Short answer. Clusters of small soft pear-shaped insects on leaf undersides and stems, white cast skins, and sticky shiny leaves mean aphids. They give birth to live young without mating, so a few become hundreds in a couple of weeks. Squash or wash early colonies, release parasitoid wasps or predatory midges matched to the species, and keep sprays off flowers.
What's happening
Aphids suck phloem sap and excrete the surplus sugar as honeydew, which coats leaves, grows black sooty mould and attracts ants [1][2]. Heavy feeding slows growth, causes wilting and yellowing [2]. On cannabis the main species are: - Cannabis aphid (Phorodon cannabis): feeds and develops only on Cannabis [4]. Cream to pale yellow early in the season, turning light green, pink or brown as days shorten [2]. It can transmit cucumber mosaic virus and alfalfa mosaic virus to hemp [2]. - Green peach aphid (Myzus persicae) and other generalists, more common indoorsbeing reviewed. - Root-feeding species are covered in kb-pests-root-aphids.
Lifecycle. Indoors and in mild climates aphids reproduce asexually: females bear live nymphs, sometimes up to 12 a day. Nymphs moult about four times with no pupal stage, and in warm weather mature in 7–8 days; one adult can produce up to ~80 offspring in a week [1]. Winged forms appear when colonies crowd and spread to new plants. Outdoors, cannabis aphid produces sexual forms from mid-September and lays overwintering eggs; volunteer hemp sustains it between seasons [4].
How to recognise it
- Colonies on leaf undersides, along veins and stems, and later in flowers.
- White cast skins on leaves below colonies (often mistaken for mealybugs or "dandruff").
- Sticky, shiny leaves; later black sooty mould.
- Ants climbing plants.
- Parasitised aphids look like swollen tan or bronze "mummies" [1][5]. That is good news.
Numbers that matter
| Parameter | Value | Notes |
|---|---|---|
| Body length | ~1–3 mm | Species-dependent |
| Nymph to adult | 7–8 days warm | [1] |
| Offspring | up to ~80/adult/week | [1] |
| Aphidius colemani size | ~2 mm wasp | Targets green peach and melon aphids [5] |
| Aphidoletes aphidimyza | feeds on 60+ aphid species | Keep out of direct sun at release [5] |
| Action threshold | none established for hemp/cannabis | NC State: no threshold research yet [2] |
What to check first
- Confirm aphids. Lens: soft body, two tail pipes (cornicles) at the rear.
- Identify the species if you can. It decides which parasitoid works. Cannabis aphid is best separated from hop aphid under a microscope [2]; many extension labs will ID a sample.
- Look for ants. They farm aphids and fend off beneficials [1].
- Check nitrogen. Lush, high-N growth favours aphids (Supported; USU suggests avoiding excess N for root aphids).
- Source: new clones, nearby plants, outdoor intake air.
How to fix it
Registrations differ by jurisdiction; check local regulations and follow the label. No products or rates here.
Cultural
- Quarantine and inspect new plants.
- Remove volunteer hemp and weeds around outdoor grows; culling volunteers reduces overwintering cannabis aphid [4].
- Do not overfeed nitrogen.
- Control ants.
Physical
- Squash or wipe early colonies; prune heavily infested leaves into a bag.
- A strong water spray dislodges aphids in veg (Grower practice).
- Yellow sticky cards catch winged aphids for monitoring.
Biological
- Aphidius colemani: parasitoid wasp for green peach and melon aphids; not effective on larger aphids such as foxglove or potato aphid [5]. Effectiveness on cannabis aphid is not establishedbeing reviewed.
- Aphidius ervi / Aphelinus abdominalis: for larger aphid species [5].
- Aphidoletes aphidimyza: predatory midge; larvae eat 60+ aphid species [5]. It can enter diapause under short days, so under 12/12 it may need a low-intensity night-interruption light or may not establish (supplier guidance)being reviewed.
- Green lacewing larvae (Chrysoperla spp.) and lady beetles (e.g. Hippodamia convergens): general predators; outdoors, native populations may make purchases unnecessary [2].
- Banker plants (cereal grasses with a grain aphid) keep Aphidius present before cannabis is attacked [6].
- Entomopathogenic fungi (Beauveria bassiana): contact, humidity-dependent.
Least-toxic options
- Insecticidal soaps (potassium salts of fatty acids) and horticultural oils: contact, coverage of undersides essential, repeat.
- Azadirachtin (neem-derived): reported to control aphids on hemp; efficacy still under study [2]. Check registration.
What not to do in flower
- Do not spray soaps, oils or botanicals onto buds; residues and moisture invite bud rot.
- Do not use systemic insecticides; they are not registered on cannabis in most places and persist into the flower.
- In flower, rely on established beneficials and removal of infested branches. Heavy honeydew on buds is a quality and mould problem: cut them out.
How to confirm it worked
- Colonies shrink; mummies outnumber live aphids.
- New growth stays clean and not sticky.
- Winged aphids on cards drop over 2–3 weeks.
Common mistakes
- Ignoring ants.
- Buying the wrong parasitoid for the species.
- Killing beneficials with broad sprays and getting a bigger rebound.
- Misreading white cast skins as a mould.
Evidence
- Biology and reproduction rates: Established [1].
- Cannabis aphid host specificity and virus vectoring: Established in extension literature [2][4].
- Parasitoids and midges for greenhouse aphids: Established in ornamentals and vegetables [5]; cannabis-specific trials sparse (Supported).
- Action thresholds on cannabis: none researched [2].
