. Thrips peak in reflectance response around 450 nm VERIFIED. Yellow is correct for whitefly, fungus gnats, shore flies, winged aphids and leafminers. A yellow-only programme systematically under-counts thrips, which in Thailand is very likely your dominant pest INFERRED.
| Setting | Specification | Why |
|---|---|---|
| Density | At least 1 card per 93 m² (1,000 ft²), in an even grid VERIFIED | Fewer than this and you are sampling luck, not the crop |
| Extra cards | At every doorway and vent VERIFIED | That is where immigration happens, and where the first hotspot forms |
| Height, canopy pests | Just above the canopy, up to about 40 cm above the crop; raise as the crop grows VERIFIED | Below the canopy top you miss the flight layer |
| Fungus gnats and shore flies | Cards laid flat on the medium surface; 1–4 small cards per 93 m² VERIFIED | Horizontal placement catches far more than vertical |
| Whitefly where parasitoids are released | Cards laid flat VERIFIED | Reduces catching the parasitoids you paid for |
| Replacement | Weekly, repositioned in exactly the same spots VERIFIED | Trend matters more than the absolute count; move the card and the trend is meaningless |
| Reading frequency | Weekly minimum, every 3 days preferred VERIFIED | See the generation-time note below for why |
Plant inspection
- Inspect at least 20 plants per 93 m², spending at least 10 minutes per area VERIFIED.
- At least three plants per bench or bed, drawn from the edge, the middle and the interior — not whichever plants are easiest to reach VERIFIED.
- Work from the bottom of the plant upward, older leaves to newer leaves, turning leaves over VERIFIED.
- Weekly at absolute minimum; twice weekly is better VERIFIED.
- Where to look, by pest: thrips — tap new growth and flowers over white paper. Whitefly — uppermost leaves for adults and eggs, lowest leaves for scales and pupae. Aphids — growing tips, buds and leaf undersides. Spider mites — undersides of older lower leaves. Broad mite — undersides of the youngest expanded leaves at the growing point. Fungus gnat larvae — potato slice on the medium, read at 48 hours. Root aphid — wash the root ball; no leaf method is reliable VERIFIED.
Quarantine every plant that arrives
Isolate incoming plant material and inspect it for 4–5 days before it goes near the production area VERIFIED. Broad mite and root aphid both arrive this way, and broad mite symptoms persist for weeks after the mites themselves are gone VERIFIED — so a plant that looks damaged may be clean, and a plant that looks clean may be carrying eggs. Inspect for the organism, not for the symptom.
Why weekly is the minimum, not a comfortable target. At Thai temperatures a melon thrips generation takes about 9.6 days at 31 °C VERIFIED, a broad mite generation about 5 days at 26 °C VERIFIED, a whitefly generation about 14 days at 30 °C VERIFIED, and a spider mite generation can be as short as five days in summer conditions VERIFIED. A fortnightly scouting round in Thailand means you are looking at the second or third generation of whatever you find. Scouting intervals copied from a Dutch or British greenhouse manual, written around 20 °C, are roughly twice too slow here INFERRED.
Thrips
Thai เพลี้ยไฟ · Vietnamese bọ trĩ · Order Thysanoptera
Which thrips you actually have in Thailand
Almost every English-language greenhouse guide is built around western flower thrips, Frankliniella occidentalis. That species is not recorded as established in Thailand — the EPPO distribution list includes Malaysia, Myanmar, Vietnam and Java, but not Thailand VERIFIED. Planning your programme around its biology means planning around the wrong insect.
| Species | Thai name | Where it is documented in Thailand |
|---|---|---|
| Scirtothrips dorsalis — chilli thrips | เพลี้ยไฟพริก | Dominant and most damaging species in sweet-pepper screen-houses in Chiang Mai VERIFIED; recorded on chilli |
| Thrips palmi — melon thrips | เพลี้ยไฟฝ้าย | Recorded on okra and eggplant; Thai specimens date back to 1947 VERIFIED |
| Thrips parvispinus | — | Native to Southeast Asia, recorded on chilli in Thailand VERIFIED |
| Thrips hawaiiensis | — | 79% of all thrips on organic roses at Pak Chong, Nakhon Ratchasima, Sep–Dec 2020 VERIFIED |
| Ceratothripoides claratris | — | Present in Thailand with a genetically homogeneous population; a confirmed virus vector here VERIFIED |
| Thrips tabaci — onion thrips | เพลี้ยไฟหอม | Onion, shallot, garlic, watermelon, asparagus VERIFIED |
Expect a mixed population, not one species INFERRED. For a protected crop the evidence points to S. dorsalis as the dominant greenhouse species, T. palmi in open-field cucurbit and solanaceous systems, and T. hawaiiensis wherever the crop flowers heavily.
What you will actually see
Not the insect. First you see a silvery or bleached fleck on a leaf or petal. Inside that fleck, at 10×, there are tiny black varnish-like specks — frass. That combination is the diagnostic feature that separates thrips damage from a nutrient deficiency or spider mite damage VERIFIED. Then tap a growing tip or flower over white paper: fast-moving slivers fall out.
