Caterpillars may seem harmless at first glance, but for cannabis growers, they can quickly become one of the most destructive outdoor pests. These hungry larvae chew through leaves, stems, and even dense buds, often leaving behind frass, mold, and severe plant damage in just a short time. Early identification is essential, as caterpillar infestations can spread rapidly during both the vegetative and flowering stages. In this guide, we’ll explain how to identify caterpillar damage on cannabis plants, the risks they pose to plant health and harvest quality, and the most effective ways to remove and prevent them before they become a serious problem.
Key Takeaways
- Caterpillars commonly damage cannabis plants by chewing irregular holes in leaves, stems, and buds, often overnight.
- Tiny black droppings known as frass are one of the clearest signs of a caterpillar infestation on cannabis plants.
- Bud rot commonly develops when caterpillars tunnel through dense flowers and leave behind moisture and pathogens.
- Outdoor cannabis plants are far more vulnerable to caterpillar infestations than indoor plants due to exposure to moths and butterflies.
- Bacillus thuringiensis (Bt) is a commonly used biological control that poisons caterpillars after they ingest treated plant material.
- Regular plant inspections during flowering help growers spot caterpillars early and reduce the risk of mold spreading through buds.
- Key Takeaways
- Understanding Caterpillars on Cannabis Plants
- How to Identify Caterpillar Infestations
- The Risks Caterpillars Pose to Cannabis Growth
- Effective Methods for Removing Caterpillars
- Preventing Caterpillars on Cannabis Plants
- Managing Caterpillars During Flowering and Harvest
- Frequently Asked Questions
Outdoor cannabis plants are naturally more exposed to caterpillar infestations, especially during the warmer months when moths and butterflies are most active. Many growers prefer hardy outdoor cannabis strains that can better handle environmental stress, fluctuating humidity, and the increased risk of mold developing after pest damage during flowering.
Understanding Caterpillars on Cannabis Plants
As children, we grew up believing that caterpillars were small, soft-bodied, cute insects that turn into butterflies and do no one any harm. You may be pleasantly surprised to know they are actually quite the menace, especially for outdoor growers. Below is an explanation of the common caterpillars that attack weed plants, covering their lifecycle and how they operate.
Common Types of Caterpillars That Attack Cannabis
You would be wrong for thinking there was only one or two types of caterpillars to worry about; there are actually six different caterpillars, and they love to devour your weed plant. Below is a list of the names so you can see which caterpillars are more prevalent in your area.
| Caterpillar Type | Main Damage Caused |
|---|---|
| Corn Earworm | Eats buds and flowers |
| Cabbage Looper | Chews leaves rapidly |
| Armyworms | Strip foliage overnight |
| Cutworms | Damage stems and seedlings |
| Tobacco Budworm | Burrows into buds |
- Corn Earworm
- Eurasian Hemp Borer (Grapholita delineana)
- Cabbage Looper (Trichoplusia ni):
- Armyworms (Spodoptera spp.)
- Cutworms (Agrotis spp.)
- Tobacco Budworm (Helicoverpa virescens)
Life Cycle of Caterpillars and How They Infest Cannabis
Caterpillars will undergo four different stages of development during their lifecycle, as shown below.
- Stage one is the egg
- Stage two is the larva (caterpillar)
- Stage three is the pupa (chrysalis)
- Stage four is the adult (butterfly/moth
Caterpillars have an enormous appetite as they grow and develop, and can chew through leaves, stems and in the worst scenario, through the buds. Indoor growers will not typically experience caterpillar problems, but when growing outdoors, they are able to locate the plants and hide on the undersides of the leaves, tucked inside of stems and inside buds.
How to Identify Caterpillar Infestations
I always recommend checking your plants on a daily basis and giving them a thorough inspection to check for any signs of insect damage. Knowing what to look for when we are talking about caterpillars may be new to you, so below are all the early signs and symptoms to look out for, knowing how to tell the difference between other insect damage and visual indicators.
