A grower with Humboldt County experience running an outdoor crop in Phuket described the pest pressure as insane, listed ants, aphids, leaf miners, beetles and spider mites, and then said something that should make growers here re-check their priorities: “The bud worms are the worst this possible IMO. BT is your best friend.” In the same thread he reported powdery mildew as effectively non-existent while bud rot remained a live threat. If those observations hold more widely, the standard imported pest calendar may have greenhouse and outdoor growers in Thailand spending their attention in the wrong order. This article sets out that reordering as a hypothesis, gives a Bacillus thuringiensis programme for flowering, and is clear about how thin the evidence for the reordering is.
Scope: this is about greenhouse and outdoor crops. Sealed indoor flowering rooms rarely get bud-boring moths. For indoor growers, the relevant parts are exclusion and the powdery mildew section below.
1. What growers in Thailand report, and how strong that evidence is
THCFarmer, “The impossible outdoor Phuket grow” is a long-running thread that started in December 2023. The grower describes the pest pressure as insane, with ants, aphids, leaf miners, beetles and spider mites all present. He names caterpillars as the worst problem and Bacillus thuringiensis as the main answer. In the same thread he reports powdery mildew as “non existent” and bud rot as “very possible”, seen specifically where insect damage had already opened the bud. [verified as reported. https://www.thcfarmer.com/threads/the-impossible-outdoor-phuket-grow.155969/]
Corroboration on the pest list: a Thailand growing guide from a seed company names aphids, spider mites, caterpillars and thrips, and calls out caterpillars specifically for greenhouses. It supports the pest list, not the ranking. [verified. https://2fast4buds.com/news/features-of-growing-cannabis-in-thailand]
The limitation. The caterpillar-first ordering rests on one very active thread from one grower in one province. Other sources back the pest list but not the ranking. The powdery mildew observation comes from a single source. [verified as reported; extending it to Thailand as a whole is inferred] We present this as a hypothesis worth acting on: acting on it costs little, and ignoring it could cost a crop. It is not an established fact.
2. Why the imported calendar may be misallocated: a hypothesis
Most cannabis integrated pest management (IPM) material in circulation, including the forum’s own IPM and mites articles, comes from North American and European indoor production. That literature’s implicit priority order is roughly:
- Spider mites
- Powdery mildew
- Thrips, aphids, fungus gnats
- Botrytis, late in flower
- Caterpillars, mentioned mainly as an outdoor footnote
If the Phuket observations hold for other greenhouse and outdoor sites, the order here may be closer to:
- Caterpillars and bud-boring larvae
- Botrytis, partly because of item 1, since larval damage opens the bud
- Spider mites, thrips, aphids
- Root-zone pathogens, driven by warm root temperature
- Powdery mildew, possibly less important in hot, humid lowland conditions
Powdery mildew is not absent from Thailand. It does occur in Thai cannabis, especially indoors under air-conditioning and during the cool, dry season in the northern highlands. Wherever conditions are cooler or drier than lowland Phuket, keep your mildew readiness exactly as it is.
Why the order matters. Powdery mildew and caterpillars need different products, scouting methods, timing and personal protective equipment. A grower on a Western calendar who checks leaf undersides weekly for mite stippling and mildew can miss larval frass and bore holes inside developing buds until the damage is done. The mildew scout looks at leaves; the caterpillar scout has to open buds.
Why the mechanism is plausible: powdery mildew on cannabis is generally favoured by moderate temperatures and is often reported to be suppressed by sustained high temperatures. Botrytis thrives in the warm, wet, poorly ventilated conditions the Thai wet season supplies. Moths breed year-round at these latitudes with no winter break, so populations do not reset each year the way they do in temperate climates. [inferred: standard epidemiological reasoning consistent with the reports, not a finding of any study we can cite]
3. The likely targets, and which Bt to use
The target species. The thread does not identify species. In Thai field and greenhouse crops, the most likely bud-boring and leaf-feeding caterpillars are Helicoverpa armigera (cotton bollworm) and Spodoptera spp. (armyworms, e.g. S. litura, S. exigua). [inferred: these are major polyphagous moth pests in Thailand. We have not seen a Thai record that names them on cannabis specifically.] Rear out or photograph a larva and identify it before choosing a product.
Bt is not one product. Bacillus thuringiensis is a bacterium whose crystal protein toxins are activated in the alkaline gut of susceptible larvae. Each subspecies has a different host range:
- Bt subsp. kurstaki (Btk): targets caterpillars (lepidopteran larvae). Usually the first choice against Helicoverpa.
- Bt subsp. aizawai (Bta): also targets caterpillars. It is often reported to work better than Btk on armyworms (Spodoptera), so consider it, or a product that combines both, where Spodoptera is the main pest.
- Bt subsp. israelensis (Bti): targets fungus gnat and mosquito larvae, not caterpillars.
- Bt subsp. tenebrionis (Btt): targets the larvae of some beetles, not caterpillars.
The caterpillar strains (Btk, Bta) do nothing against mites, aphids, thrips or adult moths. Check which subspecies is on the label.
Resistance has been reported in Thailand. A diamondback moth (Plutella xylostella) population from Bang Bua Thong was about 668-fold resistant to a Btk product and 66.5-fold resistant to a Bt aizawai product, and the resistance was relatively stable. [verified. Imai & Mori 1999, Applied Entomology and Zoology 34(1): 23–29, Levels, inheritance and stability of resistance to Bacillus thuringiensis formulation in a field population of the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae) from Thailand] This is a vegetable pest, not a cannabis one. The lesson still applies: do not rely on a single Bt strain season after season. Rotate between Btk and Bta where your product choice allows, and combine Bt with exclusion and removing infested buds.