Adults are small. T. palmi is about 1.3 mm, clear yellow with black bristles VERIFIED; another source gives 0.8–1.0 mm with a dark line along the back where the wings meet VERIFIED. T. parvispinus females are about 1 mm and brown to black, males about 0.6 mm and uniformly yellow VERIFIED — that size-and-colour split between the sexes is a useful field cue. Adults prefer young growth; larvae cluster on older leaves along the veins VERIFIED, which is why you must look in two places.
Thrips or a deficiency?
| Thrips | Nutrient deficiency |
|---|---|
| Silvery flecks and streaks with rough, uneven margins, black frass specks present VERIFIED | Smooth, even discoloration with a predictable, symmetrical distribution relative to the veins; never any frass VERIFIED |
| Clustered and patchy — hotspots near doors, vents and the edge of the block VERIFIED | Uniform across the block, following the feed line rather than the airflow line VERIFIED |
| The silvering sits on the surface: emptied cells fill with air and flash metallic when you tilt the leaf into the light VERIFIED | Chlorosis is a pigment change through the whole leaf; it does not flash |
| Deformed petals, dark scars ringed with whitish tissue VERIFIED | Zinc deficiency gives interveinal chlorosis on newer leaves with small, rosetted growth VERIFIED |
How fast they breed in Thai heat
Thrips palmi on eggplant, measured in Taiwan — the closest good tropical dataset VERIFIED:
| Temperature | Egg to adult | Eggs per female |
|---|---|---|
| 22 °C | 19.3 days | 55.8 |
| 25 °C | 14.1 days | 64.2 (maximum) |
| 28 °C | 12.2 days | 60.0 |
| 31 °C | 9.6 days | 47.2 |
Lower developmental threshold 11.25 °C; 196 degree-days egg to adult VERIFIED. At 32 °C the egg stage alone is 4.3 days VERIFIED. In central Taiwan at 25–30 °C the species runs 25–26 generations a year VERIFIED. Scirtothrips dorsalis needs 265 degree-days egg to adult and develops up to a ceiling of 33.0 °C, giving up to 18 generations a year in the tropics VERIFIED. Ceratothripoides claratris is optimal at 30 °C, lays 116 eggs per female, and only stops developing above 40 °C VERIFIED.
Isaan mean temperatures run 27.4–29.9 °C from March to September VERIFIED. That sits almost exactly on the thermal optimum of all three INFERRED.
When to act
- Greenhouse eggplant, Taiwan: economic threshold 1.05–1.5 thrips per flower, or 4.91–10.17 adults per sticky card per 4-day count VERIFIED.
- Protected and field pepper, Florida: 2–3 larvae and adults per flower or fruit is tolerable VERIFIED.
- Generic protected-ornamental card thresholds: 15 per yellow card per week per 93 m²; under 10 for sensitive crops; a threshold of 1 where thrips-borne viruses are present; and as low as 2 where biological control is your strategy VERIFIED.
Because tospoviruses are present in Thailand (see the virus section), the threshold of 1 is the one that applies to most Thai growers INFERRED. No Thai-published action threshold for thrips in protected horticulture exists that we could locate UNVERIFIED.
Control, in order
1 · Cultural and physical
Screening. Thrips are the binding constraint: the critical body dimension is about 192–215 µm VERIFIED, against 239 µm for tobacco whitefly and 340–434 µm for aphids VERIFIED. Complete exclusion needs mesh with openings under 200 µm VERIFIED.
Be realistic about ordinary insect net. A 2025 field trial measured 40-mesh at 0.922 mm, 60-mesh at 0.495 mm and 80-mesh at 0.300 mm, and found 60-mesh gave only 36–44% reduction in thrips, with 80-mesh inconsistent VERIFIED. Commercial net in the 40–60 mesh range does not exclude thrips — it slows them.
UV-blocking cover is the strongest Thai result in this whole guide — see the virus section.
Blue sticky cards for monitoring. Remove volunteer cucurbit and solanaceous plants around the structure — the species is polyphagous on over 150 plants VERIFIED.
2 · Biological
Amblyseius swirskii is the best-evidenced predator. In Florida field cucumber it significantly outperformed N. cucumeris against T. palmi, in trials run at 27 ± 1.5 °C and 78–85% RH — close to a Thai rainy season VERIFIED. Its stated limits: develops and reproduces 15–36 °C, optimum around 25 °C, RH optimum 55–85%, and eggs fail to hatch below 50% RH VERIFIED. Read against Thai seasons that makes it a rainy-season and cool-season tool, not a hot-season one INFERRED. Cornell rates: 25 per m² preventative, 50 on detection, 100–300 in hotspots VERIFIED.
Thailand has native natural enemies that already work. Surveys of Thai eggplant found the parasitoid Ceranisus menes most effective and the predatory bug Bilia sp. second — but parasitism "was usually limited to home gardens which were not sprayed with insecticides" VERIFIED. Orius minutus, Wollastoniella parvicuneis and W. rotunda are also recorded from Thailand VERIFIED. The ant-mimicking predatory thrips Franklinothrips vespiformis is present in Thailand, develops in about 3 weeks at 27 °C, lives up to 60 days and survives on pollen when prey is scarce VERIFIED — a genuinely tropical-adapted agent.
Beauveria bassiana and Isaria fumosorosea have been isolated from inside Chiang Mai screen-houses, so they are already present in Thai protected structures VERIFIED. Humidity decides whether they work: infection reached 60–88.8% at 97.5% RH but only 15.3–43.9% at 75–80% RH — although on real leaf surfaces at 75% ambient RH it still reached 71–77%, because a humid boundary layer sits right against the leaf VERIFIED. Thai rainy-season conditions are close to ideal for these fungi INFERRED.
