Micro mites in cannabis: An essential guide to combat this silent enemy

Microscopic mites that attack cannabis plants are so small that they are practically impossible to detect with the naked eye. They’re the masters of hiding and waiting, so they’re probably one of the most difficult cannabis pests to eradicate, and certainly a silent but devastating threat. In this post, we share effective methods to prevent and eliminate micro mites to ensure the health and vitality of your cannabis plants.

Microscopic mites are hitting cannabis growers hard in recent years as they’re spreading like a plague of locusts of biblical proportions in many plantations around the world. They often cause an infection in plants, since their saliva transmits potent toxins that make plants sick very quickly, causing necrosis and other physiological disorders. But the worst damage that they can cause is by sucking the nutrients from the leaves, which alters photosynthesis and slows down growth considerably.

What are micro mites?

There are thousands of different species of insects classified as mites. Like red spiders, which are another nemesis of growers, mites are also a subcategory of the arachnid family. But, unlike red spiders, which can easily be spotted with the naked eye, micro mites are the smallest of all mites. They are members of the Tarsonemidae or Eriophyidae families and can often attack cannabis plants as well as many other commercial crops.

For instance, there are several main species of Tarsonemidae that love cannabis, although the most common are the greenhouse Tarsonemidae (Polyphagotarsonemus latus) or strawberry mite, and the white spider (Phytonemus pallidus), also known as cyclamen mite. Species such as Aculops cannabicola (the red mite of cannabis, i.e. a specific host that cannot survive on other plants) and Caleptrimerus vitis (grape leaf rust mite) can also be found within the eriophids, although to a lesser extent.

Microscopic mites move quickly and are incredibly small and extremely difficult to locate. They’re so tiny that they can move to neighbouring areas using the wind, which is a big concern! They are even known to climb onto other insects to spread, so they’re often associated with flying pests such as thrips (Frankliniella occidentalis) and whiteflies, as they can simply cling on to their legs and move with them.

Micro mites can also hide in contaminated potting soil or can be introduced through the cuttings of infected plants. So don’t assume you’re safe from this plague just because you’re growing indoors.

Know your enemy: Morphology of micro mites

Adult microscopic mites are approximately 0.1 to 0.5 mm long, with an oval shape, a bright appearance, and a colour ranging from dark green to amber. They’re as tall as the thickness of a piece of paper or a human hair. When observed with a magnifying glass, their appearance and structure is similar to that of larger species: tiny legs and a fused thorax and head. Females have 4 pairs of legs, whereas the males have three legs plus two rigid pincers which are used at the time of mating and help transport the nymphs.

This pest goes through four distinct stages: eggs, larvae, nymphs, and adult males and females. Although the average female only lives for a period of two weeks, she can lay up to 75 eggs during that time. These eggs are round and translucent, with a diameter of around 0.08 mm, and have what appear to be small concentric white bumps. Look for them in the flowers and under the leaves.

Under normal circumstances, micro mites can be found in groups, and they use their sucking/ piercing mouthpieces to feed on the sap. A magnifying glass with at least 60 times magnification is required for correct identification. With this, you might spot small dots moving, which indicates the presence of these parasites. The only thing that the human eye can see are groups of eggs, a large infestation (they are so small that you can find thousands of individuals per cm²), or the symptoms of the damage they have produced.

Symptoms of a micro mite infestation

As these creatures are microscopic, many growers don’t realise they have a problem until the plants are affected. You can’t see any obvious signs, and everything seems to be under control, but suddenly the plants begin to tell you that something is wrong… So you start to wonder whether it’s a virus, a nutrient deficiency, or another form of stress.

This is because the visible symptoms of a microscopic mite infestation are very similar to those of other common issues. The fan leaves can bend slightly at the edges and have a moist and shiny appearance, almost like plastic, which is similar to when the plants suffer from heat stress. The leaves may have yellow or bronzed spots, which is also a symptom of plant viruses sometimes. New growth may appear twisted or become stunted, and the leaves may fall off, as if the plant were suffering from environmental stress or nutrient deficiency. And, even during the flowering phase, the buds can become sickly, with the pistils suffering from rapid oxidation (they turn brown) and the buds eventually dying as if they’d been attacked by a fungus.

How to detect microscopic mites

One of the main reasons why micro mites can be so difficult to diagnose is because the actual mites can rarely be seen, and the leaves don’t appear to have any bites. Therefore, you need to look in an area slightly above where the plant normally shows signs of stress. If the plant isn’t showing any signs of stress at all, start checking the lower areas. In fact, microscopic mites don’t attack all the parts of the plant uniformly: symptoms worsen in certain places where the infestation is more concentrated.

Microscopic mites prefer to devour young plant tissue, including flower buds, tender leaves, and growing tips. In fact, the males transport the female nymphs to the most tender shoots. Most of their feeding is done on the underside, near the leaf stem, which usually causes the leaves to roll up and turn brown. If the young leaves appear to have dark brown edges, it is a good indication that micro mites are present.

How to treat and eliminate a micro mite pest

Ok, so you’ve found some microscopic mites… so now what? There’s a great range of pest control strategies available; but, regardless of the strategy that you choose, you need to make sure that it is applied for long enough so that the pest can be totally eliminated. Given the life cycle of micro mites, treatments may take several weeks before the infestation is fully eradicated.

In ideal conditions, a single mite can live up to one month (or more), laying many eggs during that period. When the eggs hatch, the cycle begins again. What’s more, as microscopic mites spread easily with the wind, constant vigilance is required to prevent further infestation.

Some of the pest control options available include:

• Predator mites (such as Phytoseiulus persimilis or Amblyseius californicus), other predator insects (such as certain species of ladybugs and chrysopa), and even some entomopathogenic fungi (like Beauveria bassiana) can assist in the biological control of micro mites.
• Insecticidal soaps.
• Neem oil (when applied at the first signs of attack).
• Pyrethrin-based aerosols (but be careful with the amount you use so that you don’t exceed the permitted limits).
• Diatomaceous earth (it won’t eliminate an infestation but will slow down its spread).
• Sulphur powder / wettable sulphur (but it shouldn’t be used on flowering plants!).
• Chemical control with an acaricide product may work, but due to their high toxicity levels, it is not advisable in plants intended for human consumption.
• Prune diseased branches, remove infected plants, and quarantine affected areas.
• Removal of weeds and any other vegetation close to your crop that may house microscopic mites.
• If you have an outside air inlet, make sure you have some sort of filter so that pests can’t get into your indoor growing environment.

Keeping a sterile and balanced environment is crucial as a method of prevention. In areas with mild temperatures and high relative humidity levels (which are the typical conditions of many greenhouses), micro mites can reproduce rapidly, so it’s important to control these factors through adequate air circulation and ventilation.

Low temperatures can slow down the reproduction of microscopic mites but won’t eradicate them. However, these mites can’t develop with temperatures above 30ºC and relative humidity below 30%. Maintaining relative humidity levels between 50 and 60% can be an effective method to halve their reproduction rate, which is a critical factor to consider in the fight against this pest without making your plants suffer too much.

And remember: one of the most common sources of infestation is the introduction of infected cuttings or plants. Therefore, if you’re introducing plants from other sources, make sure you quarantine them for a few weeks first. But if you want to avoid this problem altogether, then you can always plant from seeds!