Pest Guide: Caterpillars
Caterpillars are marvelous insects in nature, but unfortunate they love marijuana plants. This isn’t a problem for indoor growers, who can usually preclude these pesky critters, but for those who maintain outdoor gardens, it can become a serious challenge. This guide explores how to identify caterpillars and the damage they cause, and offers some effective solutions to prevent them from destroying an otherwise perfect crop.
What Are Caterpillars?
Caterpillars are the larval stage of moths and butterflies in the Lepidoptera order. There are over 21,000 species of caterpillars, but only a few of them selectively feed on marijuana crops. The most common pest species afflicting cannabis gardens are Corn Borers and the aptly named Hemp Borers, so this article will focus primarily on these species. However, the advice offered below can be applied to other types of caterpillar infestation as well.
Some caterpillars eat only the leaves of marijuana plants, while others, including the Corn Borers and Hemp Borers, can cause more substantial damage. When young, the larvae feast on low-hanging leaves, but as they grow, they bore holes in the plant’s stalk to give them access to the marrow found within. A minor infestation will cause unsightly and annoying stem cankers, while a serious one can actually kill the plants. If left unchecked, just five to ten of these pests can demolish a marijuana plant.
The reason boring caterpillars cause so much damage is that they also open the door to other pests. Once they’ve begun carving holes into the stalks, other insect pests will also enter into the equation. Damaged plants are less able to withstand insect infestations, so the results can be devastating.
To make matters worse, Hemp Borers don’t confine themselves exclusively to the stems of marijuana plants. When infestations occur later in the season, they’ll also eat the flowers. Given that those maturing buds are a grower’s cash crop, that’s a serious problem. Let’s take a look at how to solve it.
How to Spot Caterpillars
Growers need to know what to look for at each stage of a caterpillar’s life. The signs of an infestation vary based on severity and species, but learning the basics gives growers the knowledge they need to ensure early detection.
Butterflies and moths lay their eggs on the undersides of cannabis leaves. The eggs can be yellow, white, or translucent. They can be either ovular or round depending on the species and will be very small. If there have been butterflies or moths loitering around the garden, break out a magnifying glass and check the leaves for eggs to catch the caterpillars before they can even emerge.
Early Larval Stage
Leaf-eating caterpillars, including the early larval stages of Hemp Borers and Corn Borers, can usually be seen by the naked eye since they only attack the plants’ external structures. For best results, go caterpillar hunting at night with a flashlight. Most species of caterpillars are more active from dusk til dawn, so they’ll be up and about and therefore easier to spot.
Even if growers don’t see any actual caterpillars, they should still keep an eye out for bite marks on the leaves. Some species of caterpillar feed exclusively on the leaves throughout their larval stages, and since they’re prolific feeders, they tend to leave behind easily recognizable signs of damage.
Don’t assume that if the caterpillars are only eating the leaves of your plants, there’s nothing to worry about. Stem borers typically begin their larval stages on leaves, and even those species that won’t go on to eat the plants’ stems or buds can cause significant crop losses by reducing the plants’ ability to photosynthesize efficiently.
Later Larval Stage
As stem boring caterpillars mature, they retreat to the inside of the plant’s stalk, which makes them harder to find. The caterpillars themselves may be hidden away, but the damage they cause will make itself apparent quickly.
Look for holes in the plant’s stems that have brown trails around them. These visible holes are a sign that the caterpillars have already entered the late larval stage and wreaking havoc on the crop. Growers may also notice other pests congregating in or near these holes.
Cocoon or Chrysalis Phase
Caterpillars that have consumed enough plant material to form cocoons or chrysalises have likely already caused substantial damage to a grower’s crop. It’s still important to be able to identify this stage of a moth or butterfly’s life cycle, though, as they may appear in other areas around the property. Removing them will disrupt the insect’s life cycle, reducing the population of adult moths or butterflies around to lay eggs and perpetuate the cycle of destruction.
