Biological control means using living natural enemies to keep pests below damaging levels. These enemies are predatory mites, predatory insects, insect-killing nematodes, fungi and bacteria. This part covers which species to use, at what rates and temperatures, and how microbial products work. It also covers what companion plants do and do not do, a weekly scouting schedule, spray rotation and residue testing. It belongs to a guide on integrated pest management (IPM): prevent first, monitor, remove pests by hand, release natural enemies, and spray chemicals only as a last resort.
Sources are United States university extension services, published studies and international guidance. Each is introduced where it first appears and listed under References. Notes on Thai and Southeast Asian conditions are marked “Translator’s note”.
This is Part 2. Part 1: Integrated Pest Management for Cannabis: Controlling Pests Without Toxic Chemicals
1. Tier 4: biological control
The release rates below are the ranges suppliers recommend, not fixed scientific values. Use the low end to prevent an outbreak. Use the high end when pests are already present.
Translator’s note: biological control does three things for you. It means fewer sprays, no chemical residue on the buds and less work in flowering.
Its use in Thailand depends on two things. The first is whether a species can be bought here. Ask suppliers which species they can deliver in Thailand and whether an import permit is needed. The second is whether your room’s temperature and humidity suit it, and the tables below show that.
To start, run one tent or one room for one crop with a single predator species. Keep the scouting log from Part 1 running alongside it.
1.1 Predatory mites
| Species | Target | Rate per release | Conditions |
|---|---|---|---|
| Phytoseiulus persimilis | Two-spotted spider mite and other Tetranychus spider mites | 2–50 per m², once or twice a week | Best 13–27 °C; no activity above 30 °C; needs 70% humidity |
| Neoseiulus (Amblyseius) californicus | Two-spotted and other spider mites | 25–125 per m², repeated weekly 5 times | Best 13–32 °C; sensitive below 60% humidity |
| Amblyseius andersoni | Thrips larvae, whitefly eggs, spider mites that do not spin webs | 50–250 per m², repeated weekly 3 times | Above 14 °C, best 20–28 °C; needs 65% humidity |
| Amblyseius swirskii | Young thrips larvae, whitefly eggs and larvae | 25–300 per m² | Best 20–32 °C; no activity below 18 °C; sensitive below 60% humidity |
Phytoseiulus persimilis eats spider mites and almost nothing else. It clears a hot spot fast. Once the spider mites are gone, it has no food and starves. So it must be re-released when mites return.
Climate limits this mite. University of Connecticut figures (from a United States university IPM programme) show how long it takes to develop: about 5 days at 30 °C, 9 days at 20 °C and 25 days at 15 °C. Below about 60% humidity, its eggs dry out and shrivel.
In a hot room, Neoseiulus californicus is the usual choice. Amblyseius swirskii also feeds on pollen. Because it can live on pollen, it can be released and settle in before any pest arrives.
Translator’s note: a Thai grow room in April at 30–35 °C goes above the best range of every mite in this table. An air-conditioned room held below 30 °C suits N. californicus and A. swirskii. P. persimilis needs the room cooler still and humid. An open greenhouse at 33–35 °C suits none of them. A heat trial comparing two of these species is discussed in Amblyseius swirskii vs Phytoseiulus persimilis at 30–32 °C.
What to do: read the maximum temperature and minimum humidity from your log for the last two weeks. Choose only species whose range covers those figures.
1.2 Buying, receiving, storing and releasing natural enemies
Translator’s note: these steps are general practice. Always follow the supplier’s label first.
- Before ordering, tell the supplier your room temperature and humidity. Ask how long delivery takes, how the product is packed for heat, and how long and at what temperature it can be stored.
- Never leave a parcel in a parked vehicle or on a sunny doorstep. Thai daytime heat there is far above the ranges in the tables.
- On arrival, open the pack in the shade. Under a 30× loupe, check that the mites are moving. If most are dead, photograph them and contact the supplier the same day.
- Release as soon as possible, in the evening or with lights off. For an outbreak, place them on the infested leaves. For prevention, spread them evenly across the canopy.
- Record the species, number, date and zone in the log. Check again at 3 and 7 days to judge whether they settled.
- Do not spray over released predators unless the supplier has confirmed the product is safe for them (see Part 1).
