by Alex Robles
Spider mites are one of the most common pests that cannabis growers are going to run into. If they’re left unchallenged and allowed to get comfortable they will be devastating to a crop. So instead of doing battle with these little bastards I suggest developing an Integrated Pest Management (IPM) program suited to your conditions. A good place to start is to have clean grow area, don’t give pests a reason to want to hang out. The next step is to know your enemy.
What are they?
- Tetranychus urticae Koch (Two spotted spider mite) is considered a general feeder, which means it attacks a wide variety of plants.
- It’s sometimes also known as the “red spider mite” and the “greenhouse spider mite”.
- This pest gets its name because it typically has at least two spots. One on each side of its body, just behind the eyes, while the top of the abdomen is free of spots. These spots are made by pigments in the digestive system, that’s why the number of spot and the pattern can be different between individual mites.
- Adults and nymphs have four pairs of legs and typically have a yellow or greenish color. They’ll also have hairs or spines on there body.
- The adult male is smaller than the female and has a pointed abdomen. It sometimes has an orange or brown color and is more active than the female.
- The adult female is about 1/60 of an inch long but will be bigger than the male and more oval shaped.
- The female is usually a pale leaf-green, but it can have a yellow, brown and orange tint. .
- After a single mating, females are fertilized for life and will lay her eggs on the underside of leaves at the rate of 2-6 a day. Each female may lay as many as 100 during her lifetime, about 75% will be hatch female to 25% male.
- The eggs are spherical, clear, and colorless when laid but will become pearly white by the time they hatch.
- The red eyespots of the embryo are visible just before they hatch.
- The young mites that hatch are larvae and they look exactly like the adults except they’re smaller and have only three pairs of legs
- Right after it hatches it’s kind of translucent (except the red eyespots), but after it starts feed on the plant juice, it’ll turn pale green and the black spots begin to form.
- Depending on the weather or indoor growing conditions, a complete generation of this pest could be completed in 7-20 days.
- With the right conditions generations will overlap and all stages can be found on most outdoor plants during the summer months. Hot dry weather and low humidity helps the development of severe pest infestations.
- The young mites shed their exoskeletons three times before becoming mature.
- When tempuratures drop outdoors the twospotted spider mite adult female will change to an orange color and overwinters in places like underneath bark scales or ground cover around the base of plants and trees (most other species of spider mites overwinter as eggs on the host plant).
- Only the females are known to overwinter. As the temperatures increase the females start feeding again and slowly lose their orange color and get their normal greenish color and the spots return.
- Overwintering females lay an average of 39 eggs over a lifespan of 23 days, alot less than their summer born sisters.
- These eggs could take up to 3 weeks to hatch, depending on temperature. After that, the generations start to overlap. Summer-form females can lay about 100 eggs over a period of 30 days.
- They love the warm dry summer months and thrive in artificial environments that are between 70 – 80 F with low humidiyt. A high mite populations often shows up with hot, dry weather.
- They like to hang out and lay their eggs on the underside of the cannabis leaf. Because of their small size and their love for feeding on the underside of leaves, it’s easy for them to go undetected until the little buggers have caused serious damage to a plant.
- Some of the plants that they love to live on are field crops, house plants, vegetables, small fruits, shade/fruit trees, shrubs, flowers, CANNABIS and some grasses, just to name a few.
- Mite and their eggs are often hard to see clearly without using magnification. But they’re not hard to find, just hold a white sheet of paper under a branch and sharply tapping the branch against the paper. If mites are there, they will show up on the paper as tiny, moving specks.
What does damage look like?
- Two Spotted Spider Mite damage looks like Whitefly damage because they feed in a similar way.
- Mites feed by inserting their mouth parts into leaf and sucking out the life juice, including the chlorophyll (that’s how they get the greenish color). The damage will first look like white spots, giving the leaves a stippled appearance. As they continue feeding, the affected leaves take on a brown hue, commonly called bronzing.
- Light infestations usually go unnoticed, it’s only with a heavy infestations that the leaves will look chlorotic (yellow), stippled, or mottled in appearance.