3 · Chemical, last
Rotate by IRAC mode-of-action group, never by trade name, and never use the same group twice in a row. Do not spray if you have released predators — the same Thai survey above shows what spraying does to the parasitoids already on your farm VERIFIED.
Use only a product carrying a DOA hazardous-substance registration number on its label, and observe the label's pre-harvest interval. See the legal section before buying anything.
On resistance: the Arthropod Pesticide Resistance Database contains no Thailand records at all for Thrips palmi — only Canada, Japan and China VERIFIED. The widespread belief that Thai populations carry heavy documented resistance is plausible but not documented in any source we could reach UNVERIFIED. Treat it as a real operational risk; do not quote a resistance ratio to anyone.
Spider mites
Thai ไรแดง · Vietnamese nhện đỏ · Family Tetranychidae — arachnids, not insects
Which species, in Thailand
A survey of 61 districts across 33 Thai provinces, October 2021 to December 2023, recorded 15 pest mite species in four families and found Tetranychus truncatus — not T. urticae — "infesting in almost every area surveyed" VERIFIED. T. urticae and T. kanzawai are both present and common; the full tetranychid list also included T. piercei, T. neocaledonicus, T. gloveri, T. phaselus, T. hydrangeae, Panonychus elongatus and Eotetranychus suvipakiti VERIFIED.
You cannot separate these in the field — at 30× they are all ไรแดง. The practical consequence is that published T. urticae life tables and biological-control pairings are an approximation for Thailand, not a match INFERRED. We could not find a temperature-resolved life table for T. truncatus at all UNVERIFIED.
What you will see
Adults are about 0.4 mm, oval, eight-legged, yellow to greenish amber, with two dark spots on the back that appear once feeding starts VERIFIED. Eggs are translucent white spheres on leaf undersides, usually under fine silk, hatching in about three days VERIFIED. They start on older, lower-canopy leaves and work upward, favouring leaves in direct sun and low humidity VERIFIED. Minimum 10× to count them VERIFIED.
The first thing a grower notices is a dull, dusty, faintly bronze cast on the lower leaves. Turning the leaf reveals the rest.
Spider mite or magnesium deficiency?
Both start on older leaves, which is exactly why they get confused. The stipple is the discriminator. Put a 10× lens on the pale area: if it resolves into hundreds of discrete pinpricks, it is mites; if it is a continuous wash between the veins, it is nutrition VERIFIED. Mite damage also changes the texture of the leaf — dull, bronzed, sandpapery, later brittle, with webbing VERIFIED. A deficiency never changes the texture and never produces eggs or webbing.
Speed and thresholds
Development can be as brief as five days in summer conditions, against about three weeks when cool; up to 100 eggs per female; adults live one to four weeks VERIFIED. At Thai hot-season canopy temperatures, a generation in under a week is the realistic planning assumption INFERRED. Do not use Dutch 20 °C figures.
Published thresholds, none of them Thai: 7 mites per leaf for a protected crop more than 5 weeks in production, 2 per leaf under 5 weeks VERIFIED; 5 or more per leaflet from a sample of 10 leaflets weekly in a field crop VERIFIED. UNVERIFIED for Thailand.
Control, in order
1 · Cultural and physical
Screening does not work for mites. They arrive on plant material, on clothing and on tools, not through vents — which is why no mesh figure exists for them INFERRED. Sticky cards do not monitor them either; leaf inspection is the only method.
They damage the lower leaves first, yellowing them before desiccation VERIFIED, so stripping and bagging senescent lower leaves removes the reservoir INFERRED. Raise canopy humidity where the crop allows it — mites favour hot, dry leaf surfaces VERIFIED.
2 · Biological
Neoseiulus longispinosus is the predatory mite actually used in Thailand. Thai extension guidance gives release rates of 2–5 mites per plant (roughly 5,300–13,300 per hectare) at two-week intervals, rising to 30–50 per plant over 3–4 applications for severe infestations VERIFIED. It was also one of only two predatory mites recovered in the nationwide Thai survey VERIFIED. This is the locally produced, locally adapted option.
Neoseiulus californicus works at 15–35 °C and 65–75% RH and tolerates humidity down to about 40%, and is described as more tolerant of heat and low humidity than Phytoseiulus persimilis, which "performs better in cooler climates" VERIFIED. For the Thai hot season that makes N. californicus the better-supported import and P. persimilis a rainy- and cool-season tool INFERRED.
Beauveria bassiana is a poor choice here specifically — in a controlled greenhouse study, two-spotted spider mite "showed minimal susceptibility across all conditions" VERIFIED. It works on thrips, whitefly and aphids; it does not work on spider mites.
3 · Chemical, last
We could not verify Thai registration for a single miticide. Not hexythiazox, etoxazole, spiromesifen, fenpyroximate, pyridaben, bifenazate or abamectin UNVERIFIED — the DOA registered-product database was unreachable, so this is a gap in our checking, not a statement that none are registered. It does mean we will not name one.