1.3 Soil and growing medium: the half of the room people forget
| Agent | Target | Rate | Conditions |
|---|---|---|---|
| Stratiolaelaps scimitus (long sold as Hypoaspis miles), a soil-dwelling predatory mite | Eggs, larvae and pupae of fungus gnats (sciarid flies); thrips pupae | 100–500 per m², once or twice | At least 15 °C; moist, loose medium |
| Steinernema feltiae nematodes | Fungus gnat larvae, thrips in their soil stage, leafminers | Depends on the product: follow the label | Active 10–31 °C, best 14–26 °C; above 35 °C kills them |
Steinernema feltiae enters the pest’s body and releases bacteria that live with it, usually Xenorhabdus. The bacteria cause a blood infection (septicaemia) that kills the pest within days. Two rules decide whether nematodes work:
- They are damaged by ultraviolet light, so apply them in the evening.
- The medium must stay moist for several days after you apply them, because they move through the water film around the particles of the medium.
Translator’s note: a black pot standing in the Thai sun can heat its medium well above the air temperature. Before a nematode drench, push a probe thermometer into the medium. Do not apply if it reads above 31 °C, the top of the active range.
What to do: if your yellow cards laid flat near the medium show fungus gnats, treat the medium, not the leaves. Use one of these two agents, then compare card counts after two weeks.
1.4 Predatory insects
| Agent | Prey | Rate per release | Caution |
|---|---|---|---|
| Chrysoperla carnea, green lacewing | Mainly aphids; also mealybugs, thrips, moth eggs | 2–20 per m², into infested areas, weekly | Only the larva hunts. Adults feed on nectar and mostly fly away |
| Orius laevigatus, a predatory bug | Various thrips species | 0.5–10 per m², once or twice | Best above 20 °C; less effective above 30 °C |
| Hippodamia convergens, convergent lady beetle | Aphids | See 1.5 | Most leave very soon after release |
Supplier figures say a Chrysoperla carnea larva eats on average 300 to 400 aphids while it develops, depending on prey and temperature. It works best at 20–28 °C.
1.5 Lady beetles: the honest picture
The University of California IPM programme gives a plain account. Commercial Hippodamia convergens lady beetles are wild beetles collected from winter clusters using portable vacuum machines. Most fly away within one or two days, even when aphids are plentiful. UC IPM says purchased beetles naturally tend to disperse.
Work cited by University of California authors found that about 95% had left within 48 hours. Each adult eats about 100 aphids per day before leaving. Controlling aphids on roses took roughly 1,500 beetles per shrub, released every one to two weeks.
So use lady beetles for one quick reduction of aphids, released in the evening. Do not expect them to stay and build a population.
Cornell Cooperative Extension (the outreach service of Cornell University in New York State, USA) issued a 2022 hemp field-day handout on biological control in high tunnels (plastic-covered hoop houses) growing Cannabis sativa. It notes that the predatory gall midge Aphidoletes aphidimyza lays up to 70 eggs inside an aphid colony. It also stresses that biological control must be used preventively, early in an outbreak.
What to do: for aphids, start lacewing larvae or gall midges as soon as cards show winged aphids. Do not wait for a colony to build.
2. Microbial products
| Organism | How it kills or protects | Must the pest eat it? |
|---|---|---|
| Beauveria bassiana | Spores stick to the insect’s outer skin (cuticle), break through it with enzymes, then grow inside the insect | No, contact is enough |
| Bacillus thuringiensis (Bt) | Its protein crystals dissolve in the alkaline gut of the larva. The insect’s own enzymes activate the toxin, which punches holes in the gut wall | Yes, it must be eaten |
| Trichoderma harzianum | Grows on roots and releases enzymes that dissolve the cell walls of disease fungi, then feeds on them | Not applicable |
| Bacillus subtilis strain QST 713 | Grows on roots, attacks disease organisms directly and triggers the plant’s own defences | Not applicable |
2.1 How insect-killing fungi work: Beauveria bassiana and Metarhizium
Insect-killing fungi (entomopathogenic fungi) infect pests through the skin. A spore lands on the insect, sticks, dissolves the cuticle and grows inside until the insect dies. The pest does not need to eat it. So it can reach pests that never chew the leaf, such as aphids, thrips and whiteflies.
A Cornell biocontrol fact sheet gives the main facts for Beauveria bassiana:
- It infects over 700 insect species, including aphids, thrips and whiteflies.
- It works best at 18–29 °C with high humidity.
- Ultraviolet light destroys the spores. Spore survival dropped sharply after 15 minutes of ultraviolet-B exposure, so apply it with lights off.
- Signs of control take 6 to 14 days.
- It may harm beneficial insects and honeybees.
Do not assume it controls spider mites. Check that the product label lists them.
Metarhizium is a second group of insect-killing fungi that infects through the skin in the same way.
