Growing

Spider Mite Prevention & Treatment for Cannabis

spider mite on plant

An integrated pest management solution for the two-spotted spider mite.

By far the most common grow room pest we’ve come across is the Tetranychus urticae or the two-spotted spider mite. Ridding a crop of this pest can be difficult due to a colony’s resiliency, adaptability, and impressive reproductive capabilities.

We’ve seen this pest afflicting both home gardens and state-of-the-art facilities. We’ve even spotted spider mites on plants that were professionally cloned and for sale to home growers. Talk about setting customers up for failure!

If you have spider mites on plants at home, you can take a few effective measures and eliminate this pest from your grow space.

About the Two-Spotted Spider Mite

Semi-translucent spider mites eggs tucked away on the bottom side of a fan leaf.

Reproductive Capabilities

The first step towards developing an effective management solution, is to learn about the pest. With spider mites, the nuisance begins with their reproductive capabilities. Let’s take a look:

Spider mite males are attracted to pheromones female adolescent (nymphs) secrete. This causes the male mite to wait nearby (called “guarding”) until the female mite is mature. Once mating is complete, the female mite is provided with enough sperm to fertilize her eggs for the rest of her life (Helle 1967). One female spider mite can then go on to lay 5-20 eggs per day, or hundreds throughout her life.

As if that weren’t bad enough, spider mites can also reproduce asexually. If the female lays unfertilized eggs, they still develop into male spider mites (C. C. M. Feiertag-Koppen 1976). It’s easy to see why a spider mite colony can quickly overtake and entire crop.

Spider mite eggs can be hard to spot because they are translucent and very small, coming in at only about 0.14mm in diameter (Cagle 1949). Additionally female spider mites tend to hide them on the underside of leaves. Spider mite eggs are resistant to many oils, detergents, and pesticides.

Egg incubation time is highly dependent on temperature. In most grow room conditions spider mite eggs take about 3 days to incubate.Young eggs are quite clear and become more opaque as incubation progresses (Cagle 1949). Egg resiliency and the time-frames on the below table (Table 1.1) are important to take into consideration when developing an effective control schedule for eradicating spider mites.


 

Resiliency in Hibernation

When spider mites are exposed to a lack of food, cooler temperatures, and winter photoperiods the females begin to hibernate (diapause). The trigger for hibernation is based on the cumulative influence of all three factors. Because of this, hibernation is much more likely in flowering rooms (12h photoperiod).

During hibernation the mites enter a sort of “suspended animation,” where they can live much longer without food or oxygen. Female spider mites may also abandon the plant in search of winter shelter (Veerman, 1985). In the case of your grow room, the insides of your walls might make a good shelter. It can be a good strategy to target these insects before or between grows with stronger measure, such as insecticidal fumigation (e.g. pyrethrum).

Prevention

Spider bite marks (caused from puncturing and draining leaf cells) are a tell-tale sign of a late stage spider mite infestation.

As with many problems in the grow rooms – treatments begins with prevention, attention to detail, and diligence.

Inspection

Enough cannot be said about this step. While routine inspections are a part of any successful garden, extra diligence must be employed when dealing with a spider mite infestation.

Naked-eye visible signs, like the tell tale yellow/grey bite marks, mite webbing, and even seeing adult mites themselves, are signs of a late stage infestation. Growers should spend time also looking for larva/nymphs (smaller, more transparent mites without markings).

Early stages of mite infestation are very difficult to spot with a naked eye. It is recommended that a lens between 60x and 120x magnification be used for inspection.

Leaflet samples should be thoroughly inspected from the top, middle, and bottom of the canopy. Special attention should be given to the leaf underside, including all veins, and stems.

Prevention

Preventing this pest from invading your garden is by far the best means of dealing with it. As with any Integrated Pest Management solution, entrance protocols should be implemented. When growing indoors, gardeners should never enter the grow space directly from the field. When possible include another room as a buffer between the grow space and the field, and doors should be kept closed as often as possible. The “buffer” room should be a zone for further entrance protocols, such as removing shoes, changing clothes, and washing before entering the indoor grow space.

Effective prevention also includes proper spacing of plants. Spider mites are quite mobile and readily move from plant to plant via branches and grow room walls. If possible, plants should be spaced as to not touch the walls or each other. This alone can prevent or drastically slow the spread of spider mites.

Pruning/defoliation can also be employed as an effective control measure to decrease pest harborage.

Effective pruning measures may include cutting away thick and overlapping vegetative growth and overly-infested leaves. This creates a structure that is easy to inspect and treat. Effective pruning also includes removing lower axillary branches. These are spindly branches that weave up through the canopy, are difficult to inspect, provide low yield, have high foliage, and are the perfect for housing undetected mite expansions.

NOTE: Good judgement should be used whenever removing leaf matter, especially from a sick plant. Keep in mind that removing any leaf reduces a plant’s photosynthetic potential, so don’t go overboard! Always consider if your action will benefit the plant more than it will harm it.

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Treatment

There are generally two conventional approaches to killing spider mites, foliar sprays and natural predators. However, no treatment is effective without due diligence and commitment to a thought-out schedule.

Foliar Sprays

Using a foliar spray is by far the most common approach to killing spider mites. At this point the Colorado Cannabis School is not recommending brands. However, when used properly, most commercial miticides (acaricides), oils (neem), and insecticidal soaps are very effective at killing spider mites.