Choose by IRAC group with your supplier, looking at the registration number on the actual label. Mites develop resistance faster than almost anything else, so rotating groups is not optional. A Thai study on acaricide resistance in two-spotted spider mite on strawberry exists VERIFIED but we could not retrieve its compounds or ratios UNVERIFIED.
Broad mite
Thai ไรขาว · Vietnamese nhện trắng · Polyphagotarsonemus latus
This is the most misdiagnosed problem in protected cropping in this region, and it is not close. The damage is routinely read as a calcium or boron deficiency, herbicide injury or cold damage VERIFIED. Growers correct the feed for weeks while the crop stalls. The mite is 0.2 mm and invisible at 10×.
Confirmed in Thailand
Recorded in the 2021–2023 nationwide Thai mite survey, where it "caused tops of leaves to become small and curled" VERIFIED. Thai literature also documents ไรขาว as a cause of leaf-curl disorder in bird's-eye chilli VERIFIED.
How to actually find it
Adults are about 0.2 mm, oval and broad, pale yellow to yellow-green, the females with a white stripe down the centre of the back VERIFIED. But you will not find the adults. Find the eggs. They are about 0.07 mm, transparent and elliptical, and covered with rows of whitish bumps VERIFIED. Nothing else on a plant looks like that. They sit on the underside of leaves near the growing points VERIFIED.
You need a 20× hand lens at minimum; a microscope is better VERIFIED. At 10× you will see nothing and conclude it is nutritional. That is the whole failure mode.
Broad mite or a calcium/boron deficiency?
| Broad mite | Calcium or boron deficiency |
|---|---|
| New growth thickened, hardened, leathery and stiff; bronzed lower leaf surfaces; leaf edges puckered downward VERIFIED | Calcium: necrotic, papery, hooked or scorched leaf margins with tissue collapse. Boron: brittle, corky, cracked tissue and hollow stems VERIFIED |
| Growth stalls, or the terminal bud is killed VERIFIED | "If new leaves continue to expand normally, cultural stress is more likely" VERIFIED |
| Clustered — a few plants first, then spreading plant to plant VERIFIED | Uniform across the block; tracks the feed EC and pH or the transpiration pattern VERIFIED |
| Symptoms persist for weeks after the mites are gone VERIFIED | Corrected nutrition shows improvement in the next flush |
The decisive test: 20–30× lens on the underside of the youngest expanded leaf, looking for the white-bumped transparent eggs. That is faster and more reliable than a tissue analysis INFERRED.
Speed — and why the Thai rainy season is its season
Measured in India at 26 ± 2 °C and 65–75% RH, which is a fair Thai proxy VERIFIED: egg 2.05 days, larval 1.61 days, quiescent larval 1.47 days, egg to adult 5.13 days, 35.5 eggs per female (range 26–44), female lifespan 15.6 days.
The species thrives at 21–27 °C and 80–90% RH VERIFIED. That is a precise description of the Thai rainy season. Expect roughly five-day generations and explosive growth from May to October INFERRED.
When to act, and how
There is no published action threshold for broad mite on any comparable crop UNVERIFIED. The operational threshold is first symptom. Extension guidance is that predators "must be released before populations are high and plant damage occurs" VERIFIED — by the time you can see the distortion, you are late.
1 · Cultural and physical
Broad mite arrives on cuttings and on staff, not through vents. Quarantine and inspect all incoming plant material for eggs, not for symptoms — symptoms persist for weeks after the mites are gone, so symptom-based culling of incoming stock is unreliable VERIFIED. Males carry immature females on their hind legs until mating VERIFIED, which is why it moves across a bench so fast.
2 · Biological
Neoseiulus cucumeris, Neoseiulus californicus and Amblyseius swirskii are all effective and commercially available VERIFIED; broad mites and russet mites are explicitly within A. swirskii's prey range VERIFIED. The temperature and humidity limits in the thrips section apply here too. The high-humidity rainy season suits both the pest and these predators — the predator has to be established first INFERRED.
3 · Chemical, last
Same position as spider mites: we could not verify Thai registration for any miticide UNVERIFIED, so choose by IRAC group against the registration number on the label. See the legal section.
Russet and other eriophyid mites
Eriophyid mites are confirmed in Thailand — an Aculops species was recorded in the nationwide survey, along with Brevipalpus californicus (false spider mites) VERIFIED. Northern Thailand records include Eriophyes sandorici and Aceria litchii VERIFIED. Russet mites are wedge-shaped with only four legs, far smaller than broad mites, and produce the same top-down signature — thickened, leathery, twisted new growth with no stippling and no webbing VERIFIED. Separating russet from broad mite needs a microscope, not a lens INFERRED. Fortunately the control is the same, so the distinction matters less in practice than the diagnosis of "top-down mite damage" itself.
Whitefly
Thai แมลงหวี่ขาว · Vietnamese rệp phấn trắng · Bemisia tabaci
Which whitefly is in Thailand
A survey of 40 Thai locations in 2012–2013 found only indigenous cryptic species: Asia 1 dominant at about 95% of sequences, plus Asia II 6 and Asia II 10 at single sites. No invasive MEAM1 ("B biotype") and no MED ("Q biotype") were detected VERIFIED. Vietnam by contrast already had MEAM1 at around 20%, concentrated in the north VERIFIED. In the laboratory, MEAM1 completely displaced an indigenous species within four generations VERIFIED.
