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How to Keep Caterpillars Off Your Cannabis Plants

A healthy insect population contributes to healthy biodiversity in your garden. However, caterpillars can do some damage to your weed plants given the chance. This is how to deal with them.

Growing cannabis outdoors comes with many benefits: more space, bigger yields, the sun on your skin… However, these positives don’t come in isolation. Outdoor growers have to keep on their toes to prevent the local wildlife from feasting on their crop.

While aphids and whiteflies are some of the most infamous cannabis pests, caterpillars can do some damage too. Learn how to prevent and remove them below.

WHAT IS A CATERPILLAR?

Caterpillars represent the larval stage in all moth and butterfly species, and researchers estimate there are over 175,000 species around the world.

These soft-bodied insects range anywhere from 1mm to 14cm in length. The majority of them are strictly herbivorous, except for a carnivorous and cannibalistic minority.

Caterpillars are differentiated from other larval species based on specific anatomy. Their key features include:

• Maximum of five pairs of legs
• Twelve stemmata (simple eyes)
• The presence of crochets on the prolegs (limbs that allow them to climb well)
• “Y” or “V” shaped cleavage lines on the front of the head

We rarely see the interesting behaviour of caterpillars with the naked eye, however, closer inspection reveals the fascinating world of these creatures—from their chemical defences to unique social behaviours such as forming alliances with ants.

WILL CATERPILLARS KILL MY CANNABIS PLANTS?

At the very least, caterpillars will do some damage to your cannabis plants. A minor infestation will leave holes scattered throughout your fan leaves, and a larger problem can lead to fatal damage that cannot be remedied.

Of the thousands of caterpillar species, a select few are more frequently problematic to cannabis growers. The cabbage looper [1] , for example, prefers to graze on the surface of fan leaves, using its siphoning mouthparts to strip away pieces of tender tissue.

Other species go directly to other parts of the plant’s anatomy, such as the stems. If this major structural component becomes compromised, a plant may never recover. For these reasons, you should constantly be on the lookout for caterpillars, ready to remove them from your growing space.

HOW TO SPOT A CATERPILLAR INFESTATION

A caterpillar infestation can range from a few inconvenient holes in foliage to total devastation of a specimen and subsequent loss of yield. Regardless of the severity, these hungry critters always leave some of the below symptoms in their wake.

Irregular holes: Caterpillars will chomp your leaves, but they won’t be neat about it! They’ll leave oddly shaped holes in your fan leaves that range in size from several millimetres to several centimetres across.

Stem damage: Species such as the Eurasian hemp borer bypass the leaves and decide to feast on the denser material found in the stems and branches. They make a hole in the surface of the tissue and bore through the interior of the structure.

Chewed flowers: Borer species will also have a go at your flowers—a heartbreaking sight for any grower to endure. They’ll burrow into your buds and relish in the supply of resin and precious phytochemicals. They primarily aim for the base of developing buds, resulting in wilting and death of the flower.

Leaf yellowing: Damage to your plants—particularly the stems—can affect water and nutrient transportation. Leaves located above the point of damage will likely begin to yellow due to a lack of these vital components.

Stunted growth: Chances are you’ll have noticed the infestation by the time you realise your plants are becoming stunted. Caterpillar damage can stress plants out, reducing their size and productivity.

REMOVING CATERPILLARS FROM CANNABIS PLANTS

Although they have the potential to do some real damage, caterpillars are relatively easy to get rid of. Many conventional gardeners reach for a bottle of pesticide as a solution, but most cannabis growers aren’t keen on spraying their plants and living soil with noxious chemicals.

Fortunately, there are numerous natural and organic means of putting an end to a roaming caterpillar horde in your garden. Here are the easiest ways.

Physical removal: Caterpillars are perfectly visible to the naked eye. Unlike aphids, they are easy to spot. If you’re only dealing with a handful of specimens, simply pick them up, place them in a jar, and relocate them to a nearby natural setting. Many species are nocturnal, so prepare yourself for some nighttime hunting.

Parasitic wasps: Why spend potential hours picking caterpillars from your crop when your own army of beneficial insects can do the job for you? Parasitic wasps prey on caterpillars and use their bodies to lay their eggs. Although the means of dispatch is nothing short of horrific, nature has engineered this species to take down caterpillars.

Praying mantises: Masters of stealth, these cool-headed assassins wait patiently underneath leaves, hidden in flowers, and behind stems until their prey makes the first move. These brutal yet swift predatory insects do an effective job at reducing caterpillar populations.

Bacterial sprays: These natural pesticides contain bacterial species that harm caterpillars, without posing a risk to cannabis plants or the people that smoke them. Avoid administering these sprays too close to harvest time. Formulas containing Bacillus thuringiensis will do the trick.