There are several reasons foliar sprays fail:

  1. Applying foliar sprays without an electronic fogger can be tedious and inconsistent. This inconsistency can lead to surviving mites. When applying, ensure you are quite thorough in your application, especially when hand-spritzing. Spider mites tend to hide in the nooks and crannies of the underside of leaves. Spray the walls as well.
  2. Applying foliar sprays against their labels can cause more harm. This may include spraying the wrong frequency, mixing the wrong ratio, or applying too much. This can lead to severe foliage burns and can even destroy an entire crop. Read the entire label carefully.
  3. As we discussed above, spider mite eggs are hard to spot and resilient to most treatments. It is important to understand the relative incubation rates in your environment. Reference the above table (Table 1.1) and consider your grow room’s environment.
  4. Some insecticidal oils work by creating a barrier that spider mites can’t bite through. This creates a lack of food and may trigger hibernation, where the females may leave the plant only to come back later.

NOTE: Most foliage spray should be rinsed off with plain water after a set treatment time. Skipping this step may lead to leaf tissue burn.

After many failed exposures, spider mites can develop a resistance to most foliar sprays.

Many growers find success by applying an oil or soap every fifth day until total eradication. Please always consult the label directions for product you are using. Please also consider what you’re spraying in relation to the health and safety of your end consumer.

Natural Predators

Another very effective measure for killing spider mites is to introduce a natural predator into the grow room. Spider mites have many natural predators to choose from. Some of these include predatory mites, Amblyseius, Metaseiulus, and Phytoseiulus; predatory gall midges, Feltiella acarisuga; lady bugs, Stethorus; minute pirate bugs, Orius; the thrips, Leptothrips; and lacewing larvae, Chrysopa.

We consider the predatory mite Phytoseiulus persimilus and predatory gall midges Feltiella acarisuga as optimal natural mite killer. One adult predator mite can eat 20 mite eggs or five adults daily (Denmark, 2009). Additionally, these predators disperse well among plants and in a similar way as the target two-spotted spider mite (McMurtry, 1982).

Carbon Dioxide

Another, less conventional method is to enrich the greenhouse environment with very high levels of CO2. Spider mites can be effectively controlled with CO2 when used properly and in conjunction with other methods as very high CO2 levels suffocate and poison mites.

However, like other methods, CO2 strategies can be foiled by hibernation. For instance, it can take nearly 5 days at 20% CO2 (200,000 PPMS) to suffocate hibernating spider mites (Whiting and van den Heuvel, 1995). However, at a temperature of 40C/104F it only took 15.5 hours to achieve the same results with the same amount of CO2 (a 99% kill rate). This may be due to the high temperatures forcing the spider mites our of hibernation, and also increasing their need for respiration. More information is needed on the exact temperature and CO2 amount needed to effectively suffocate spider mites.

NOTE: Growers should exercise extreme caution when utilizing CO2 generators. Please read all warning labels and understand the risks to you, those around you, and your plants.

A Spider Mite Free Garden

Killing off a spider mite colony requires one to to consider multiple methods that consider the pests’ life cycle and incubation rates. Stick to a schedule that targets new generations of spider mites… before they lay eggs!

Once the adults disappear, remember, your work is just beginning. Your focus will then switch to larva and eggs, continue to inspect for and note the developmental progress.

Staying on top of inspections (LOOK FOR LARVA with a lens! see above) and employing the preventative measures covered in this article will lead to a pest-free, worry-free, high-yielding garden.

Happy growing!

Mike S.
Colorado Cannabis School Instructor

Promoting Cannabis Education

REFERENCES CITED

  1. Cagle, L. R. 1949. Life history of the twospotted spider mite. Virginia Agr. Exp. Stn. Tech. Bull. 113:1-31.
  2. C. M. Feiertag-Koppen. 1976. Cytological studies of the two-spotted spider mite Tetranychus urticae Koch (Tetranychidae, trombidiformes). I: Meiosis in eggs., Genetica, Volume 46, Issue 4, pages 445-456, doi:10.1007/BF00128090
  3. Helle, W. 1967. Fertilization in the twospotted spider mite (Tetranychus urticae: Acari). Entomol. Exp. & Appl. 10:103-10.
  4. McMurtry, J.A. 1982. The use of phytoseiids for biological control: Progress and future prospects. In M.A. Hoy, [ed.] Recent advances in knowledge of-the Phytoseiidae. Agricultural Sciences Publications. University of California, Berkeley, CA. 92pp.
  5. Sabelis, M.W. and H.E. van der Bean. 1983. Location of distant spider mite colonies by phytoseiid predators: demonstration of specific kairomones emitted by Tetranychus urticae and Panonychus ulmi. Entomol. Exp. & Appl. 33:303-314.
  6. Thomas R. Fasulo & H. A. Denmark. 2009. “Twospotted spider mite”. Featured Creatures. University of Florida / Institute of Food and Agricultural Sciences.
  7. Whiting, D.C. And J. Can Den Heuvel. 1995. Oxygen, carbon dioxide, and temperature effects on mortality responses of Diapausing Tetranychus urticae (Acari: Tetranychidae). J. Econ. Entomol. 88: 331-336

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