Two things follow. Your resistance and control assumptions should not be copied from MEAM1-dominated regions. And the buffer is thin: the invasive is already next door. That survey is now over a decade old and we found no more recent Thai one UNVERIFIED — treat "no MEAM1 in Thailand" as accurate as of publication, not as current.
Greenhouse whitefly, Trialeurodes vaporariorum, is the other species foreign guides lead with. We could not confirm whether it occurs in Thailand at all UNVERIFIED, but the physiology argues strongly against the lowlands: its population growth peaks around 24 °C and at a constant 32 °C, nymphs and puparia suffered 100% mortality — the life cycle could not be completed VERIFIED. B. tabaci, meanwhile, has its highest rate of increase at 30 °C VERIFIED. In lowland Thailand, expect B. tabaci overwhelmingly; if greenhouse whitefly occurs at all it would be in cool-season or highland production INFERRED.
Telling them apart in the field
B. tabaci adults are slightly smaller and yellower, and critically hold their wings vertical and parallel along the body like a steep tent roof, where greenhouse whitefly holds its wings flatter and more triangular VERIFIED. That posture is visible at 10× and is the field-diagnostic character.
Scout in two places: uppermost leaves for adults and eggs, lowest leaves for the scales and pupae VERIFIED. The final immature stage is a yellow, almost circular "pupa" in which the red eyes and white wings of the adult are already visible VERIFIED.
Damage, and the unambiguous test
Sap removal, then honeydew, then black sooty mould, stunting, irregular ripening, and virus transmission — in Thailand B. tabaci Asia 1 transmits Tomato yellow leaf curl Thailand virus VERIFIED. Whitefly feeding produces diffuse chlorosis that looks slightly mottled and appears progressively, where nutrient patterns are predictable and sequential VERIFIED. But the discriminator is simple: a sticky, shiny leaf with sooty mould is never a nutrient deficiency INFERRED.
Speed
On cucumber in a hot climate VERIFIED: 34.8 days egg-to-adult at 20 °C, about 24 days at 25 °C, and 14.1 days at 30 °C, with the intrinsic rate of increase peaking at 30 °C. Fecundity peaked at 25 °C with 264 eggs per female. The authors concluded the species "is well adapted to high temperatures." Expect a 14-day generation through most of the Thai year INFERRED. No Thai or Asia-1-specific life table was located UNVERIFIED.
When to act, and how
There is no published numeric threshold for whitefly on a comparable protected crop; extension guidance is explicit that whiteflies and aphids "require direct plant inspection rather than threshold-based management" VERIFIED. Do not invent one.
1 · Cultural and physical
Screening works: critical dimension 239 µm for B. tabaci, 288 µm for greenhouse whitefly VERIFIED. A screen built for thrips automatically excludes both; the reverse is not true VERIFIED. Yellow sticky cards, laid flat if you are releasing parasitoids VERIFIED. Remove and destroy heavily infested lower leaves — that is where the pupae are — and clear all crop residue between cycles INFERRED. UV-blocking cover material: see the virus section.
2 · Biological
Amblyseius swirskii takes whitefly eggs and first instars VERIFIED, and its 15–36 °C tolerance makes it better supported for Thai conditions than Encarsia formosa INFERRED. Encarsia's performance above 30 °C is a real open question — we could not retrieve the comparison study, so treat any claim about it at Thai temperatures as unverified UNVERIFIED. Franklinothrips vespiformis, present in Thailand, attacks whiteflies among other prey VERIFIED. Beauveria bassiana works well here given humidity: 60–88.8% infection at 97.5% RH, dropping to 15.3–43.9% at 75–80% RH VERIFIED.
3 · Chemical, last
No Thai whitefly resistance data exists that we could find; a global review covering sixteen countries omits Thailand and Southeast Asia entirely VERIFIED. The closest proxy is Indian data on the same Asia-1 cryptic species, showing resistance ratios up to 70× for cypermethrin, 44× monocrotophos, 25× chlorpyrifos, 18× imidacloprid and 7× thiamethoxam VERIFIED. That is a strong reason to assume resistance exists in Thailand too — but it is an assumption, not a Thai measurement INFERRED. Rotate IRAC groups aggressively and see the legal section.
Mealybug
Thai เพลี้ยแป้ง · Vietnamese rệp sáp · Family Pseudococcidae
We are going to be blunt about this section: the Thai evidence is thin. We could not locate a systematic Thailand-specific mealybug survey for protected or intensive horticulture UNVERIFIED. What follows separates what is actually documented from what is merely likely, and we would rather say so than hand you an invented species list.
What is documented for Thailand
- Phenacoccus manihoti (cassava mealybug) — first detected in Thailand in October–November 2008, then spread aggressively through the cassava region and into neighbouring countries VERIFIED. Critically for a horticultural grower: "only cassava is known to experience significant damage" VERIFIED. It is almost certainly not your greenhouse mealybug. It was brought under control by the classical introduction of the parasitoid Anagyrus lopezi, one of the largest biological-control successes in Asia VERIFIED.
- Dysmicoccus neobrevipes — present in Thailand and the subject of Thai control research VERIFIED.
- Paracoccus marginatus (papaya mealybug) — the genetic invasion chronology for Asia lists Indonesia and India 2008, Pakistan 2009, Malaysia and Taiwan 2011; Thailand is not on that list VERIFIED. Presence is highly likely given the geography, but we found no Thai first record UNVERIFIED. Do not repeat the claim that it is an established major Thai invasive without one.
