Cannabis Pests - Spider Mites

If there's one kind of pest a grower hates seeing in his or her garden, it's the dreaded spider mite. Spider mites are particularly hard to keep under control but luckily there is plenty we can do to stop them taking over. This article shows you how to prevent spider mites, and what you can do to get them out of your grow space in the event of an infestation.

What are Spider Mites?

Spider mites are tiny pests that belong to the Acari family (mites), which includes over 1000 species of mite. They are known to feed off many types of plant, not only cannabis. Spider mites spin their webs to create a protective home for themselves for feeding and to lay hundreds of eggs.

There are 2 common types of spider mite which can affect cannabis plants:

Once spider mites have made their way into a grow space, they begin to feed off the sap/nutrients stored in the cannabis plant's leaves. They actually puncture the cells of a plant, which can result in serious damage. Initially, they can be found lurking around the undersides of leaves but as they multiply they spread to the flowers.

Identifying Spider Mites

When spider mites are not on your cannabis plants, they can hide in all sorts of places. They tend to favour high spots, so you may notice small colonies grouping in small corners around the grow room. Look out for their silky web, which glistens when the light catches it.

Morphology

Spider mites are very hard to see because they are so small (less than 1mm), even as an adult. They vary in colour but cannabis spider mites tend be either red, green, or brown. Even though they're mites, they are considered spiders due to their 8 legs and ability to spin webs.

The red spider mite multiplies in huge numbers and females can lay around 20 eggs per day when the conditions are optimal. Eggs can hatch in just 3 days and the larva are sexually mature only 5 days after that. They live for up to around 4 weeks, and their lifecycle is as follows:

• Egg - Larva - Protonymph - Deutonymph - Adult

Plant Symptoms

Spotting spider mites before they've established themselves is almost impossible and often our plants give us the first signs. It may look like your plant is deficient and leaves will begin to show symptoms. It's important to monitor plants daily, even on an automated system, so we can catch the spider mites as soon as possible.

Symptoms of a spider mite infestation:

• Yellow/white dotted speckles on leaves from bites
• Leaf yellowing (chlorosis), curling and drying
• Slow plant growth
• Silky, web forming around stem nodes and undersides of leaves
• Heavy infestations begin to show on flowers, normally at high points in the canopy (top buds).

Spider Mites Pest Control

Spider mites can be eradicated if they are spotted early on in their development. However, if you run a large grow op this can be like finding a needle in a haystack. You need to thoroughly examine your plants when you're on the hunt for spider mites. Check everywhere and each following day after you begin any kind of treatment. 

Due to their fast reproduction spider mites can adapt quite quickly, meaning treatments can become ineffective after multiple uses. This makes them even more of a challenge to control. If you've already got an infestation you might be wondering what to do next. So, here are a few ways to deal with spider mites:

Neem Oil - Neem oil is more of an organic preventative measure but can work to keep the population of spider mites down. It may stop the mites from reproducing as quickly and they definitely don't like it.

However, you might think the spider mites have left after applying neem foliar, but soon realise they are back. Neem oil treatments need to be applied every 2-3 days, but be careful not use strong solutions and avoid spraying on buds in the last 3-4 weeks of flowering. 

Insecticidal Soaps - Insecticidal soaps are normally mixed with neem oil and water to create a foliar spray solution, but they can also be used without neem to safely control spider mites. They do not stay on the plant for long so used on their own requires more regular application. Check the instructions for strengths.

Organic Insecticides - There are many products on the market that can help kill spider mites and not leave residue on your plants. Organic insecticides may need more regular application and aren't as strong as chemical pesticides, but they are less harmful for your plants and there is less risk of flavour becoming affected. Feel free to check out products like Spinosad Products or Essentria IC3.

Predatory Insects - You can use predatory insects such as lady bugs or predatory mites (Phytoseiulus persimilis) to get rid of spider mites. They won't fully eliminate the spider mites but they can do a good job of keeping the numbers at bay.

Pruning - If you catch spider mites in their early stages and only small areas of your plants are infested, consider removing those parts completely and burning them. You may also think about discarding a plant completely if it means the spider mites don't spread to other plants.

Treating Spider Mites

Indoors

Low humidity and high temperatures tend to be the main attraction for spider mites. So, one of the first things you should do is check your hygrometer and see how you can change the conditions. The spider mites already have a reason to leave if they don't like the environment.

Once you discover the mites, you might be better off moving the plant to a separate space or outside to deal with it. Pruning off the infected areas and hosing down the plant outdoors can help to remove some of the mites before you proceed to other treatments. Make sure the water is at room temperature to avoid shocking your plants and be careful not to use too much pressure.

Outdoors

Add tall, black stakes or poles around your pots or containers. The spider mites try to find the highest point and crawl to the tops of the stakes, making them easy to spot. This way you can help to 'distract' them from your top buds while you focus on treatments.

Place outdoor plants in a breezy area where air does not stagnate often. Watch out for high temperatures and protect your plants with shade if necessary.

My FIRST OUTDOOR

Finally, if you do choose to apply chemical insecticides, choose high quality products specifically for the problem your are trying to treat so you can avoid potentially damaging beneficial organisms.

Tips To Prevent Spider Mites

As always, the best method of controlling spider mites is by prevention. They love the heat and do not like air movement. Maintaining the right environment before spider mites get comfortable on your cannabis plants is worth every second. Any grower who has dealt with spider mites before will know how it goes.

Tips to prevent spider mites:

• Keep temperatures under 26-27°C.
• Maintain humidity at 55-60%.
• Circulate the air using oscillating fans.
• Apply routine neem foliar spray once every 7-10 days until the flowering stage (indoors).
• Apply neem oil directly on and around the base of the stem to prevent mites climbing back onto the plant.
• Keep a clean grow space.
• Apply treatments to the whole grow space.

Spider mite problems on PEAKYPLANTERS' Amnesia (World Of Seeds).

Conclusion

Spider mites are a real nuisance but they don't have to be the be all and end all of your grow. Pay close attention, provide the right treatments and you should be able to take control of the situation. After you harvest, make time to thoroughly clean your equipment and grow room so you can start the next cycle with a fresh, sterile space.

If you've got any tips regarding spider mites, why not share your thoughts with fellow users down in the comments section?

External References

Biocontrol of Tetranychus urticae in greenhouse tomato crops. Acta Horticulturae. - Cédric, Camps & Gilli, C. & Fischer, S.. (2014).

Effects of Temperature on Demographic Parameters of Bryobia praetiosa (Acari: Tetranychidae). Journal of economic entomology. - Ullah, Mohammad & Kamimura, Tatsuya & Gotoh, Tetsuo. (2019).

The development of the clover mite (Bryobia praetiosa, Koch, Tetranychidae) in relation to the nitrogen, phosphorous, and potassium nutrition of its host. - Morris, & Nathaniel, O.. (2020). 

The stealthiness of predatory mites as spider mite biological control agents. Biological Control. - Otsuki, Hatsune & Yano, Shuichi. (2019).

Tetranychus urticae and Tetranychus cinnabarinus: Synonymous or not?. Suid-Afrikaanse Tydskrif vir Natuurwetenskap en Tegnologie. - Ueckermann, Eddie & Navajas, Maria & Auger, Philippe & Migeon, A. & Tiedt, Louwrens & Yanar, Dürdane. (2013).

This article was updated November 2020.