Hemp Growers Pest Reference Guide
This reference guide is designed to help identify potential insect pests that could be affecting your hemp crop. Use it in your initial scouting and identification stages while waiting for feedback from agricultural extension services or other agencies. Always consult your local agricultural department for identification of pests and possible treatment.
The insects covered in this guide are categorized by how they are most likely to impact the plant, some can be found indoors, outdoors, and in the greenhouse. It is not a comprehensive list of all potential pests, but covers commonly observed insects affecting hemp crops.
Petiole: this is the small “stem” that connects the leaf to the stem or stalk of the plant.
Stippling: refers to light colored specks left on leaves as a result of insects sucking nutrients and chlorophyll out of that area.
Skeletonization: this refers to extreme defoliation where an insect has eaten all leaf tissue except the vein structure. The “skeleton”of the leaf remains.
There are three main categories of mite and insect pests that affect hemp production:
(Not including seed damaging insects)
Piercing: Insects and mites with needle-like mouth parts that pierce plant tissue and extract nutrient rich sap from the leaves or stems. This can cause yield reduction over time due to nutrient loss, compromising overall plant health, and creates a wound for viruses to enter the plant.
Borers: These insects bore into the stem of the plant, they primarily use the plant during the larval stage for habitat and nutrition. They are not accessible for treatment using topical insecticides. Stem obstruction can inhibit nutrient and water flow to upper parts of the plant and cause stems to break.
Defoliators: Any insect that chews the leaves of a plant leaving holes where they have eaten the tissue. Damage can appear consistent and round or in irregular patterns and on different types of leaves.
Common Indoor and Greenhouse Hemp Pests
Fungus Gnats
Where to find on plant: Adult stage fungus gnats are often found flying around the base of the plant, they may appear to fly out of the soil when disturbed. They lay eggs in growing media and feed on roots or other microorganisms in the media.
Associated risk: Potential virus vector, larvae are known to feed on roots, which can be detrimental to young newly rooted cuttings or seed, adult fungus gnats (seen above) are usually not a significant threat to healthy plants.
(Jim Baker, North Carolina State University, Bugwood.org)
Two Spotted Spider Mite
Type: Piercer
Plant Stage Affected: Vegetative and Flowering
Where to find on plant: Undersides of leaves and petioles until populations are large enough and can be seen clustering at leaf tips.
Associated risk to plant: Extract nutrients from plants leaving small light spots known as “stippling.” Spider mites can transmit viruses, and large populations will produce enough webbing to bind up large sections of the plant.
(Whitney Cranshaw, Colorado State University, Bugwood.org)
(Whitney Cranshaw, Colorado State University, Bugwood.org)
Hemp Russet Mite
Type: Piercer
Plant stage affected: Vegetative and flowering
Where to find on plant: The hemp russet mite can be found on undersides of leaves and petioles.
Associated risk to plant: Extracting nutrients from plant, and can be a vector for plant viruses. The insects are difficult if not impossible to spot with the naked eye. Other noticeable signs may include distortion of color and shape of leaf.
(Whitney Cranshaw, Colorado State University, Bugwood.org)
(Whitney Cranshaw, Colorado State University, Bugwood.org)
Thrips
Type: Piercer
Plant stage affected: Vegetative and flowering
Where to find on plant: Thrips can be found on the underside of leaves, they are very small and blend very well into the plant they are feeding on.
Associated risk to plant: Extract nutrients from plant causing stippling, and can be a potential virus vector.
(Whitney Cranshaw, Colorado State University, Bugwood.org)
(Whitney Cranshaw, Colorado State University, Bugwood.org)
Cannabis Aphid
Type: Piercer
Where to find on plant: Aphids are generally found on the underside of leaves and along stems and petioles of the plant.
Associated risk to plant: High populations of aphids can lead to sticky residue on plants called honeydew which attracts black sooty mold. This residue is aphid waste from feeding on nutritious plant sap. High populations can lead to stunted growth, chlorosis, and potentially spread viruses among plants.
(Whitney Cranshaw, Colorado State University, Bugwood.org)
(Whitney Cranshaw, Colorado State University, Bugwood.org)
Whiteflies
Type: Piercer
Where to find on plant: Look for whiteflies on the undersides of leaves, they will often fly off the plant all together when leaves are rustled
Associated risk to plant: Whiteflies extract nutrients from plants, they are mostly of concern as a common vector for viruses.
(Central Science Laboratory, Harpenden , British Crown, Bugwood.org)
(Lesley Ingram, Bugwood.org)
Common Outdoor Hemp Pests
Flea Beetles
Type: Defoliator
Plant stage affected: Vegetative
Where to find on plant: Flea beetles will be visible dark specks moving on the leaves.
Associated risk to plant: Chew small holes in leaves, damage is not usually significant enough to reduce yield except on young seedlings.
(Whitney Cranshaw, Colorado State University, Bugwood.org)
Corn Earworm
Type: Defoliator (chews into buds)
Plant stage affected: Flowering
Where to find on plant: Corn Earworm can be difficult to spot early in their life cycle. Once full grown, you’ll easily spot them on the leaves, climbing stems, or through flower buds as in the photo above.
Associated risk to plant: Caterpillars damage a plant by disrupting vasculature and eating large portions of the leaf tissue.
(Whitney Cranshaw, Colorado State University, Bugwood.org)
(Whitney Cranshaw, Colorado State University, Bugwood.org)
Grasshoppers
Type: Defoliator
Plant stage affected: Vegetative and Flowering
Where to find on plant: Grasshoppers can be easily spotted anywhere on the plant, primarily around flowers and stems.
Associated risk to plant: Grasshoppers primarily do damage by chewing on leaves and stems of the plant, they are most concerning when plants are small and not fully developed
(Whitney Cranshaw, Colorado State University, Bugwood.org)
Fire Ants
Plant stage affected: Vegetative and Flowering
Where to find on plant: Fire ants colonize the root zone of plants, they mound around the base of the plant sometimes crawling up through the stem.
Associated risk to plant: Fire ant damage to outer and inner parts of the plant’s stem cuts off nutrients to the rest of the plant, a substantial outbreak can cause the entire plant to die. Small and young plants are particularly at risk.
(Russ Ottens, University of Georgia, Bugwood.org)
Japanese Beetles
Type: Defoliator
Plant stage affected: Vegetative and Flowering
Where to find on plant: Japanese beetles feed primarily on leaves, they can be found on the top or the underside of leaves.
Associated risk to plant: Japanese beetles are known to “skeletonize” plant leaves. Often not severe enough to damage yield, but are challenging to control. They are most active in mid summer.
(Sterling Southern, NC State University)
Approved Insecticides for use on Hemp
The US EPA keeps an updated list of pesticides approved for use on hemp Always confirm a product is either listed on the US EPA Registered pesticide list or is exempt from EPA designation.
Check-in with your local state regulations as well, not all EPA registered pesticides are approved in every state.
Final Notes
The best integrated pest management (IPM) protocols focus heavily on prevention and scouting. Insects are more attracted to weak plants that may already have some damage, nutrient deficiency, or immune weakening due to pathogen presence. Ensure a well prepared environment with airflow, moisture maintenance, nutrient rich growing media at the proper pH, and sufficient light availability. Regularly check through your plants for any signs of insect presence or damage, catching them early is the best way to minimize a surge in population growth.
Related Posts