- Phenacoccus solenopsis, Planococcus spp., Ferrisia virgata and Pseudococcus jackbeardsleyi in Thai horticulture: all widespread tropical Asian species and all unsurprising, but no Thai record located UNVERIFIED.
Field identification and damage
Soft oval insects covered in white mealy wax with lateral filaments, sitting in protected sites — leaf axils, under the crown, under calyces, along stems, at the medium surface and in the root zone. Honeydew, sooty mould and attending ants come with them.
Mealybug is a visible insect problem, not a symptom-diagnosis problem, so it is rarely confused with nutrition. The real confusion is with fungal mycelium or downy mildew — white fluff. Prod it: mealybug wax moves and has a discrete insect body under it. Where a plant is stunted and chlorotic with no visible insects on the foliage, the population is in the root zone INFERRED.
Control
No temperature-resolved life table and no published action threshold were located for any comparable protected crop UNVERIFIED. Sticky cards do not monitor mealybugs; inspection of leaf axils and the medium surface is the only method INFERRED.
Cultural control carries most of the weight: quarantine incoming material, control ants (they protect the colonies from natural enemies), remove heavily infested tissue, and clean between cycles. For biological control, Amblyseius swirskii lists mealybugs in its prey range VERIFIED though it will realistically only take first instars INFERRED; Cryptolaemus montrouzieri and Anagyrus spp. are the standard commercial agents worldwide, but we did not verify their availability or field performance in Thailand UNVERIFIED. The Thai success with Anagyrus lopezi on cassava at least proves mealybug biological control works under Thai conditions VERIFIED.
Aphids
Thai เพลี้ยอ่อน · Vietnamese rệp muội · Family Aphididae
The Thai reading is counterintuitive
Aphis gossypii (cotton/melon aphid) and Myzus persicae (green peach aphid) both occur in Thailand, though we found no Thai survey quantifying which dominates UNVERIFIED. The physiology, however, is well measured, and it points somewhere most growers do not expect VERIFIED:
| Temperature | Immature development | Nymphs per female | Rate of increase |
|---|---|---|---|
| 15 °C | 10.8 days | — | 0.208 |
| 25 °C | — | 82.1 | 0.526 (maximum) |
| 30 °C | 4.1 days | — | — |
| 32.5 °C | 4.1 days | 2.3 | 0.132 |
| 35 °C | Lethal to the immature stages | ||
The optimal range for population growth is 22.5–30 °C VERIFIED. Since a constant 35 °C kills the immatures, aphids in Thailand should be a cool-season and shaded-microclimate problem, not a hot-season one INFERRED. Isaan cool-season means of 24.2–26.5 °C sit squarely inside the optimum VERIFIED.
Identification at 10×
Look at the cornicles — the two tailpipes at the rear VERIFIED:
- Myzus persicae: cornicles long, about the length of the body, pale with only the tips black; a distinct indentation at the base of the antennae; generally under about 3 mm.
- Aphis gossypii: cornicles short, about one third the width of the body; no indentation at the antennal base; smaller on average; colour from pale yellow to very dark green or black.
They sit on new growth, at the base of buds, and on the undersides of mature leaves VERIFIED. The white cast skins on the leaf below the colony are the earliest reliable sign VERIFIED — look for those before you look for the insects. Honeydew never comes from a nutrient problem INFERRED.
Speed, thresholds and control
Under favourable conditions, 7–10 days between generations with 60–100 nymphs per female over 20–30 days VERIFIED. No published greenhouse threshold exists; yellow cards catch the winged forms but do not reliably indicate population level, so management is inspection-driven VERIFIED.
1 · Cultural and physical
Critical dimension 340–355 µm for melon aphid, 434 µm for green peach aphid VERIFIED — comfortably excluded by any screen built for thrips or whitefly INFERRED. Control ants: ants tending aphids for honeydew actively disrupt natural enemies VERIFIED.
2 · Biological
Aphidius colemani targets both species VERIFIED, operating at 18–25 °C and 70–85% RH VERIFIED. The temperature study is the important part: parasitisation "increased almost linearly with increasing temperature to reach a maximum at 25 °C and decreased at 30 °C", with high mortality at higher temperatures VERIFIED. So the parasitoid's window is the Thai cool season — which is also when the aphids are active INFERRED. That is a convenient alignment: use it. In the hot season both pest and parasitoid are suppressed, but the parasitoid more so. Beauveria bassiana was effective against both aphid species in the greenhouse humidity study VERIFIED.
3 · Chemical, last
No Thai aphid resistance data located UNVERIFIED. M. persicae is one of the most resistance-prone insects in the world, but that is general knowledge, not a Thai citation. Rotate IRAC groups; see the legal section.
Caterpillars
Thai หนอน · Vietnamese sâu ăn lá · Order Lepidoptera, larval stage
Spodoptera litura — the one that matters most here
Thai หนอนกระทู้ผัก. This is the single most important caterpillar for Thai horticulture. It is native to Southeast Asia VERIFIED, present throughout Thailand VERIFIED, and the USDA datasheet records that most US port interceptions of this species — 595 instances — originated from Thailand VERIFIED.