Four things people get wrong:
It only works if eaten. Bt is not a contact insecticide. You have to cover the surfaces larvae actually chew. For bud-boring species, that means the bud and surrounding bracts, not the fan leaves.
It only works on larvae, and works best on small ones. A large late-stage larva already inside a bud may cause the damage before it dies. Timing against egg hatch is the whole game, so you need to watch for adult moths rather than wait for damage.
It breaks down in sunlight. Ultraviolet light degrades the crystal proteins. Residual activity lasts a matter of days, and less in full tropical sun. You have to repeat applications, and evening application is standard practice for exactly this reason. [established practice]
It does not touch botrytis. Bt is an insecticide. It helps against bud rot only indirectly, by preventing the wounds that let botrytis in.
4. A flowering programme
This is set out as a programme rather than a fixed schedule, because moth activity is the trigger and moth activity is local.
Before flowering: exclude, do not treat. Insect netting on greenhouse openings, with mesh fine enough to stop adult moths, is worth more than any spray programme. A moth that never reaches the crop lays no eggs. This is the highest-value measure and the one most often skipped, because it is a construction cost rather than a consumable.
Monitor adults with traps. Species-specific pheromone traps for Helicoverpa armigera and Spodoptera spp. tell you when a flight starts and which species is flying. Light traps catch a wider range of moths at night. Check traps at least twice a week from late vegetative growth onward. A rising catch is your signal to treat the hatch rather than the aftermath.
Scout by opening buds, weekly, from the start of flower. Look for frass (larval droppings, which look like dark granular material), bore holes, and patches of discoloured or dying bracts. A single dying bract in the middle of an otherwise healthy bud is the classic sign and is easy to dismiss.
Apply Bt on hatch: practical parameters.
- Product and rate: use a Bt product registered with the Thai Department of Agriculture, at the rate on its label. Do not under-dose.
- Water: mix in water at about pH 6–7. Bt loses activity in alkaline water, so check your source water and adjust if it is above 7.
- Mix fresh: spray within a few hours of mixing. Do not leave the mixed tank standing overnight.
- Sticker or adjuvant: add a sticker or spreader that the product label says is compatible, to improve retention and coverage on waxy bracts.
- Timing: apply in the evening, when larvae feed and UV is lowest.
- Interval: repeat roughly every 5–7 days while moths are flying, or at the interval the label gives. Do not use a fixed fortnightly calendar.
- Coverage: use fine droplets and enough pressure to get into the buds and bracts, spraying from several angles, not just across the top of the canopy. Coverage on the outside of a dense bud does nothing for a larva already inside it, which is why timing against hatch matters.
Licensed and export growers: watch the certificate of analysis (COA). Bt is a live bacterial product. Spores sprayed onto buds late in flower can raise the total aerobic microbial count or Bacillus count on a COA and cause a batch to fail its microbial limits. Before you include late-flower Bt in the programme, check your testing lab’s limits and methods. Set your own pre-harvest cut-off for the last Bt application, and lean on exclusion and removing infested buds in the final weeks.
Remove and destroy infested bud tissue. Cut it out and take it out of the growing area. An infested bud is both a source of larvae and a site where botrytis gets in. The forum’s botrytis article explains why one infected bud in a dense canopy does not stay one.
Do not stop the botrytis programme because the caterpillar programme is working. The two are linked but not substitutes. Below-canopy airflow, lower-canopy clearing and humidity management remain the botrytis controls. The article on monsoon humidity gives the measured cost of getting them wrong.
Legal status under Thai GACP. Licensed cultivation must follow Thai GACP, which is certified by the Department of Thai Traditional and Alternative Medicine (DTAM). Pest management is part of that standard. We have not been able to check the official DTAM text on which substances are permitted, so we make no claim here that Bt is specifically allowed. Any Bt product used in Thailand must be registered with the Department of Agriculture and used according to its label. Licensed growers should confirm with their GACP certifier before using it. The article on what you may legally spray under Thai GACP (bai241) covers this question.
5. Where this leaves the rest of the pest list
Mites are still real, and the predator choice changes in the heat. See the article on predatory mite selection above 30 °C (bai234). Powdery mildew: keep readiness, especially indoors under air-conditioning and in the northern cool season. Root-applied silicon is covered in the silicon article (bai239). Broad and russet mites: we found no Thai evidence either way, which does not mean they are absent. Ants were on the Phuket list. They farm aphids and scale insects, so controlling ants is often the most efficient way to control the insects they protect. [inferred]
6. Help us test the hypothesis
If you grow in a greenhouse or outdoors in Thailand, reply with:
- Which pest cost you the most flower last wet season, and in which province: caterpillars, bud rot or mites.
- Whether you see powdery mildew, and where, in what season and under what conditions.
- Whether your bud rot started at insect wounds.
- Which Bt subspecies you use, at what rate and interval, and whether you have seen any effect on your COA.
A dozen answers would turn one grower’s observation into real evidence.
Related reading: the articles on integrated pest management and on spider mites and thrips; predatory mite selection above 30 °C (bai234); botrytis; what you may legally spray under Thai GACP (bai241); silicon (bai239); monsoon humidity and cannabinoid loss.
This article is technical horticultural information for readers operating legally in their jurisdiction. Pesticide and biological control product registration differs by country. Check that any product named is registered for your crop and use in your jurisdiction before applying it. In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis) B.E. 2568, and GACP requirements apply to licensed cultivation. Flower sold or exported must come from a GACP-certified site. The Cannabis Control Act is still a draft (Cabinet approved 22 September 2026, sent to the House). Nothing here is legal advice.