- Mite feeding substantially reduces bud production.
- Eventually the infested leaf will die and fall off.
- With closer examination an infested plant will have fine, silken threads over the foliage, twigs, and branches.
- These silken threads are produced by the mites as they move across the leaf surface. When a plant is severely infested they will usually have a fine cobweb appearance on the underside of the leaf and buds.
- The twospotted spider mite loves to producer of webbing.
How to fight them?
- Predatory mites are more effective than chemical control because the pest mites do not build up resistance to predatory mites and growers can reduce or eliminate the use of chemical pesticides.
- Predatory mites do not bother plants, people or animals.
- Because predatory mites do not give immediate control, they need to be released at the first signs of a pest mite problem. If there are no pest mites for food, predatory mites will not survive or reproduce.
- Other insects are also good predators, such as the six-spotted thrips, Feltiella acarisuga, and general predators, such as ladybugs and lacewings.
- The predator mite that I’m able to get from a local insectary is the Phytoseiulus Persimilis. This bug specializes in killing web-spinning mites like the carmine red, two spotted and Pacific mite. They’re most effective when the temperatures are between 59-86F and about 60% humidity.
- The Persimilis can reproduce pretty fast, which helps if your spider mite population is getting worse but you don’t have an infestation yet. Rincon-Vitova suggests treating sudden hot spots by releasing 1 mite per 100 spider mites for control within 1 week. Don’t forget once the food is gone they will eventually die of hunger.
- Bean Plants are a great tool to use to use against spider mites I’ve learned. They’re really good at attracting mites away from infested plants. It’s suggested to have single bean plants growing in small container or disposable cups. That way when the bean plant looks full of mites you can just remove it and replace it with a fresh one. Also, mite damage shows up within hours on bean plants, where it may take a day or two to show up on cannabis.
- Video of spider mite interactive study https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4211727/
“Since Spider Mites hate high humidity and if your not battling Botrytis or other molds. Try mistinting the affected areas on a regular schedule for a few week” Rincon-Vitova ‘Marijuana Bio-Control’
- The populations of pests could increase after you use certain insecticides because natural predators may be killed.
- Protect natural enemies as much as possible by choosing insecticides and miticides that are the least harmful to natural predators. All of the suggested chemicals below are broad spectrum killers, which means they don’t discriminate, they will kill or affect pests as well as predators.
- When treating for mites, choose the most targeted and specific miticide and alternate it with a miticide of a different chemistry or mode of action to avoid the development of resistance.
- Aza Max is good miticide to use. Apply it every 6-10 day to avoid any serious damage from pest.
- Neem Oil is good because it sprips the cuticle (outer wax layer) off the most insects, which doesn’t allow them to molt. Does nothing to the eggs so you will need to reapply every 6-10 days.
- Pyrethrum is made from some chrysanthemum flowers. They work by exciting the nervous system of insects that touch or eat it. This quickly leads to paralysis and ultimately their death. It’s often mixed with another chemical to increase the effectiveness.
If your growing in containers, you can smear TangleFoot on the rim to help keep crawling pests from migrating from one plant to another. –paraphrasing Jorge Cervantes-
I hope this helps you better understand what your up against if these mites show up in your garden. What you just read is a small amount of the information and research that is out their on the Two Spotted Spider Mite.
Grow Learn Teach
I used many different sources along with some personal knowledge to put this together. Below is a list of those sources.
- UC-IPM: https://www2.ipm.ucanr.edu/agriculture/strawberry/spider-mites/
- Washing State University – Elizabeth H. Beers and Stanley C. Hoyt (originally published 1993) http://jenny.tfrec.wsu.edu/opm/displaySpecies.php?pn=260
- Penn State Department of Entomology: https://ento.psu.edu/extension/factsheets/twospotted-spider-mite
- Pyrethrins fact sheet: http://npic.orst.edu/factsheets/pyrethrins.pdf
- Rincon-Vitova Insectary: https://www.rinconvitova.com/
- Jorge Cervantes “Marijuana Horticulture, The indoor/outdoor Medical Grower’s Bible”