Pest identification and control

What this guide is (and is not)

This is a plain-language field guide to identifying and controlling the eight pest groups that most commonly attack cannabis: spider mites, russet and broad mites, thrips, fungus gnats, aphids and root aphids, whitefly, and caterpillars. It tells you what each pest looks like, how fast it breeds, how to monitor for it, and how to treat it with bugs (biological controls) and sprays.

It is the ‘who’s who’ companion to the IPM SOP, which is the separate decision process that tells you when to act and which lever to pull. Read this guide to recognise the enemy. Read the IPM SOP to run the program.

• Covers 8 pest groups by name with tell-tale signs, life cycle, monitoring, biocontrol and treatment
• Beginner-first: every technical term is defined the first time it appears
• Pairs with the IPM SOP (the decision framework), this guide is the identification reference
• Bias is toward prevention and early detection, because every pest below breeds faster than you can spray

Key terms, defined once

A handful of words appear throughout this guide and the IPM SOP, so define them now. You do not need to memorise them, each one comes back in context.

Key terms, in the facilityNano Banana 2

Leaf sap-suckers: spider mites, thrips, aphids and whitefly

These four feed on leaves and stems by piercing cells and sucking sap, so they share a look: pale stippling, distortion and loss of vigour^[2]. Telling them apart is about the secondary signs each one leaves.

• Spider mites: leaf-underside stippling plus fine webbing. The cycle runs as fast as 3 days at 27C / ~20% RH, and one female can lay up to ~200 eggs.
• Thrips: silvery rasping streaks plus tiny black frass dots. They drop to the medium to pupate, so leaf sprays alone miss a whole life stage.
• Aphids: soft-bodied clusters on new growth, cast white skins, honeydew that grows black sooty mould. They can give live birth, so buildup is explosive.
• Whitefly: tiny white moth-like adults that flush up in a cloud when disturbed, with uniform yellowing and lower-canopy decline.

Note that the aphid most associated with cannabis is its own species, the cannabis aphid (Phorodon cannabis), now recognised as a pest in North America^[3]. The shared driver across all four is environment: warm plus dry shortens every life cycle, so climate control is the first lever, not the spray bottle.

The hidden ones: russet and broad mites, fungus gnats, root aphids and caterpillars

This group is dangerous because the damage shows before the pest does. Russet mites (Aculops cannabicola, under 1 mm, needs 80-100x) and broad mites (needs ~60x) are invisible to the naked eye and announce themselves through cupped, glossy or ‘wet-looking’ new growth, twisted tops and stunting. Those symptoms are often misread as nutrient or heat stress^[6].

• Russet/broad mites: cupped, glossy, twisted new growth. Sub-millimetre, need 60-100x. Easily mistaken for a nutrient problem.
• Fungus gnats: larvae eat root hairs in wet media, adults are weak dark fliers over the medium. The larvae open the door to root-rot pathogens.
• Root aphids: feed at the crown and roots, causing yellowing and stalled growth with no visible canopy pest. Check the root zone and pot base.
• Caterpillars / budworms: frass and bore holes in flowers, a rapid trigger for bud rot. Scout at dusk when larvae feed.

Fungus gnat larvae weaken plants and wound roots, and the damage they cause interacts with soil-borne disease, which is why a wet-media gnat problem so often turns into a root-rot problem^[7]. Cannabis is also a confirmed host of the rice root aphid, which lives below ground and is missed entirely by canopy scouting^[4].

Monitoring: traps, loupes and a weekly scouting routine

You cannot control what you do not measure, and early detection is the single biggest lever you have. Run a fixed weekly scouting walk on the same day, inspecting leaf undersides, growing tips, flowers and the root zone, and step up to twice weekly as rooms warm up.

Hang yellow sticky cards at canopy height for thrips, whitefly and fungus-gnat adults at roughly one card per 1,000 square feet as a minimum, with more cards giving a better signal. Place fungus-gnat cards low, near the medium surface, where the adults fly. Record the count on each card every week so you track the trend, not just the snapshot.

• Scout weekly on a fixed day, spot-check twice weekly when temperatures and pest development rise
• Yellow sticky cards: minimum ~1 per 1,000 sq ft, low at the medium for fungus gnats, canopy height for thrips/whitefly
• Log card counts each week, a rising trend (not a single number) is the alarm
• 30x loupe for spider mites, 60-100x scope for russet/broad mites, inspect undersides, tips, flowers AND roots
• Action thresholds are facility-specific, e.g. one grower tolerates 10-15 thrips/card/week, another with virus history tolerates under 5

Biological controls and treatments, pest by pest

Match the tool to the pest, and to whether you are preventing or reacting. For spider mites the specialist predator Phytoseiulus persimilis is the fast curative, but it wants humidity above ~60% RH, while Neoseiulus californicus or Amblyseius swirskii establish preventatively. Thrips are managed with Amblyseius cucumeris or swirskii at about 100-300 mites per square metre, with control typically visible around 3 weeks^[5].

Aphids respond to species-matched parasitoid wasps (Aphidius colemani for green peach aphid, Aphidius matricariae for cannabis aphid) plus the insect-killing fungus Beauveria bassiana. Whitefly are hit with Encarsia or Eretmocerus wasps and swirskii. Fungus gnats are drenched in the medium with Steinernema feltiae nematodes and the bacterium Bti every ~2 weeks. Caterpillars are killed with Bacillus thuringiensis kurstaki (Btk), which only kills larvae that eat it. Several of these biopesticides have been tested directly on cannabis pests^[8].

Troubleshooting and common pitfalls

Most pest disasters are diagnosis and timing failures, not product failures. The classic mistakes repeat across facilities, and all of them are avoidable with a scope and a calendar.

Realistic expectations and prevention payoff

Eradication is rarely the goal. Durable suppression below the action threshold is. Expect biological control to take time: thrips control is often visible only around 3 weeks after release, and breaking a fungus-gnat cycle by trapping adults while killing larvae usually takes 4-8 weeks^[7].

The cheapest ‘treatment’ is prevention. Quarantine and inspect every incoming clone, keep mother plants clean (an infested mother makes every cutting infested), control humidity and airflow, sanitise tools and rooms, and run the weekly scout without fail. A facility that prevents and detects early spends far less on curatives and loses far less crop than one that fights outbreaks reactively.

Track your outcomes against the thresholds in the IPM SOP so you know whether the program is actually holding, and keep the room itself working for you with good airflow design that denies pests the warm, still, humid pockets they love.