What you should know about spider mites in cannabis cultivation

What you should know about spider mites in cannabis cultivation

by Mary de Soto

If you’re reading this, you probably have at least a passing interest in spider mites, and, more specifically, how to keep them from infesting your cannabis plants. If that’s the case, you came to the right place!


In this article we will cover some of the main information you should know about this pest and several of its natural predators, so you can get Two-spotted spider mites off your plants and keep them off. First things first, you need to know about the enemy... 


IDENTIFICATION OF TWO-SPOTTED SPIDER MITES


Spider mites belong to the family tetranychidae, and they’re actually not insects at all. They’re a member of the arachnid family, along with spiders and ticks. 

Our focus today will be on one of the most common mites that cause problems for growers, the Two-spotted spider mite, Tetranychus urticae. 


They’re easily identified by the two large dark spots on their bodies, hence their name. Two-spotted spider mites are about .5 mm long and oval in shape, with eight legs and colors that can vary from yellow to orange to green to brown and anything in between. Because of their size, they’re difficult to see clearly and look like little moving dots unless looked at with a hand lens. The eggs are extremely small, .14 mm, and are nearly transparent when freshly laid. They gradually turn opaque and straw colored when ready to hatch. 

Two-spotted spider mites with eggs
Image: Two-spotted spider mites with eggs.

TWO-SPOTTED SPIDER MITES POPULATION GROWTH


Their population growth depends on several factors, but temperature is the most important. The Two-spotted spider mite can reproduce from 54°F to 104°F, which means that they can survive in most climates. They can go from egg to adult in just 9-14 days at 77°F, and a population can double in three days. This means that in ten days, a population can increase ten-fold! 

Two-spotted spider mites and leaf damage
Image: Two-spotted spider mites and leaf damage.

DAMAGE OF TWO-SPOTTED SPIDER MITES


Even if you haven’t seen the spider mites themselves yet, you might see the damage on the leaves. 


Here’s what to look for: small whitish pinpricks on the surface of the leaves like shown in the picture above. In severe infestations, white webbing forms over the plants, resembling cobwebs (they’re related to spiders, remember?). The mites and eggs themselves will usually be found on the underside of the leaves.

 
The light-colored pinpricks are caused by the spider mites feeding on the sap. They feed by piercing the plant cells with their mouthparts and sucking out the contents. This causes the cells to change color, which, in turn, causes reduced photosynthesis and affects plant growth. As the infestation gets worse, the entire leaf will turn yellow and drop off. Eventually, if the infestation is severe enough, the whole plant may die.

Thankfully, there are beneficial insects that prey on spider mites and can keep your cannabis plants infestation free if applied preventatively or reduce current levels of infestation.

NATURAL ENEMIES OF TWO-SPOTTED SPIDER MITES

Image: Predatory mite Neoseiulus californicus.

Phytoseiulus persimilis and Neoseiulus californicus are two of the most widely used beneficial mites when it comes to combating Two-spotted spider mites. 

Persimilis is a voracious feeder and the most important natural enemy of spider mites. It can eat up to 500 spider mites in a lifetime and can significantly reduce an infestation. They are sensitive to humidity and need a relative humidity of 70% or greater, making them an ideal choice for most indoor grows and some green houses as long as they are within moderate temperature ranges. Above 89°F, they struggle to develop. 

Californicus is more tolerant of higher temperature and lower humidity than persimilis. They’ll work up to 100°F, making it a better choice for outdoor grows. It can consume up to 150 spider mites in a lifetime. They’re also more tolerant of pesticides than persimilis.

For warmer, drier climates, Mesoseiulus longipes is a good choice. They are effective up to 100°F and 40-60% humidity, making them ideal for greenhouses and outdoor grows. Like persimilis, they have a huge appetite and can consume up to five spider mites per day.

Another predatory mite that does well in warmer weather is Galendromus occidentalis. It can hunt in 80 - 115°F weather and functions better at lower humidity, down to 30-50%. Below 80°F it may go into hibernation, so it should only be used in areas where the temperature is quite high. 

Neoseiulus fallacis falls into the middle ground. It tolerates lower relative humidity than persimilis at 50%, and prefers warmer temperatures. They also diapause, a sort of hibernation, with reduced light, and so are not recommended for use in flower rooms. 

Finally, there’s Feltiella acarisuga. It’s a gall midge and feeds exclusively on spider mites. Relative humidity must be at least 75% for it to work optimally, and the length of its lifespan actually depends on the humidity level. At 64% humidity, it will live 5.5 days, and at 95% relative humidity, it can live 10.5 days. Due to the specific nature of Feltiella acarisuga, consulting a Shale Peak Horticulture Specialist before purchase is suggested.

Usually a combination of beneficial insects is recommended, such as persimilis and californicus, in order to fully combat the spider mites. It’s like a move in a video game. A punch or kick works fine, but a combo move works so much better. 

The last thing that you need to figure out is how severe the infestation is and what rate of predatory insects you need to release to properly combat the Two-spotted spider mites.

Below are a list of the beneficial insects we sell mentioned in this post. For those not listed, please send us a message inquiring about them. On each page is a release guide listing the amount of each beneficial that would be needed for your degree of infestation, should you have a Two-spotted spider mite infestation.

# per 1000 sq ft

N. californicus

P. persimilis

M. longipes

N. fallacis

G. occidentalis

Preventative

2,000

N/A

N/A

2,000

N/A

Light Curative

10,000

2,000

2,000

10,000

2,000

Heavy Curative

20,000

4,000

4,000

20,000

4,000

Determining your degree of infestation can be tricky, though, so here’s some general rules of thumb. 

Preventative is pretty self explanatory. This is the rate you release the beneficial insects at to avoid getting an infestation in the first place. Californicus is a good choice for this, depending of course on your growing conditions, as they’re less voracious than persimilis and are available in sachets (packets that contain all stages of life and hatch continuously over a three week period) from some suppliers. 

Curative light is when you just start to see damage on the plant. Light pinpricks start to appear and you may find a spider mite or two after inspecting several leaves.

Curative heavy is when you see around 3 spider mites per leaf. This is a full on infestation and requires immediate attention. If you already have webs, targeted sprays followed by heavy curative predatory insect releases is needed. 

There! Now you’ve got all of the information necessary to combat those Spider mites and keep your plants healthy and happy. 

Are you already using biological control in your cannabis cultivation? What are your preferred natural enemies to control Two-spotted spider mites? Let us know in the comments below! If you have any questions about Two-spotted spider mites, which predatory insects to use for your grow, or anything else, you can also send us an email or on reach out to us on social media. message. We’re also more than happy to help make a customized release plan specifically for you!


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