Find the eggs, not the larvae. Egg masses are laid in clusters and covered with buff-brown hair scales from the female's abdomen — a fuzzy felt patch on a leaf underside VERIFIED. That is the earliest and easiest thing to find, and removing one patch removes a hundred larvae.
Mature larvae are 40–50 mm, highly variable dark grey to black, often with yellow stripes. The diagnostic markings are a Y-shaped mark across the head capsule and thoracic shield, two large black spots on the first and eighth abdominal segments, and a small yellow dot at the base of the black patch — the last separating it from related species VERIFIED. Adults have a wingspan of 28–38 mm, dark brown forewings and whitish hindwings with a violet sheen VERIFIED.
Young larvae skeletonise leaves gregariously near the egg mass, giving a "window-paned" patch; older larvae disperse, feed at night and hide in the medium or leaf litter by day, with losses exceeding 50% at high infestation VERIFIED. If you scout only in daylight you will find the damage and not the animal.
Speed: first instar to adult in 23.4 days at 28 °C; a complete cycle in about 4 weeks at 30 °C; 8–12 generations a year in the tropics; 1,500–2,500 eggs per female VERIFIED. A meta-analysis of 17 studies across Asia and West Africa found the optimum range 30–35 °C, peaking at 35 °C, with egg-to-adult optimum at 33 °C and oviposition declining sharply above 35 °C VERIFIED. It is optimised almost exactly for the Thai hot and rainy seasons INFERRED.
Resistance — and here Thailand is documented. Field populations collected in the 2017 summer season from three Thai sites showed cypermethrin resistance ratios of 6.78 at Kamphaeng Saen (Nakhon Pathom), 5.09 at Wang Nam Khiao (Nakhon Ratchasima) and 4.73 at Bang Len (Nakhon Pathom), with elevated detoxification enzymes — cytochrome P450 1.3–1.7×, carboxylesterase 1.9–2.7× VERIFIED. This is one of the few Thai resistance measurements that exists for any pest in this guide. Pyrethroid failure against S. litura in Thailand is not a theory.
Helicoverpa armigera — the boring one, literally
Thai หนอนเจาะสมอฝ้าย. Eggs are 0.4–0.6 mm, pomegranate-shaped, laid singly — the contrast with S. litura's masses is a useful field character. Larvae reach 30–40 mm with variable colour and distinctive dorsal bands. Adults have a 3.5–4 cm wingspan. It is present in Thailand, polyphagous on over 300 plant species with a preference for nitrogen-rich fruiting parts, completes egg-to-adult in about 11–15 days with an optimum of 32.5 °C, and runs up to 12 generations a year in the tropics VERIFIED.
It bores into flower buds and fruit, with secondary rot following VERIFIED. One larva ruins one flower or fruit, so the economic damage per individual is far higher than for a defoliator INFERRED. EPPO notes widespread pesticide resistance and recommends pheromone traps plus natural enemies VERIFIED.
Spodoptera frugiperda — fall armyworm
Arrival in Thailand is precisely documented: the official IPPC pest report from the Department of Agriculture records first detection on 14 December 2018, in maize in Kanchanaburi and Tak provinces on the Myanmar border VERIFIED. By mid-August 2019 roughly 236,000 hectares were infested VERIFIED.
It is primarily a grass and maize feeder. Its status as a pest of broadleaf protected horticulture in Thailand is UNVERIFIED — expect it as an occasional invader from adjacent maize rather than a resident greenhouse pest INFERRED. If you are surrounded by maize in Isaan, that matters.
Monitoring and control
Monitoring is by pheromone trap plus visual egg-mass and frass search, not sticky cards VERIFIED. The USDA protocol for S. litura uses pheromone-baited bucket traps at about 1.2 m above ground, with lures effective for 84 days VERIFIED.
No published action threshold exists for a comparable protected crop UNVERIFIED. Thailand's DOA published fall armyworm thresholds tied to maize growth stages VERIFIED, but those do not transfer.
1 · Cultural and physical
Any insect screen excludes a 15–20 mm moth outright. The failure mode in a screened house is eggs and small larvae arriving on plant material, and adults coming through open doors INFERRED. Hand-remove egg masses on sight. Search the medium surface and litter by day for hiding larvae.
2 · Biological
Bacillus thuringiensis is the standard microbial against young larvae and is listed in Thailand's Hazardous Substances List as a Type 2 substance, meaning products containing it are registrable with DOA VERIFIED. It is IRAC group 11A. Bt works on small larvae and poorly on large ones, so it depends entirely on catching the infestation early. Trichogramma egg parasitoids and nucleopolyhedroviruses are used worldwide; we did not verify their availability in Thailand UNVERIFIED.
3 · Chemical, last
Given the documented Thai cypermethrin resistance above, pyrethroids are a poor first choice against S. litura here INFERRED. Rotate IRAC groups and see the legal section.
Fungus gnats
Thai แมลงหวี่เชื้อรา · Vietnamese ruồi đục nấm · Bradysia and related genera (Sciaridae)
No Thai species records were located for either sciarids or shore flies in Thai protected cropping UNVERIFIED — this appears to be genuinely unstudied in the accessible literature, so do not accept a Thai species name from anyone without a citation.
