Caterpillars in your cannabis/marijuana garden and how to fight them.

by Alex Robles

One of the biggest pest problems when we’re growing cannabis outside or in a greenhouse, is caterpillars.  These little demons will devastate a crop and break your heart. Caterpillars are the voracious larvae of butterflies and moths and are part of a large family of insects known as Lepidoptera.  Now when I say large, I mean 21,000 members strong in that family.

The Corn Borer and the Hemp Borer are two of the most damaging caterpillars know to feast on cannabis because they feed on the marrow in the stalk of our plants.  Now this type of attack is especially sinister because as they drill holes in the stems it leaves the stocks open vulnerable for other sap suckers and disease.

Even though there are many species of caterpillar, most of them share some basic characteristics.  Below are just a few general notes about caterpillars and how fight them.

What are they:

  • Caterpillars are the larvae of insects in the Lepidoptera family, which eventually become beautiful butterflies and moths.  Some moths are considered pest borers because of their larval activity.
  • Caterpillars have long segmented bodies and are sometimes very colorful, these bright colors warn predators of a bad taste.  Others will avoid being eaten by camouflaging themselves to look like the surrounding vegetation.
  • They move by using 6 pairs of legs but they will also have upto 10 pairs of false legs, depending on the type of caterpillar.
  • Caterpillars don’t breathe through their mouth, instead they have small holes on the sides of their bodies called spiracles. These holes are interconnected tubes or trachea that give oxygen directly to the cells.
  • Even though they have 6 small eyes in a “U” shape on their face, they don’t see very well.     Instead, they use their antennae to find leaves, buds and stems to chomp on.
  • Butterflies usually lay their eggs on the upper leaves and buds of our canopy, where it’s harder to inspect.  They do this so the newly born caterpillars have something to eat after they hatch.
  • The eggs are tiny and look like a small cluster of dots that are yellow or white in colour. The shapes range from round to oval depending on the type of caterpillar.
  • You will need some magnification to get a good look at the eggs.
  • Butterflies usually lay their eggs before the cooler fall and winter temperatures move in.
  • The caterpillars phase usually lasts 2 week to a month, depending on type of caterpillar.   They will feast on your plant until they pupate or cocoon up.


  • It doesn’t take a lot of caterpillars to do serious damage to your plants.  When your inspecting your plants, look for signs and symptoms of caterpillar activity.
  • Things like holes in the stalks and branches with brown trails around them,  If you find these holes in your stems, the chances are pretty high that you have a serious problem.  Bite or chew marks on the leaves are easy signs of this pest as well.
  • Some caterpillars also leave behind a slime trail when they crawl across the leaf surface.
  • When they start to chew or tunnel through the cannabis buds, dead spots will show up, which are signs of rottenness.  They will easily chew up that bud to get to the soft tissue in the branches.
  • Caterpillar droppings combined with the tunneling will usually let fungi like botryis and mold like bud rot to quickly spread throughout your plants.

How to fight them:

Trichogramma Life Cycle: Rincon-Vitova
  • Visually inspect your plants leaves and stem, starting at the top and working your way down.  When you find one, pick it off and get it out of your garden.
  • You’re going to want to inspect your plant daily if you do find caterpillars.  It’s easier to find them in the later part of the day when the Sun is low and the relative humidity is high.
  • If you have a history of caterpillars in your garden, have a plan to prevent butterfly and moth eggs from hatching and a way to fight the larvae if they hatch.
  • Green Lacewings and Ladybugs are great general predators because they will eat the eggs of many different pests.
  • If you live in a warm climate, praying mantis are also a great solution for caterpillars because they have big appetites and are very active hunters.
  • Trichogramma is a very tiny parasitic wasp that lays its egg in the eggs of moths and butterflies.  It kills them before they can enter the plant-eating larval stage and uses smell to determine the suitability of a host to lay its egg in.  Wasps adults hatch in about a week ,after eating the content of the host egg.  It remember the smell of the destroyed egg and uses it to find another suitable host.
  • The emerging adults are mostly female and are ready to mate with the males as soon as they hatch. This and the short life cycle allows the wasp’s population to quickly reach effective numbers to fight pests.
  • Because of this quick turnover, trichogramma wasps multiply quickly when conditions are right (80-90F).  Eggs will hatch slower in cooler weather (40-60F).
  • During her 9-11 day life, the female wasp will seek out and destroy about 100 pest eggs by laying an egg inside of it.
  • I use Trichogramma egg cards that I can cut into smaller squares and hang them around my garden.  They usually hatch 2-5 days later depending on the temperature.
  • To use them effectively, put them out when you see the first flight of moths whose caterpillars cause damage to plants
  • The County Ag Commissioner or University extension agent can tell you the usual time of moth flight. You can monitor moth activity during that time with simple light traps, pheromone traps to tell you when moths are flying before they start laying eggs.

Growers Note:  Ants can be a problem, they like to eat the eggs on the squares if found. Hanging each square from a piece of thread is a good solution.

  • You can spray your plants with neem oil as a preventative measure. It won’t be very effective if you already have a large caterpillar population, it’s mostly meant to discourage the caterpillars from getting comfortable and chewing on new leaves.  Don’t soak your plants with this because it can cause them to wilt. If you are in flower, you can wipe the fan leaves down with paper towels soaked in this mix so you don’t get any on buds. Don’t spray in direct sunlight either because it can cause them to burn.
  • Pyrethrins are a good natural insecticide since it’s made from chrysanthemums.
  • Remember not to spray anything during the last 15 days before harvesting your cannabis plant.  This way the treatment has time become inactive and hopefully not affect the taste.
  • Bacillis thuringiensis kurstaki (BTK) strain is a gram-positive, rod-shaped bacterium native to the soil.  It’s a bacteria that works as an insecticide without harming the beneficial insects we like having around. It is a subspecies of Bacillus thuringiensis, and helps controls Lepidoptera.
  • BTK does not harm other animals or insects outside of the order Lepidoptera once it’s been sprayed or ingested by the pest.  Similar to Bacillus thuringiensis israelensis, birds and other predators can feed on the infected pests without toxic effects.  As with most biological control, BTK will be most effective when used early in the pest’s life cycle, particularly during the larvae stage.
  • The caterpillars have to feed on the treated leaves for it to be effective and it does break down rapidly, so you may need more than one treatment.  Once ingested, the alkaline environment of the caterpillar’s digestive system triggers the BTK bacterium to release a crystalline protein, a type of endotoxin, which paralyzes the caterpillar’s digestive tract. The caterpillars will stop feeding and die shortly after this occurs.
  • Some farmers also use bat boxes to help attract bats that will eat the caterpillars.

Identification is crucial if you want to develop a specific strategy for your caperpiller problem.  So keep a look out for moths and butterflies because they are nice to look at, but they bring death and destruction to cannabis.

Grow Learn Teach

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