Translator’s note: Thai rooms at 30–35 °C are above the best range of Beauveria. Rainy-season humidity helps the fungus, but a canopy that stays wet invites bud mould. Before buying a product, ask the seller for its Department of Agriculture registration number.
What to do: test it on one hot spot first. Mark the plants and count pests at 7 and 14 days before you treat a whole room.
2.2 Choosing the right Bacillus thuringiensis
With Bt, the subspecies matters more than the brand:
- Bacillus thuringiensis subsp. kurstaki carries the Cry1A and Cry2A toxins and targets caterpillars.
- Subsp. aizawai adds Cry1C and is described as more toxic to Spodoptera armyworms.
- Other subspecies target Diptera (flies), which includes fungus gnats.
The larva must eat Bt, so cover the exact tissue it is chewing. Feeding can stop within 30 minutes, and death usually follows in one to three days. Sunlight’s ultraviolet breaks the proteins down, so the effect on leaves lasts only a short time.
What to do: identify the caterpillar first. Check that the subspecies on the label matches it, and spray in the evening.
2.3 The root-zone organisms
Trichoderma harzianum and Bacillus subtilis QST 713 work by settling on roots and surfaces first. Disease organisms that arrive later find the space already taken. They protect healthy roots but do not cure an existing infection. They disappoint against Botrytis cinerea, the grey mould that rots buds (see Botrytis (Gray Mold)), once it is established. Against Botrytis, airflow and humidity control do the real work.
None of the common nutrient lines sold in Thailand is a pest control product. A nutrient mix can also kill a living inoculant. So check whether the product survives your tank mix rather than assuming it does.
What to do: ask the inoculant supplier which nutrients and additives it can be mixed with. Apply it from the first weeks, before any root problem appears.
3. Companion plants: what the evidence supports
Companion plants are other plants grown near the crop in the hope they protect it. The topic is covered in Companion Planting for Outdoor Cannabis.
| Claim | Evidence |
|---|---|
| Marigolds repel flying pests from a nearby crop | Weak. Trials against several fly and butterfly pests found no repellent effect. United States Department of Agriculture listings note that several pests are attracted to marigolds |
| Marigolds suppress root-knot nematodes (soil worms that damage roots) | Real but conditional. Studies credit natural compounds called thiophenes, including alpha-terthienyl. It works when marigolds are grown as a cover crop and dug in weeks before the main crop, not as a plant in the next pot |
| Basil and other herbs repel mites and thrips indoors | Not proven. Plant scents change insect behaviour in laboratory tests, which is not the same as protecting a neighbouring plant |
| Companion plants give pests somewhere extra to live | Well supported but usually ignored. Each companion is extra host tissue that nobody checks |
3.1 The version that works: banker plants
The method with published support does not repel pests. It feeds your predators. A banker plant supplies pollen or other prey, so a general predator settles in before the pest arrives. A 2012 study in the journal Biological Control tested ornamental pepper as a banker plant for Amblyseius swirskii against several greenhouse pests.
Translator’s note: marigolds (ดาวเรือง) grow easily across the region. For outdoor fields with root-knot nematodes, grow them as a cover crop and dig them in weeks before planting. The 2012 study used ornamental pepper cultivars. We found no test of common Thai chilli varieties as banker plants. Every banker plant must be checked in your weekly scouting like the crop.
What to do: trial one banker plant in one room. Compare A. swirskii numbers on nearby leaves with a room that has no banker plant.
4. A weekly scouting and treatment schedule
At room temperature, most pests take longer than a week to complete a generation. A weekly check means a pest population cannot double twice between checks.
| When | Scouting | Typical action |
|---|---|---|
| Every week, same day | Change cards; check leaf undersides on a fixed set of plants; check the medium | Log counts, including zeros. Zero rows make a rise easy to see |
| Vegetative weeks 1–2 | Baseline counts | Settle general predatory mites in; root microbials with the feed |
| Rest of vegetative stage | Watch for rising counts | Targeted predator releases, repeated weekly |
| Early flowering | Last safe window for spraying leaves | Foliar biologicals with lights off; nematode drench |
| Mid to late flowering | Scouting and removing infested parts by hand | No leaf sprays. Dense buds hold residue and moisture |
| After any treatment | Check again at 3 and 7 days | Judge the product and record the result |
Translator’s note: spider mites develop faster in Thai heat (see Part 1), so check known hot spots twice a week. Keep the full count weekly.
