How To Spot And Deal With Caterpillars On Cannabis Plants

If you’ve ever walked out to your beautiful flowering cannabis plant one morning and noticed a chunk missing from a cola, or worse, found bud rot spreading through what used to be your best top, you might’ve met your first caterpillar. Or at least the trail of destruction they leave behind.

They don’t get the same hype as spider mites or powdery mildew, but honestly? Caterpillars can do just as much damage, fast. The worst part? You often don’t even see them until the damage is done.

So What Are We Dealing With Here?

Caterpillars are just the larvae of moths and butterflies. They come in a bunch of varieties, but the most common culprits in cannabis are:

• Helicoverpa armigera (African bollworm) – a major pest in South Africa and a real problem for outdoor cannabis.
• Cutworms – chunkier guys that stay lower down, often attacking stems and lower leaves.
• Looper-type caterpillars – those inchworm-style ones that arch their backs when they move.

They usually show up during warm months, especially in outdoor or greenhouse grows where moths can freely lay eggs. The moth comes in, lays a few eggs on the undersides of leaves or right on the buds, and boom, you’re suddenly a host.

How Do You Spot Caterpillars?

It’s not always obvious. You won’t see a web, and they’re not flying around in plain sight like whiteflies. You’re more likely to spot what they leave behind:

• Holes in leaves are a classic feeding damage
• Frass (bug poop) looks like tiny black grains or pepper scattered on leaves and buds
• Browning patches inside colas, often accompanied by mold
• Bud rot that seems to start from the inside out
• A single chewed-off bud top, like it got decapitated overnight

If you’re lucky, you’ll catch a green or brown caterpillar nestled inside the buds when you peel them open. But most of the time, you’ll only realize you had one after it’s ruined a cola.

How Bad Are They?

Real bad, especially in flower. A single caterpillar can hollow out a top cola in a night or two. And the damage they cause creates the perfect environment for botrytis (bud rot) to take hold, which spreads like wildfire once it starts.

If you’re running an outdoor grow or even a semi-open greenhouse, you’re definitely at risk. Once moths start laying, you could be dealing with a new generation of bud-chompers every few weeks.

What’s the Difference Between Caterpillar Damage and Other Bugs?

Caterpillar damage is pretty distinct once you’ve seen it:

• It’s messy. They rip and chew through soft plant tissue, especially flowers.
• They leave poop everywhere. That frass is one of your best early warning signs.
• You’ll often find bud rot not long after. Especially if humidity’s high or airflow is weak.

Compare that to something like spider mites, which cause stippling and webbing, or thrips, which leave silver streaks, caterpillar damage is more “chunky” and destructive.

Can You Prevent Them?

You can try. And it’s worth doing, because once they’re inside your flowers, your options get very limited. A solid IPM (Integrated Pest Management) plan helps keep them out in the first place.

Here’s what makes a difference:

• Netting or screening outdoor plants, especially during moth-heavy seasons
• Using pheromone traps to monitor moth activity
• Inspecting plants daily during flower, especially deep in the buds
• Blue or UV insect lights outside your grow to lure adult moths away (don’t put these inside, you’ll just attract them in)

There’s also the natural predator route. While not as surgical as you’d like, there are beneficial insects and bio-controls that help:

• Trichogramma wasps – tiny parasitic wasps that lay their eggs inside moth eggs, stopping the caterpillar before it starts
• Predatory beetles – some will eat caterpillar eggs or young larvae, but results can be hit or miss
• Birds – if you’re growing guerrilla-style or on a permaculture plot, letting natural predators into the ecosystem can help. Not perfect, but still a factor.

How to Deal With Caterpillars Once They’re In

Okay, so the little creeps made it through. Now what?

Manual Removal

It sounds basic, but it’s often your best shot, especially in flower. Grab a pair of gloves and go hunting. Peel open suspicious buds, look for poop, check every cola. If you find one, there’s probably more.

BT Spray (Bacillus thuringiensis var. kurstaki)

This is a game-changer for organic growers. It’s a bacteria that specifically targets caterpillars, when they ingest it, it destroys their gut lining. Doesn’t hurt bees, ladybugs, or beneficials. Works best on young larvae, so timing and coverage matter.

Spray it on:

• In early flower (safe window)
• After rain or heavy dew
• Repeatedly, every 5–7 days during moth season

If you’re only going to do one thing for caterpillar control, make it BT.

Spinosad

Also organic-ish, derived from soil bacteria. Works on caterpillars and thrips, but again, only on contact. Use in early flower only. After that, it can leave residues.

Neem Oil

Not ideal here. It’s more effective as a deterrent against egg-laying than a treatment for active caterpillars. And like Spinosad, it’s not flower-friendly.

Synthetic Options

There are some, but you really need to be careful.

• Carbaryl (Sevin) or pyrethroids – will kill caterpillars, sure, but you don’t want to be spraying that on something you’re going to smoke. These are for desperate pre-flower interventions, and even then… think twice.

If you’re growing for personal use or anything clean-label, just skip the synthetics. The risk isn’t worth it unless you’re in a massive outdoor setup and under heavy pressure.

Final Thoughts

Caterpillars are one of those pests that can take your grow from thriving to heartbreaking overnight. What makes them so frustrating is how stealthy they are, no buzzing, no webbing, just silent destruction deep inside your buds.

But with a little prevention, regular inspection, and the right tools (seriously, get BT), you can keep them under control. The key is catching them early. Once they’ve tunneled into your colas, there’s no spray in the world that’s going to save that flower.

So get used to checking your plants. Know what frass looks like. Learn when the moths in your area are active. And don’t be afraid to get your hands dirty when it comes to picking them off.

Because at the end of the day, nothing ruins a harvest faster than a bud full of caterpillar guts and mold. And you only have to lose one cola like that to never want to risk it again.