How to

22 March 2021

How to Control Pests when Growing Cannabis

It’s happened to every grower: vegging plants stop growing as quickly as the rest of the crop, or maybe leaves are yellowing and stressed despite the plants receiving nutrients regularly. A closer look at a stunted plant reveals a grower’s worst nightmare – an outbreak of pests!

A Matter of ‘When’ and not ‘If’

Every cannabis grower will deal with pests in their crop at some stage. The reality for anyone growing anything is that the environment that best suits their plants will also attract pests. By the time the grower discovers an infestation, it is often too late, but there are proven techniques that can be applied to prevent and control pests.

There are many ways cannabis pests can enter and infest a garden, regardless of the growing environment. Small insects that are airborne for part of their lifecycle can enter a clean grow room on clothing or can be borne by the wind into a garden. Pests can lay eggs in soil or on clones that hatch when plants are vegging.

Good Practices to Prevent Pest Infestation

Here are some of the most common ways pests can find their way into cannabis plants and some of the best preventative measures and control methods you can use.

Clothing: Clothes can carry pests indoors from outside. A single aphid clinging to clothes worn outdoors can be a vector for a quickly growing colony. The grower can reduce risk by changing into clean clothes before entering their grow environment.

Planting mix: Unsterilised soil may contain eggs and diseases that can hinder plant growth. Most high-quality potting soils are sterilised before being sold. Smaller amounts of garden soil can be sterilised at home by heating the oven to 82C and baking for 30 minutes.

Infected stock: If the cultivator is growing seedlings or clones sourced from friends, grow shops, or other growers, a good practice is to quarantine the plants away from the growing environment for one week. Observe the plants for any pests or infections and if necessary, spray them with natural pesticides like neem oil or pyrethrum.

Pets: Pets can bring pests into a growing environment in the same way as clothes. Do not let pets enter the grow space.

Air: A good practice for indoor cultivators is to filter the incoming air, as aphids and fungus gnats that are airborne for part of their lifecycle can ride currents straight into a grow room.

Plant debris: Pests are likely to multiply under plant debris, which is also a vector for powdery mildew and other pathogens. Dispose of any pruned foliage, dead leaves, and plants as soon as possible.

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How to Scout for Pests

The grower can prevent a pest problem by implementing a regular scouting practice. Scouting plants every few days can be tedious, but it is a small price to pay to ensure a crop’s overall health. A microscope or hand-held lens (x10-x15 magnification with light) is also a handy tool to have for definitive pest identification.

Cannabis plants are susceptible to pests at all stages of their growth, but they are most vulnerable during transition periods such as the first days after transplanting or the first weeks of flowering. Plants are using energy to stabilise themselves and may not have the reserves needed to fight off an infestation. Growers should be extra vigilant during these phases.

The grower can start their scouting by assessing the uniformity of their crop. Are there plants in the crop that appear less vigorous, stunted, or yellowing? Inspect those plants more closely for any pests and identify affected plants with tape or flags to determine if they are spreading to the rest of the crop.

The most common areas to check for pest activity are the underside of leaves, the axils of leaves and branches (space/angle between the upper side of leaves and the stem), and new growth. Another location to check is the soil and plant’s root zone, as pests can thrive in these moist, warm areas.

Winged insects can be monitored via sticky traps placed around plants (3-5cm above the canopy). Every week, record how many are caught in the traps to get a sense of the severity of infestation.

Cannabis plant is sprayed with insecticidal soap to target pests

The Usual Suspects

The most common pests that infest cannabis plants include:

Spider mites

Aphids

Whiteflies

Fungus gnats

Aphids and spider mites pierce the surfaces of leaves, branches, stems, and flowers and suck out juices, robbing plants of nutrients. Fungus gnats primarily live off of roots in planting media, stunting plants by making it more difficult for roots to uptake nutrients.

Pest populations can explode in a very short time if left unchecked. They can spread any number of diseases, viruses, bacteria, and fungi throughout the entire crop.

Aphids are 1-3mm in size, pear-shaped, and soft-bodied. They can range in colour from green and yellow to black. Aphids’ key distinguishing feature are two “tailpipes” or horns that extend from their abdomens.

Infestations of aphids can cause cannabis leaves to curl at the tips or to wilt and for the plants themselves to have stunted growth and reduced vigour. The presence of individual aphids on plants is a sign for the grower to start applying population controls; if the grower is observing clusters of aphids, especially on stems and the undersides of leaves, there’s a good chance the population has exploded.

Spider mites are some of the most serious pests that can infest marijuana plants. They are no more than 0.4mm long and favour the warm, dry conditions found in grow rooms, where their numbers can rapidly explode if left unchecked. Like their arachnid cousins, they have four pairs of legs, no antennae, and produce webbing that they use to travel from plant to plant, which can dramatically reduce bud quality.

Spider mites feeding will leave tiny brown spots on the underside of leaves, stunting plants by robbing them of vital nutrients, and if the grower can identify minuscule cobwebs in their colas, chances are it’s a severe infestation. The most likely vector for these tiny pests is unclean plants sourced from other growers (mother plants usually transfer mites to their clones). The best way to prevent an infestation is by quarantining plants before growing them out, spraying them if necessary with neem, pyrethrum, or sulfur. One more way indoor growers can mitigate spider mites is by keeping grow room environments humid and cool (55-60% humidity, 18-20C).

Fungus gnats are grey/black airborne pests, 3-4mm in size featuring long legs and delicate long wings. Their larvae feed on root systems, and organic matter in planting media, pupating in the soil to emerge as adults in 4-10 days. Indoors, fungus gnats breed year-round and reproduce very rapidly.

These root feeders severely weaken plants by inhibiting nutrient uptake, stunting plants, and making them more susceptible to diseases and viruses. Fungus gnats thrive in high humidity, so the grower that identifies them can mitigate the population by letting their planting media dry out entirely between waterings.

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Spraying and Biological Controls

Control small amounts of pests by using water in a pressurised sprayer outside of the growing environment to physically knock them off of the plant (remove any leaves or buds that appear heavily infested). The grower can also practice integrated pest management (IPM) by releasing beneficial insects such as ladybugs and green lacewings into the plant canopy. Nematodes can be watered into soil to feed as they are natural predators of fungus gnat larvae.

Large outbreaks can be countered by spraying plants with insecticidal soap or plant-derived organic insecticides like neem oil (Azadirachta indica, known as azadirachtin) or pyrethrum (Chrysanthemum cineraria folium). Prioritise spraying the underside of the leaves where pests live, insecticidal sprays kill pests on contact and evaporate quickly and will need to be reapplied 5-10 days after the initial session.

Do not compromise bud quality by spraying during flower, spraying is ideally done veg. If spraying outdoors, the best times to spray are early in the morning or from dusk onwards when the day’s heat has abated. Before spraying the entire crop, test the strength of the dilution recommended by the manufacturer by spraying a handful of pests away from any plants.

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Post author
Martin
Martin is a production horticulturist with experience in commercial cannabis cultivation and sustainable farming from his time with Emerald Cup Award-winning farmers Esensia Gardens in northern California's Emerald Triangle.
See more from Martin

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