Fighting Thrips in Your Cannabis Grow

Cannabis growers, like all gardeners, should be prepared to deal with a variety of pests (hopefully by means of prevention), but for cannabis growers, no pest may be as pesky as thrips. These tiny bugs can hide for weeks eating and breeding before conclusive evidence of their presence can even be identified. A few stowaways into your greenhouse or grow room can explode into an overwhelming population in a hurry, and when they do, it takes vigilance and tenacity to completely eradicate them.

In this article, we review what thrips are, how to identify them, how to get rid of them, and how to prevent them from coming back.

Thrips are tiny insects, only about 1mm long, some of which can fly, though not all species. Thrips feed on plants by puncturing the skin of the leaves or soft stems and eating the nutritious sap inside, like little plant vampires. Not only do they feed on a plant’s lifeblood, but they can also carry several different diseases between plants.

There are over 4,000 species of thrip, and most often appear black or brown. But cannabis also attracts western flower thrips which are orange to yellow. Larva are a lighter green. Thrips are less of a problem for outdoor growers where they have to compete with predators and the elements, but tend to thrive indoors, where they can eat freely in a climate-controlled environment. Greenhouses, in particular, are very susceptible to them.

The average lifespan for a thrip is a mere 45 days. But in those 45 days, females can lay up to 80 eggs, meaning a few thrips can become an infestation in a hurry. Females lay eggs in leaf tissue, where they are protected from predators, harsh temperatures, and most insecticides.

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The eggs hatch within a week. The larva will spend another week or two feeding inside the plant before some species drop to the soil for their pupal stage, before returning to the plant for adulthood.

Further complicating matters is that thrips will not necessarily die off over the winter. Because they are adept at finding secure crevices, they can hide in any manner of plant material to keep warm, and have even been found wintering in walls and furniture.

A few of those species are actually beneficial to growers, feeding on mites, aphids, and even other thrips. If you manage to find thrips but no thrip damage, then congratulations, you somehow lucked out. Otherwise, it’s best not to wait to see who’s team they’re on, and just move to get rid of them.

Thrips are tiny, elongated insects that resemble thin grains of rice. They tend not to hang out in the open like spider mites or aphids, but instead hide on the underside of leaves, or nestle their minuscule bodies into tight crevices in the plant material. For this reason, you’re likely to notice thrip damage before you notice the thrips themselves.

Thrip damage may present as yellow or white patches on leaves where nutrients have been drained. Similarly, white streaks may occur from thrips feeding in a line down the leaf or stem (though these should not to be confused with the more winding trails left by leaf miners). Afflicted leaves may become thin and flattened, and feel brittle like dried paper, as the nutrients inside the tissue are drained away leaving only the dried skin behind.

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Black freckles the size of a pin poke are thrip droppings. And in extreme cases, galls like little warts may appear where thrips are nesting. Chances are you’ll see other evidence before that.

Thrips can be hard to see with the naked eye, and some actually recommend inspecting your plant with a magnifying glass to definitively identify them. If you suspect a leaf is infested, you could also shake or tap the leaf over a white surface. If any small, dark grains of rice collect on the white surface, you’re probably looking at thrips!

Thrips are hardy little buggers, and can quickly develop resistance to pesticides. In fact, for this reason pesticides are commonly tested on thrips. But one thing that seems to work consistently is insecticidal soap, such as Safer Soap. These soaps work by way of fatty acids actually penetrating the insect’s cellular walls, generally causing death by desiccation, not unlike salting a slug. A gruesome death, to be sure. NOTE: Insecticidal soap affects almost every insect the same, so it will also kill ladybugs, lacewings, bees, and other insects that actually benefit your garden.

Be advised that insecticidal soaps are only effective for the time the liquid is present on the plant. Any bugs arriving after the spray has dried will not be affected, so be sure to saturate the plant, making sure the soap gets into every crevice where thrips may be hiding. And be prepared to repeat this process at regular intervals.

Thankfully, insecticidal soap is not very toxic to mammals, especially large mammals, so it is safe to use around children and pets, and may be used up to the day before harvesting. That said, a quick rinse of your harvested product is a good idea.

Neem oil may also be used similarly to insecticidal soaps. Neem oil came under scrutiny in the past decade as a potential culprit for Cannabinoid Hyperemesis Syndrome, but in recent years those suspicions have not held up to scrutiny. Neem oil is considered safe for healthy mammals and is frequently used by organic gardeners.

Another organic spray is pyrethrin, which is a potent insecticide derived from chrysanthemums. Pyrethrin kills insects on contact and breaks down quickly, so no chemical compounds linger on the plant or in the soil. The drawback, similar to Safer Soap, is that there are no residual effects. Thrips arriving an hour late to the party will be fine.

A liberal dusting of Diatomaceous earth (DE) may be used as a preventative measure to keep thrips from establishing. DE works at once by drawing moisture from the insect’s exoskeleton, but at a microscopic level it also acts like razor wire, or craggy terrain to very small insects. The granules feel like powder to humans, but for tiny bugs, those granules have sharp edges that can cut their exoskeleton, so bugs don’t like crawling through it, and could die trying. Remember though, similar to insecticidal soaps, DE will not discriminate between harmful and beneficial insects, and can slice up your ladybug friends as well.

An alternative to diatomaceous earth is kaolin clay. Kaolin clay has been used in hair and skin care products and toothpaste, so it’s certainly safe for humans. But a dusting of this over leaves creates a barrier that insects will not want to work through to get to the leaf material. It also sticks to insects' bodies, and they would rather find a new feeding ground than put up with the stuff.

Sticky traps like yellow fly paper won’t resolve an infestation, but they can work as a preventative measure, catching some of them before they can establish, and alerting you to their presence.

The best way to prevent thrips is simply by keeping a clean grow room or garden area. Closely inspect any plants or plant material you may introduce to your grow room for stowaways, change clothes if possible when coming into an indoor grow room from outside, and promptly remove any refuse plant matter like trimming or fallen leaves. 

How do I get rid of thrips?

Thrips can be dealt with in a number of ways, most of which involve an insecticide foliar spray. Products like Safer Soap are effective and safe around children and pets. Neem oil, pyrethrin, and spinosad are other common remedies. Be sure to saturate the plant so these liquids get into every crevice. But remember: once these products dry, their effects do not persist, so be prepared to repeat this process about once a week to continue to kill off newcomers, or new hatchlings.

What will thrips do to cannabis?

Thrips puncture the skin of leaf and other soft plant material, then their saliva turns the contents in the leaf into a nutritious soup they then drink. A colony of thrips will drain enough nutrients from a plant to stunt growth, and eventually kill them. 

Can I spray Spinosad on buds?

You probably shouldn’t. Spinosad is an organic insecticide derived from soil bacteria, which will infect and kills thrips. However, it has also been banned from cannabis operations in five states and Canada. While it may be safe to consume after harvest, no information exists regarding how safe it is to smoke. Ingesting a chemical is much different than combusting it and inhaling the vapors. Insecticidal soaps have not been shown to be harmful to smoke.

Growers, how have you dealt with thrips in your cannabis? Share your experience in the comments below!

Photo Credit: 2H Media (license)