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Don’t get complacent with spider mites


May 17, 2021
By Gagandeep Singh Bhatoa and Mohyuddin Mirza
Photo: Mohyuddin Mirza

It was a shock to receive this picture from an experienced grower. 

He thought the distinctive tiny spots or extensive webbing of a spider mite infestation is some sort of nutrient deficiency, not realizing it’s actually something far more sinister.

Cannabis greenhouses and grow rooms are literally a hot bed for pests and pathogens. The warm and humid conditions, plus ample crops for food make perfect homes for pests and pathogens. Pest problems can develop and spread rapidly in a cannabis crop. 

In particular, there are two different types of mites species known to feed on cannabis – the two-spotted spider mite (Tetranychus urticae) and the clover mite (Bryobia praetosia) – both of which have evolved more recently with powerful adaptations that make them persistent and resistant pests. 

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Problem identification in general requires critical thinking, but solving a problem requires creative thinking. 

This applies more to spider mites because they can hide, blend easily with the surroundings, and are resistant to most of the products used to kill them. And they can explode into a web. That is why we must understand their life cycle and must get creative. 

We mean don’t just focus on killing them. Instead, look at vulnerabilities of these mites. We must eradicate them completely or manage them so that they don’t reach damage threshold. 

This is how an adult female, two-spotted spider mite looks like (Figure 1). The two dark spots are their identification mark. The size of an adult is less than one milimetre. They can be seen with the naked eye but it is preferred to use a hand lens to spot them.

Mites have a lifecycle that varies directly with the environment. In a very hot, dry climate, they can complete a life cycle in a few weeks. If the environment is cooler, and has more humidity, it can take them over a month to hatch, mature and leave their own offspring for the next wave of attacks on your plants. 

A female spider mite can lay anywhere from 100 to 300 eggs in their lifespan of 30 days. The eggs hatch and the first stage is a larva with six legs, followed by stage one nymph, then second stage nymph, and then an adult. The lifecycle depends on temperature and relative humidity. The drier the climate, the quicker the hatching will be. The table below describes the relationship to temperature.

This is the most important information to understand, the effect of temperature and relative humidity in the facility. Many growers have a tendency to use a relative humidity of 40 per cent because of fear of developing grey mold Botrytis. 

So if you are growing cannabis crop at 23 C and a relative humidity of 40 per cent, the Vapor Pressure Deficit (VPD) will be 16.86 millibars. VPD above 9.0 mbars is an invitation to spider mites to live happily from laying eggs to adulthood in a short period of time.  

Fortunately, you can take steps to take charge. Pest management should start a month before a cannabis crop is brought into any part of an indoor or greenhouse facility. At this stage, the facility should be inspected for maintenance, proper water drainage and evidence of pests. 

Previous pest issues and treatments in that area of the facility should be reviewed. Incoming plants must also be inspected before they are placed in the cultivation area. If there is history of spider mites before, then pay particular attention to nooks and corners. We have seen buds going into the lights and if webs have been formed there, then you have a major task to clean them. 

Perform daily scouting through the crop, checking leaves, buds and every portion of the plant. The first symptoms of spider mite infestation will appear as “pinhead” type of spots in the lower portion of the leaf (Figure 2).

Check the underside of the leaves with a hand lens and you might see a few eggs,  some nymphs and may be few adults. The symptoms can be confused with magnesium deficiency. 

This is the time to release biological control agents and this is also the time to tag these plants so that other staff are alerted to the problem. Cross contamination should be avoided. 

Have a list of all registered products handy with rates and availability. We have had situations where the products to control the spider mites were ordered late and infestations had spread.

Have all the knowledge about biological controls agents. They have to be released in every plant if infestations are serious. The suppliers have very good recommendations as to how many predators are needed, when to release them and how to monitor their progress.

Having proper tools are necessary for scouting. Hand lens of at least 10X, an optivisor which can be mounted on head, stakes and tags, a map of the facility and trained professionals are all essential part of scouting. 

One of the biggest challenges is to avoid cross contamination. Spider mites, unlike thrips and whiteflies cannot “fly” from one plant to other. They latch on to your clothes and thus, carry over to young plants. 

Establish a traffic path so that infected plants are handled at the end of work schedule. Change gloves and coveralls after handling the infested plants. Pay special attention to clones and mother plants, so that they are not infected.

Once plants are removed, and then thoroughly clean the area with registered disinfectants. Many a times if multiple crops are grown in the same area, then extreme care is needed to avoid cross contamination. 


Gagandeep Singh Bhatoa is a plant heath care specialist in Lethbridge, Alta. He can be reached at gaganagrico@gmail.com.
Mohyuddin Mirza is a greenhouse specialist in Edmonton, Alta. He can be reached at drmirzaconsultants@gmail.com.


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