Early Signs and Symptoms on Leaves and Buds
When checking your plants, if you notice any of the signs below, then there is a good chance that you may have caterpillars in your grow room.
| Sign | What It Looks Like |
|---|---|
| Leaf Holes | Jagged or irregular holes in leaves |
| Frass | Tiny black droppings on leaves or buds |
| Windows | Thin transparent patches on leaf tissue |
| Stem Damage | Chewed or weakened young stems |
| Bud Rot | Brown or moldy areas inside buds |
- Holes in the leaf tissue - Tiny patchy holes on the leaf will appear overnight
- Caterpillar Droppings - These tiny black droppings are found on the leaves
- Windows - These appear as a small, square, transparent layer of leaf tissue
- Moldy Buds - The buds will begin to mold where the caterpillar has entered
- Stem Damage - Caterpillars will chew the stems of younger plants
Differentiating Caterpillar Damage From Other Pests
The damage caused by caterpillars is actually easier to spot in comparison to the damage caused by thrips, aphids and even spider mites. Caterpillar damage is much more sudden, and overnight, they are able to chew through developing healthy leaves. To help you differentiate between the different types of damage, below is a list of signs of infestations from the other commonly known pests.
Signs of Thrip Damage
Thrips are slender, 1-2 mm-sized pests that also love cannabis plants and can inflict a tremendous amount of stress on the foliage of your plants.
- Silver and bronze dots on the leaf surface
- Small black specks of their excrement
- Leaves will slowly turn brown and have a dry appearance
- Wilting and claw leaf may occur
- Growth during the vegetative period is slowed down
- Flower development is slowed, and stunted growt
Signs of Aphid Damage
Aphids come in a bunch of different colours, ranging from black, brown, yellow and red. They tend to cluster together in large numbers, making them easy to spot.
- You will see clusters of 1-3 mm-sized aphids on the undersides of cannabis leaves
- Leaves can be curly and appear distorted
- Leaves will begin to turn yellow
- A shiny and sticky sap residue is left on the leaf
- Cannabis plants can wilt due to aphid damage
- Growth will be slowed down and stunted
Signs of Spidermite Damage
Spider mites are a grower's worst nightmare, and if you have ever experienced them, you will know why! Many growers end up with spider mites due to contaminated clones.
- Yellow dots on the leaf that, over time, strip the leaf
- Bronze and shiny markings on the leaf surface
- Spider webs are forming between the internodes and buds
- Small, usually red, 0.5 mm-sized spiders are inside the webs
- Wilting plants once the webs are well established
- Affected leaves become lifeless and brittle
Visual Indicators: Holes, Frass, and Bud Rot
If you are still not totally sure about whether you may have caterpillars on your plants, then below is a breakdown of the key visual indicators.
Leaf Holes
Caterpillars chew through leaves and leave behind a very distinct mark. The edges of the leaf will have jagged edges, or the centre parts of the leaf may have been eaten.
Frass
Frass is another name for the droppings that caterpillars leave behind as they manoeuvre through the plants. They appear as tiny black granules of dirt and are found around the leaves.
This is easily the worst case scenario when it comes to caterpillar damage. Caterpillars will work their way through the buds, leaving a small trail in their path. Buds will start to mold where they have eaten and where they spread pathogens from their excrement.
The Risks Caterpillars Pose to Cannabis Growth
There are some pests that rank much higher than others when it comes to the damage they can do in one sitting to your plants. Caterpillars are at the top of the list, and whilst they may not be as tricky and difficult to deal with as dreaded spider mites, they sure can cause havoc with cannabis growth, as explained below.
| Growth Stage | Common Risk |
|---|---|
| Seedling Stage | Stem chewing and collapse |
| Vegetative Stage | Heavy leaf damage |
| Early Flowering | Caterpillars entering buds |
| Late Flowering | Increased bud rot and mold |
Impact on Plant Health and Photosynthesis
Cannabis plants produce green, healthy and vibrant foliage with the goal of capturing as much light energy as possible for photosynthesis. When caterpillars work their way through fresh leaf growth, the amount of leaf tissue significantly reduces over time, thus lowering the amount of potential energy a plant can store when growing indoors or outside.
