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Cannabis Cultivation: Spider Mites In The Crop

If left unchecked, they destroy an entire crop quickly. Females can lay up to 20 eggs per day, and these can hatch in as few as three days. You can see why these mites are so dreaded.

March 1, 2018  By Dr. M. Ishtiaq Rao

Predatory mites are mixed in a carrier material to spread on the leaves.

As a grower, one of your worst nightmares is finding out that you have a potential pest in your grow space. The sight of your hard work looking sickly can be very disheartening to say the least.

Your first step is to troubleshoot the problem, and the best way to do that is to first start looking at some of the most common offenders and the types of damage that they cause.

Tetranychus urticae, commonly known as the spider mite, will leave evidence that can be seen by the naked eye. An individual spider mite can be hard to spot at first as they are less than a millimetre in size and can vary in colour.


One thing to look for is tiny off-white to yellow specs on the tops of your plants leaves. This is caused by the mites feeding on the chlorophyll.

From there, your next step is to look at the underside of the leaves for signs of silk webbing that they spin to protect themselves from predators (hence the “spider” reference). If your eyesight is really good, you can possibly see the spider mites themselves. In extreme cases, the webbing can cover the crown of the plant.

Now if you are still unsure if spider mites are the pests at hand then the one way to tell for sure is to get a grower’s/jeweller’s loupe and look along the veins of the underside of the affected leaves, as that is where they like to congregate.

By now you have determined that it is indeed the dreaded spider mite.

If left unchecked, they destroy an entire crop quickly. Females can lay up to 20 eggs per day, and these can hatch in as few as three days. You can see why these mites are so dreaded.

You now need to plan your strategy for battle. Normally, we suggest starting off with a good rinse of your plants with cold water before your lights-out period. Make sure to get the underside of the plants as thoroughly as possible as again that is the spider mite’s preferred place to hide out. This has two functions.

  • Firstly, spider mites do not like humidity so this practice slows their reproduction.
  • Secondly, P. persimilis (a predator of spider mites) in turn likes humidity so their reproduction would increase.

We would suggest waiting a day or two, then you should spray the plant again just before the lights out period with a registered insecticidal soap. Make sure to get the underside of the leaves as this is the only way that it will come into contact with the spider mites.

Some growers swear by home remedies such as dish soap mixtures and tobacco teas or even cayenne pepper concoctions. They may kill your spider mites, but potentially risks your product being recalled. The commercial product methods are reliable, but usually they are also the ones that cost you some of your hard earned money, unfortunately.

Above all, preventive measures are always the best method of fighting these harmful pests. The most simple preventive tactic is to keep a clean grow space at all times. At the end of your crop, do a thorough cleanout, and during the crop stay on top of dead, fallen leaves and weeds.

The best strategy is to take care of spider mites in the earliest plant stages. The farms that have older plants and new plants in the facility at the same time will likely see carryover of spider mites onto those new plants. This can be a real problem, especially if workers move from older plants to new plants in the same day. If you’re one of these farms, as soon as you bring your new plants in, direct your workers to handle the new plants first thing in the morning, and make sure that you have a strong preventative barrier.

Other methods of fighting these pesky buggers is by way of predatory mites such as  Hypoaspis miles, Amblyseius andersoni, A. fallacis, A. californicus, and Phytoseiulus persimilis, among others. Predatory mites come packed in a bottle or tube with either sawdust, corn grit or vermiculite and need to be applied directly to the affected leaves. Just shake a few onto the top of the affected leaves and the predatory mites will work their way to the back of the leaves to find the spider mites.

With so many species to choose from, how do you decide when to use what?

Amblyseius species make great preventive tools because they are general predators, and in absence of mites they can feed on other unwanted insects. Rather than waiting to have a spider mite outbreak and experiencing the sad sight of your plants covered in webs, you can simply sprinkle these mites around on your plants evenly as a blanket.

Similarly, Hypoaspis miles can work wonders for preventing any damage on your plants at all. It works preventively and can be distributed in the greenhouse well before the plants are even brought in. Since it is a soil-dwelling predatory mite, it can be added along the posts, and along the walkways and outer perimeter of the grow facility. These are all the places where spider mites tend to hide for diapause, when nothing else can reach them.

If you’re in the situation where spider mite has become established on your plants, the only thing that can rescue the situation is Phytoseiulus persimilis because it is host-specific and the only food in medical marijuana for this predatory bug is spider mites. Each could handle up to 20 spider mites. The earlier you can detect and treat those spider mites, the cheaper the treatment will be.

Keep in mind that persimilis are more effective under prime conditions. The temperature should ideally be between 70-80 F (21-26 C) and the relative humidity should be approximately between 60-90 per cent.

These conditions, however, don’t necessarily coincide with all the growth stages of the cannabis plant. In the flower stage, you to want lower temperatures as well as much lower relative humidity levels, because humidity can cause mould issues. Persimilis applied during the post-flowering stages will not perform well because they are being thrown into unfavourable conditions. If, however, you did these applications against spider mites prior to the flowering stage, your resident population of persimilis will be better able to adapt to this temperature and humidity shift. To add persimilis early is a recipe for success. Bare this in mind when planning your attack strategy.

Make sure to take the time to inspect your plants on a regular basis and rinse them off once in awhile as well. A proactive and dedicated pest scout will stay on top of every single pest spot in the facility, and can save you time and money in the long run. A grower that is in touch with their plants will likely be more successful in the long run.

Dr. M. Ishtiaq Rao, CEO of Crop Defenders Ltd., is a PhD in entomology who is dedicated to the eradication of horticultural pests through IPM methods. Crop Defenders Ltd. utilizes biological knowledge of pest and beneficial insect species in order to manage pests cost-effectively and in a way that is cohesive with a healthy environment.

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