Fungus gnat or shore fly? They need different answers
| Fungus gnat (Bradysia) | Shore fly (Scatella) | |
|---|---|---|
| Adult | 3–4 mm, slender, long legs and long antennae, clear wings, weak fluttery flight VERIFIED | Stockier, short antennae, short legs, red eyes, five pale spots on each dark wing, fast strong flight VERIFIED |
| Larva | Up to about 6 mm, white/translucent with a distinct shiny black head capsule VERIFIED | Opaque tan-brown, no head capsule VERIFIED |
| Where | Larvae in the top few cm of moist medium; adults run on the medium surface | Larvae in algae on benches, floors, pot rims and drip lines |
| Damage | Larvae feed on root hairs and fine roots, causing stunting and opening wounds for pathogens VERIFIED | No direct plant damage — a nuisance and a pathogen carrier |
This distinction is not academic: "beneficial nematodes and predaceous mites used for fungus gnat control do not appear to work as well against shore flies" VERIFIED. If your cards are catching spotted-wing flies rather than long-legged gnats, your biological programme will look like a failure when in fact you are treating the wrong insect.
The deficiency trap
Fungus gnat larval damage presents as generalised stunting and lower-leaf yellowing that looks exactly like nitrogen deficiency or over-watering. The discriminator is physical: pull a plant, wash the root ball, and look for translucent black-headed larvae — or use the potato test INFERRED.
Monitoring and thresholds — the only clear numbers in this guide
- Potato disc test. Place raw potato discs or wedges on the medium surface; larvae congregate underneath. Potato recovers 38% of larvae against 23% for carrot, and a 48-hour exposure is more efficient than 24 hours VERIFIED.
- Propagation areas: 3–5 larvae per potato disc after 48 hours. Production areas: 15–20 larvae per wedge after 48 hours VERIFIED. Note that one extension source takes the contrary position that no reliable thresholds exist VERIFIED — these are the only published numbers on offer, so treat them as indicative rather than exact.
- Yellow cards laid flat on the medium, 1–4 small cards per 93 m² VERIFIED.
Control
Water management is the control. Larvae need consistently moist medium; letting the surface dry between irrigations is the primary tool. Control algae on floors, benches and drip lines and the shore flies go with it VERIFIED. Adults are excluded by any screen — but they arrive inside bagged growing medium and in rooted plugs, a documented pathway VERIFIED, which is why a brand-new, fully screened house gets fungus gnats in week one.
Biological agents: Steinernema feltiae (entomopathogenic nematode), Stratiolaelaps scimitus (predatory mite, optimal 15–30 °C), Atheta coriaria (rove beetle) VERIFIED. A Thai caution: S. scimitus's stated 15–30 °C optimum means a hot-season Thai root zone may sit at or above its ceiling, and we could not find an upper thermal limit for S. feltiae at all UNVERIFIED — it is generally regarded as a cool-adapted species. Verify before relying on either in a tropical root zone INFERRED.
Root aphid
Thai เพลี้ยราก · Vietnamese rệp rễ · Rhopalosiphum rufiabdominale and relatives
Honest statement of the evidence gap: we found no Thailand-specific record of any root aphid in protected horticulture UNVERIFIED. What is verified is that the best-documented candidate, the rice root aphid Rhopalosiphum rufiabdominale, is native to East Asia, is a major pest of rice there, and has become a serious problem in hydroponic and greenhouse systems in North America, infesting plants across 22 or more plant families and "often attacking plants outside its typical host range" VERIFIED. Thailand is inside its native range and is a rice-growing country, so presence is close to certain — but that is an inference, and it should be stated as one.
Identification
Wingless forms are 2.0–2.6 mm, dark green or brownish on secondary hosts, with characteristic rusty-red colouring around the bases of the cornicles VERIFIED. They feed on roots, not shoots. Winged forms appear on foliage but do not establish there.
How growers actually detect it, which is the most useful detail in this section VERIFIED:
- Dead winged adults trapped on the hairs of the upper leaf surface is typically the first thing noticed.
- Yellow sticky cards catch the mobile winged forms.
- Sticky bands around the lower stem intercept both emerging aphids and winged forms migrating down to the roots.
- Winged females emerge in numbers as plants flower and mature, colonise new plants, give birth on the stems, and the offspring migrate down to the roots — so an overlapping crop cycle is what perpetuates it.
Why it is so hard to diagnose
The source is blunt: affected plants show "generalized decline and reduced rate of growth" with possible lower-leaf discoloration, and "no unique symptoms distinguishing it from other root-feeding insects" VERIFIED. Root aphid is therefore a diagnosis of exclusion plus physical confirmation INFERRED. A plant that is stunted and chlorotic despite correct EC, pH, root-zone temperature and irrigation, and that does not respond to a feed correction, must have its root ball washed and examined. Never diagnose root aphid from the leaves.
Control
In protected settings reproduction is continuous parthenogenesis — no sexual generation, no diapause — so populations build without interruption VERIFIED. No temperature-resolved data and no published threshold exist UNVERIFIED.
Entomopathogenic fungi (Beauveria bassiana, Isaria fumosorosea) "show promise" VERIFIED. Commercial suppliers list Aphidius spp., Aphelinus ervi, lacewings and Orius — treat supplier pairings as claims, not as evidence VERIFIED. The realistic lever is sanitation: because spread runs from maturing plants to new plants via winged forms, removing finished plants promptly and not overlapping crop cycles breaks the chain INFERRED.