How Caterpillars Cause Bud Damage and Rot
Caterpillars seem to love cannabis buds as much as growers! They will mine their way through your best-looking buds and create small tunnels. At the same time, the caterpillar will produce excrement, which contains pathogens.
Another reason why caterpillars are single handedly responsible for causing infected eaten buds to become moldy, is to the fact the damaged buds are exposed to humidity and heat. This dramatically increases the chances of the buds becoming too wet and damp, especially outdoors.
Bud rot caused by caterpillars is often far more severe in dense flowers where airflow is restricted and excess moisture becomes trapped inside the buds. For this reason, many outdoor growers choose mold-resistant cannabis strains or varieties with slightly looser bud structures, particularly in regions with humid late summers and wet fall conditions.
Effective Methods for Removing Caterpillars
After close inspection and comparing the signs of caterpillar damage with other insects, you will be able to confirm if these soft-bodied hungry insects are, in fact, on your plants. In the event that they are, well, you will need to know how to effectively remove them. There are two main methods to deal with a caterpillar infestation, as explained below.
Manual Removal: When and How to Do It
This involves carefully inspecting your seedlings and vegetating plants, locating any caterpillars and removing them from the plant. It may be challenging to spot the smaller ones, as they are good at camouflaging with your plants. Throw the caterpillars far away from your plants and kill them to ensure they do not return anytime soon.
Biological Controls: Bt, Beneficial Insects, and Natural Predators
The other way to get rid of pesky caterpillars without the need to remove them by hand as and when you spot one, is to use biological controls or natural predators. There is a bacterium called Bacillus thuringiensis (Bt), which, when used as a diluted spray, enters the caterpillar's stomach after eating the leaf.
| Predator | How It Helps |
|---|---|
| Lacewings | Feed on eggs and larvae |
| Praying Mantises | Hunt larger caterpillars |
| Assassin Bugs | Attack soft-bodied insects |
| Birds | Reduce outdoor caterpillar populations |
At this point, the bacteria will act as a poison and cause a slow death for the caterpillar. Be careful when spraying buds during the late flowering stage. If you prefer not to spray your plants, then why not consider natural predators such as:
- assassin bugs
- Lacewings
- praying mantises
Organic and Homemade Insecticides
Caterpillars have a strong sense of smell, and there are certain aromas that they cannot stand that work as an excellent deterrent. You could mix any of these listed below in extract form with water and create your own homemade organic spray.
- Peppermint
- Vinegar
- Lavender
- Hot pepper
- Garlic
Preventing Caterpillars on Cannabis Plants
Eliminating caterpillars once you notice they have become a problem is one thing, but preventing them in the first place will make life much easier and significantly reduce the threat again.
Indoor growers do not need to be as concerned about caterpillars, but for the outdoor growers, prevention can help save an entire outdoor crop. Below are the best outdoor grow practices to reduce caterpillars.
Outdoor Grow Practices to Reduce Caterpillars
| Prevention Method | Purpose |
|---|---|
| Daily Inspections | Spot damage early |
| Row Covers | Block moths from laying eggs |
| Bt Sprays | Kill caterpillars after feeding |
| Remove Eggs | Prevent infestations spreading |
| Beneficial Insects | Naturally reduce pest numbers |
- Growers can hand-pick caterpillars off their plants when they can see them
- Eggs can be sprayed away using insecticidal soap and water
- Row covers create a fine mesh layer over the plants, preventing caterpillars
- Frequently spraying plants with a Bacillus thuringiensis (Bt) solution
- Introducing natural predatory insects onto your plants
Monitoring and Regular Plant Inspection
Monitoring your cannabis plants does not just mean giving them a brief glance every now and then, or checking that there are no holes or edges of leaves eaten. When determining if you have any insect problems, I would encourage you to check your plants daily using a magnifying glass.