HOW TO PREVENT CATERPILLARS ON CANNABIS PLANTS

As the age-old adage goes: Prevention is better than cure. You can take certain measures to avoid infestations taking hold in the first place, saving you time, effort, and damaged plants.

Beneficial insects: Introducing insects as a preventive measure will minimise the risk of infestation. Add these patrolling insects before you even transplant outdoors to allow them to build a substantial population.

Barrier fabrics: They might not look pretty, but they are effective. Set up a wooden frame and fabric layer around your crop to prevent caterpillars from physically reaching them to feast and lay eggs.

Neem oil: Derived from a South Asian tree, neem oil serves as a natural pesticide. It contains phytochemicals that deter insects when administered as a soil drench or foliar spray.

Neem oil serves as a completely natural way to protect your cannabis plants against pests.

Caterpillars might be fascinating—some even cute—but they'll eat your plants to the roots if they get a foothold. Learn how to defend against them.

Getting Rid of Caterpillar Infestations

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Getting rid of caterpillar infestations in cannabis grows is a relatively easy task if you catch them early enough; once those caterpillars have grown into mini snakes from hell, however, it isn’t such an easy task.

Caterpillars don’t just come out of nowhere; those little inoffensive grey moths aren’t as inoffensive as you once thought, as they lay eggs all over your plant and once they hatch your grow will be covered in tiny little 1mm worms that will devour your plants’ leaves, leaving them almost transparent. It’s almost as if they leave the fibers of the leaves and only eat the green parts.

Once they grow bigger, they become easier to detect and many times, growers don’t notice them at all until they grow a bit bigger. They will no longer only eat the green bits; they’ll begin to take entire bites out of the leaves, leaving little cuts similar to those left on apples by the same kind of caterpillars. Apart from eating the leaves, the caterpillars will begin eating the buds as well once they’re a bit bigger, making tunnels and caves through them which allow humidity into the bud and generally, all of the weed where these little buggers have been will rot.

They don’t just rot due to the bites taken out of them; caterpillars defecate all over your plants, and this can only mean bad news. The feces makes it much easier for fungi like botrytis to infect your plant, and it’s generally this kind of fungi that ends up killing off your plants when you have a caterpillar infestation. The feces are quite visible, little 1mm balls all over your plants and buds, stuck to the resin. If you can see the feces then you’ve got some monster caterpillars roaming your grow.

The caterpillars come alive at night and camouflage themselves during the day. When they’re small you can still find them during the day time, and they look like little moving pistils. You can find them under the leaves or even sitting on top of a leaf perfectly camouflaged; you can even find them showing down on your leaves in the middle of the day. Once they’re adults they eat at night time and they go down into the soil during the day so the plant can’t see them; they spin a little silk line that attaches them from the soil to the plant itself, so you won’t see them during the day until they make their cocoon underneath the leaves, but we highly recommend that you don’t let it get that far.

The best way to get rid of them is to avoid them all together by using a preventive product, which is the same product you can use to kill them off if you catch them too late. This product is Bacillus Thuringiensis, a bacterium that burns the stomach of the caterpillars and basically melts them from the inside out, drying them up completely. If you spray at the start of the summer and keep spraying every three weeks, then you should have caterpillar-free plants. If you wait, however, and your plants get infested, you might have to kill most of them by hand as the bigger caterpillars won’t be killed by Bacillus Thuringiensis, they’ll just be slowed down, so you’ll have to hand pick them off which can be kind of disgusting.

If the caterpillars have already made your grow their home, then you’ll need to remove as many as possible by hand, searching through the leaves and buds, as well as anywhere you find feces or bite marks. If you check on your plants first thing in the morning you might just manage to find most of them, as during the day you’ll only find small ones. You’ll need to spray Bacillus Thuringiensis once a week for three weeks in a row to make sure all of the small caterpillars are gone, and then all you have to do is search for the big ones until bite marks stop appearing; if you’re a grower, then you’ll know your plants from top to bottom and you’ll be able to tell when new bite marks appear. During the day it’s much easier to see them if you look at the bottom of the leaves from underneath the plant; the sunlight will highlight any possible caterpillars hiding there. If you wait too long to kill them or you don’t notice them in time, it will be too late and they’ll end up rotting your entire plant, and grow if you’re unlucky.

There’s not much else you can do other than prevent it from happening, as these creatures are natural and go through their life cycle every single year, so if you’re growing outdoors you’re going to need to start worrying about preventing these little hellish worms from appearing on your plants. If you want more information about Bacillus Thuringiensis, click here.

Author: Javier Chinesta
Translation: Ciara Murphy

Getting rid of caterpillar infestations in cannabis grows is easy enough if you catch them in time; if not, you'll need to act quickly.