Cocoons are white, translucent, or green sacks that can be found hanging from the branches or stems of plants and trees. Cocoons are constructed by caterpillars to facilitate metamorphosis, while chrysalises are actually the hardened bodies of butterfly pupae. They can be found on the undersides of leaves and twigs.
Just about everyone knows what butterflies and moths look like. While describing how to identify all species of these insects is beyond the scope of this article, it’s worth taking a look at Corn Borer and Hemp Borer moths, in particular.
Corn Borer moths are around one inch long with 0.75 to 1-inch wingspans. Female adults are light brown or brownish-yellow with dark, wavy bands. The males of the species are darker and smaller. Adult Corn Borers are most active right before dawn.
Hemp Borer moths are much smaller. They have wingspans of less than two inches. They’re drab looking and brown, which makes them difficult to see on tree trunks and branches, but it’s worth keeping an eye out for them, anyway.
How to Get Rid of Caterpillars
Growers have a few methods at their disposal for reducing caterpillar populations. The best way to deal with caterpillars is to adopt an integrated pest management (IPM) strategy. This method of pest control utilizes multiple organic and least-toxic approaches of tackling problems to reduce complications like pesticide resistance and ecological damage.
Hand Removal of Eggs, Larvae, and Plants
If the infestation is already underway, growers should immediately start checking their plants for eggs and larvae. If they’re still on the outside of the plants, pick them off by hand.
If the larvae have already entered the late stage of growth and are infesting the stems, growers will have to remove parts of the plant. Cut the stem above the damaged cells and dispose of them far away from the garden. If the plant doesn’t recover, remove it completely before the infestation can spread to other plants.
Introducing Beneficial Insects
Not all insects are pests. Some of them can actually be integrated into an IPM strategy to control caterpillars and other destructive species. Predatory insects that eat caterpillars include ground beetles, minute pirate bugs, damsel bugs, and certain types of parasitic wasps, including braconid wasps.
Growers have two options when it comes to introducing beneficial insects. They can either introduce them into the garden themselves by purchasing them from pest control companies or use methods like companion planting to attract native predatory insects to the area. Companion planting requires diverting resources like water and nutrients away from the main crop, but if done correctly, it can provide lasting relief from caterpillars and other pests.
Parasitic wasps are attracted to lemon balm, yarrow, and parsley, while damsel bugs prefer the smell of fennel, spearmint, caraway, and goldenrod. Ground beetles like evening primrose, amaranth, and clover. Minute pirate bugs are attracted to caraway, fennel, alfalfa, and spearmint.
Apply Neem Oil
Neem oil is a natural insecticide that is very effective at getting rid of caterpillars. Apply it to the tops and bases of the plants either after the sun has already gone down or anytime on an overcast day to avoid burning the leaves. One of the great things about neem oil is that while it’s toxic to insect pests, it doesn’t kill beneficial wildlife or cause substantial disruptions to the ecosystem. For best results, use neem oil applications alongside other IPM techniques such as introducing beneficial insects.
Microbial insecticides are considered an effective, less-toxic option since they’re selective. Bacillus thuringiensis (Bt) is one of the most popular microbial insecticides for tackling caterpillar problems. Bt kurstaki (BtK) is one of the most effective strains for controlling caterpillars of all kinds, including those that feed on marijuana crops.
Nematodes are soil microorganisms found in healthy soil across the world. Certain nematodes, including Heterorhabditis, are great for controlling caterpillars. They kill their hosts by releasing chemicals or bacteria into the pests, which then die from blood poisoning.
The Bottom Line
Controlling caterpillars requires a multifaceted approach. Any effective caterpillar control strategy should include frequent inspections of outdoor plants throughout their life cycles. If growers notice moth eggs, young leaf-eating larvae, or signs of late-stage larvae inside the stems, they should take action immediately to prevent the spread. Once growers have removed all visible caterpillars and damaged stems, they should combine continued inspections and hand removal with an integrated management strategy that will cover all their bases and prevent future infestations.