4.1 Spraying with lights off, and its cost
Leaf sprays are normally applied in the dark period. Ultraviolet light breaks down Beauveria bassiana spores, Bt proteins and nematodes. Wet leaves under strong light also risk leaf burn from the spray (phytotoxicity). University of Connecticut guidance on insecticidal soap says to avoid:
- hot, sunny afternoons;
- plants under heat stress above roughly 32 °C;
- soft young growth.
There is a serious cost. A canopy left wet in a dark, cool, still room invites Botrytis cinerea. So spray early in the dark period, keep the fans running, and make sure the canopy is dry before lights come on.
Translator’s note: outdoors and in greenhouses, the “dark period” is the night. In the Thai hot season, greenhouse afternoons often pass 32 °C, so spray in the early evening. In the rainy season, leaves may stay wet until morning. Spray only when a dry night is forecast and air can move through the plants.
4.2 Rotating products by mode of action
The Insecticide Resistance Action Committee (IRAC) is an international committee that sorts insecticides and acaricides into numbered groups. It groups them by mode of action, meaning the way the product kills the pest. Products with the same group number attack the same target in the pest, even under different brand names.
IRAC’s central rule is to rotate by group number only. The goal is that each new pest generation meets a different group from the generation before it.
How to apply it:
- Find the IRAC group on the label. The code is a number, sometimes with a letter. If it is missing, look up the active ingredient in IRAC’s published classification.
- Add an “IRAC group” column to your log.
- For the next pest generation, choose a product with a different group number.
Two brands with the same group number put the same pressure on the pest. Switching between them gives no benefit, even though it can look like rotation. Using the same contact product every week through a whole crop is the fastest way to breed resistance.
Insecticidal soap works by contact only. Its fatty acids damage the pest’s cell membranes, and nothing remains once the spray dries. Such products do not breed resistance the way long-lasting chemicals do. But they need full coverage, and they also hit beneficials.
5. If you still reach the last resort
Choosing chemical control is a decision, not a failure. Make it deliberately:
- Identify the pest to species, so the product matches it. Use Plant Infirmary for help.
- Check what your licence category permits before you spray, not after.
- Treat the hot spot, not the whole room.
- Record the product, active ingredient, rate, date, who applied it and the pre-harvest interval. The pre-harvest interval is the minimum number of days between the last spray and harvest.
Translator’s note: in Thailand, use only pesticides registered with the Department of Agriculture (กรมวิชาการเกษตร), and use them exactly as the label says. For a microbial product sold for pest control, ask the seller for its Department of Agriculture registration number. See What You May Legally Spray Under Thai GACP.
6. Residue testing and the medical supply chain
Flower going into a medical or export supply chain is tested for pesticide residues (see Pesticide Residues on Cannabis). The Good Agricultural and Collection Practice (GACP) framework requires two things. Flower must stay within the maximum residue limits (the highest amount of each pesticide allowed to remain) in the relevant pharmacopoeia, the official book of quality standards for medicines. Each batch must also be traceable to its source.
A residue failure fails the whole batch, and it is found only after all the growing costs are spent. Your scouting records are the evidence an auditor will read.
Translator’s note: ask your buyer or licensing authority which residue limits apply to your batch. For laboratories in Thailand, see Testing Cannabis Flower Before Export, Part 2.
What to do: before harvest, check that every product used on the batch appears in the log with its pre-harvest interval. Keep that page with the batch’s test result.
Your experience
Reply under this topic with what has actually worked in your room. Posts about predatory mites in hot Southeast Asian conditions are especially useful. The published ranges for Phytoseiulus persimilis plainly do not describe a Thai grow room in April. Please include which species survived, at what release rates, and what you changed to keep them alive. A programme that failed is the more valuable post.
References
- University of Connecticut Integrated Pest Management guidance (predatory mite biology, insecticidal soap).
- University of California Integrated Pest Management material on convergent lady beetles.
- Cornell Cooperative Extension, 2022 hemp field-day handout on biological control for high-tunnel Cannabis sativa.
- Cornell biological control fact sheet on Beauveria bassiana.
- Xiao Y. et al. (2012). Ornamental pepper as a banker plant for Amblyseius swirskii, journal Biological Control.
- Nematology literature on marigold thiophenes; United States Department of Agriculture pest listings for marigold.
- Insecticide Resistance Action Committee, principle of mode-of-action rotation.
- European Medicines Agency, guideline on Good Agricultural and Collection Practice for starting materials of herbal origin.
- Commercial biological control supplier introduction rates.
This article is part of the Asiannabis Community educational series on pest and disease management. The content is for education in jurisdictions where cannabis cultivation is legally permitted.