Short answer. Smooth-edged holes and clipped seedlings overnight, with silvery slime in the morning, mean slugs or snails, most often the gray field slug Deroceras reticulatum [1]. They feed at night in cool, wet weather and hide by day. Seedlings and fresh transplants are the ones that die. Scout with a flashlight or boards, protect young plants with iron phosphate baits and copper barriers, trap, and remove hiding places [1].
Which slugs and snails attack cannabis?
The slugs that attack cannabis outdoors and in greenhouses are the same garden slugs that attack vegetables: the gray field slug Deroceras reticulatum, banded and three-band garden slugs (Lehmannia spp.), the tawny slug Limacus flavus and the greenhouse slug Milax gagates [1]. The brown garden snail is the common snail [1].
No published study of slug damage to cannabis specifically was found; the biology below is from vegetable and lab studies and is extrapolated (Supported).
How do I recognise slug and snail damage?
Slug and snail damage on cannabis is irregular holes with smooth edges, chewed leaf margins, and seedlings cut off at the base overnight [1]. Silvery mucus trails on leaves, pots and soil confirm slugs or snails, because earwigs and caterpillars make similar holes [1]. See kb-pests-caterpillars.
Seedlings are most at risk because slugs prefer succulent tissue and a seedling has little to lose [1]. Older plants lose leaf area but survive (Grower practice).
What is the slug life cycle in numbers?
The gray field slug lives about a year, lays clutches of about 10-40 eggs and develops best near 17-20 C. All land slugs and snails are hermaphrodites and lay eggs after mating [1].
| Parameter | Value | Conditions and source |
|---|---|---|
| Eggs per clutch | 3-40 (slugs generally) | extension summary [1] |
| Eggs per clutch, D. reticulatum | mean 22 (range 9-49) | field, Carrick 1938, cited in [5] |
| Eggs per clutch, D. reticulatum | ~20 at 20 C; ~10 at 28 C | lab [2] |
| Eggs per clutch, D. reticulatum | 21-43 | lab [3] |
| Lifetime eggs | 217 at 20 C; 105 at 16 C | lab [2] |
| Eggs per year | up to ~500 | field estimate, cited in [5] |
| Egg hatch | 12-19 d | lab, 16-28 C [2] |
| Egg hatch | 23-27 d (hatchability 79-97%) | lab [3] |
| Hatch rate | 95% at 20 C (highest) | lab [2] |
| Juvenile period | 51-68 d | lab [2] |
| Time to maturity | 3-6 months, by species | [1] |
| Total lifespan | 216 d at 20 C; 193 d at 24 C; 170 d at 16 C; 151 d at 28 C | lab [2] |
| Best temperature | ~20 C for egg laying, development and longevity | lab [2] |
| Growth optimum | ~17-19 C, detrimental above | literature summary in [4]being reviewed |
| Generations per year | 1 (lab) to 2 (UK field) | [3][4] |
| Snail clutches (brown garden snail) | up to 6 per year, ~80 eggs each | [1] |
| r_m, doubling time | no published value found |
Slug populations are mixed-age. Spring and autumn peaks in D. reticulatum numbers may reflect a mixed-age population responding to good conditions rather than two distinct generations [4].
Why do temperature and moisture matter for slugs?
Moisture decides when slugs feed and temperature decides how fast they breed. Slugs are most active at night and on cloudy or foggy days, and hide from heat and bright light [1]. In cold weather they shelter in the topsoil; hot, dry periods stop activity [1].
The gray field slug performs best near 20 C: egg laying, development and longevity peaked there in the lab, and 28 C shortened life and halved clutch size [2]. Cool, wet spring and autumn weather is slug season outdoors.
In greenhouses and polytunnels, wet floors, mulch, damp pots and drip lines keep slugs active all year (Grower practice). Mild coastal climates allow year-round activity outdoors [1].
How do I scout for slugs?
Scout slugs at night and under traps, because by day they are hidden. Search with a flashlight after dark; watering the area in late afternoon draws them out [1].
Lay boards raised about 2.5 cm (1 in) on runners, or inverted melon rinds, and check underneath every morning; scrape off and destroy what you find [1]. Check daily at first, then weekly once numbers fall [1].
There is no published action threshold for slugs on cannabis. Grower practice is to act on any slug found near seedlings or transplants.
How do I control slugs and snails on cannabis?
Control slugs and snails by combining baits, barriers, traps and habitat removal; no single method holds them alone [1]. Follow the label and local law for any bait.
Iron phosphate baits. Iron phosphate baits are safer around children, pets and wildlife, and some are allowed in organic growing [1]. Slugs stop feeding soon after eating them but take several days to a week to die, usually hidden [1]. Iron phosphate has been field-tested as a molluscicide [6]. Scatter baits around seedlings, not in piles.
Other bait classes. Ferric sodium EDTA baits work like iron phosphate but faster, in about 3 days instead of 7, and are not organic [1]. Metaldehyde baits are highly poisonous to dogs and cats and should be kept away from pets, children and edible plants [1].
Bait timing. Baits work poorly in very hot, very dry or cold weather [1]. Light irrigation before baiting encourages slugs to forage; heavy watering after baiting makes pellets mouldy [1].
Copper barriers. Use a copper strip at least 5 cm (2 in) tall, buried 2.5-5 cm (1-2 in) into the soil [1]. Bands on pots and planters work until they tarnish; clean with vinegar [1]. Remove slugs inside the barrier first.
Beer traps. Sink deep-sided containers with the rim at soil level and a lid with entry holes; they attract slugs within a few feet and need refilling every few days [1]. Sugar water with yeast works too [1].
Handpicking. Pick at night with gloves and drop slugs into soapy water [1]. Repeat regularly.
Habitat. Remove boards, debris, dense weeds and thick mulch near seedlings, and water in the morning rather than the evening (Grower practice, consistent with [1]).
Predators. Ground beetles, toads, snakes, birds and others eat slugs but rarely give enough control in gardens [1]. Penned poultry can reduce snail problems [1]. Do not release decollate snails where they are restricted [1].
What should I not do against slugs?
Do not use metaldehyde baits where dogs, cats or children can reach them, and do not put any bait on the plants themselves [1].
Do not salt the soil to kill slugs; salt damages soil and plants (Grower practice).
Do not rely on beer traps alone around a field of seedlings.
What are the common myths about slugs?
"Salt is a good slug control." Myth. It kills on contact but harms soil and plants.
"Beer traps clear a garden." Myth. They draw slugs only from a few feet away [1].
"Iron phosphate does not work because there are no bodies." Myth. Slugs die over several days, out of sight [1].
"Copper always keeps slugs out." Contested. It works only tall, clean, continuous and with no slugs already inside [1].
Evidence
- Species, damage, behaviour and control options: Established (extension) [1].
- Temperature effects on D. reticulatum development and egg laying: Established in the lab [2][3][4].
- Use on cannabis: extrapolated from vegetables and gardens (Supported).
Glossary terms used here
- Glossary: slugs and snails