The reason is that you have a far greater chance of seeing any signs of caterpillar droppings (frass), ‘windows’ that appear as small square patches of eaten leaf tissue. You should take the time to inspect every leaf from the top to the undersides, as well as the stipule, which is the stem part connecting the leaf to the main stem.
Managing Caterpillars During Flowering and Harvest
We can all handle leaves being eaten during the vegetation stage, but caterpillars can be a nightmare for growers during the most important stage. Managing caterpillars properly during the final weeks of harvest when your buds are at their most dense, well-developed and swollen, will make a huge difference in the quality of your harvest.
Safe Strategies Near Harvest Time
So you are only a week or two away from the harvest date, and everything is looking fantastic as far as bud development is concerned. Wearing a pair of latex gloves, carefully check each bud at every internode. Caterpillars will typically gather around the top cola of flowering cannabis plants; however, check the lower buds and side branches.
- Remove and kill any caterpillars that you do find during inspection
- Cut away any damaged buds and moldy buds using scissors
- Be careful when spraying buds due to the increased chance of mold spreading
Post-Harvest Inspection and Pest Elimination
Even if you check every single bud from top to bottom on all of your plants, there is still a chance that some caterpillars were hiding out. I have had times in the past when trimming outdoor colas into smaller thumb-sized buds, that I only noticed the caterpillars inside the buds at that point.
If you find yourself in the same situation, inspect the buds closely with a light and see whether the levels of damage warrant throwing them away. The worst trait about Botrysis is that buds can begin to mould from the inside out, which can create a devastating loss when trimming your buds.
- Clean the grow room down with bleach after harvesting, ensuring all insects are dead
- Throw away the soil that you were using in case of any insects or eggs present
Some growers prefer fast-flowering cannabis strains outdoors because they can finish earlier in the season before caterpillar populations become more aggressive during late summer and fall. Shorter flowering periods may also reduce the amount of time buds are exposed to heavy rain, rising humidity, and the increased risk of mold developing after pest damage. Many growers also choose outdoor cannabis strains that are better suited to handling environmental stress and fluctuating weather conditions throughout the flowering stage.
Frequently Asked Questions
How do caterpillars get onto cannabis plants?
Caterpillars usually appear after moths or butterflies lay eggs on the leaves or stems of outdoor cannabis plants. Once the eggs hatch, the larvae begin feeding on the plant tissue and can spread quickly if left untreated.
Are caterpillars more common on outdoor cannabis plants?
Yes, caterpillars are far more common outdoors because plants are exposed to flying insects that lay eggs directly on the foliage and buds. Indoor growers are less likely to experience caterpillar infestations unless pests are introduced through contaminated clones or outdoor plants.
What are the first signs of caterpillars on cannabis plants?
The earliest signs typically include irregular holes in leaves, tiny black droppings known as frass, transparent “window” patches on leaf tissue, and sudden chewing damage that appears overnight.
Can caterpillars damage cannabis buds?
Yes, caterpillars can tunnel through developing buds and leave behind frass and moisture, which often increases the risk of bud rot and mold forming inside dense flowers.
What is the best way to get rid of caterpillars on cannabis plants?
Manual removal combined with biological controls like Bacillus thuringiensis (Bt) is commonly considered one of the most effective approaches. Regular inspections and early treatment usually help reduce severe plant damage.
Is Bacillus thuringiensis (Bt) safe for cannabis plants?
Bt is a naturally occurring bacterium commonly used by growers to target caterpillars without harming the plant itself. Many growers avoid spraying buds late in flowering to reduce excess moisture and mold risk.
Can beneficial insects help control caterpillars?
Yes, natural predators such as lacewings, assassin bugs, and praying mantises may help reduce caterpillar populations in outdoor cannabis gardens.
How can growers prevent caterpillars during flowering?
Frequent plant inspections, removing eggs by hand, using protective row covers, and applying preventative Bt sprays during early flowering are common methods growers use to reduce caterpillar problems before harvest.
