How to Get Rid of Spider Mites
Updated: May 23, 2023
Spider mites are tiny arachnids that wreak havoc on cannabis plants. They live outdoors and find their way into our homes and gardens during the warmer months. In terms of economic impact, spider mites are the most destructive pest associated with cannabis cultivation.
Spider mites are most commonly found outdoors and are more common in dryer regions with temperate climates and high levels of sunlight.
In this article we're going to discuss:
Signs & symptoms of spider mite infestations
How to get rid of spider mites
Garden cleanliness & pest prevention
Table of Contents
What Kills Spider Mites?
Neem oil will kill spider mites reasonably well; and is all-natural. However, the drawback to using neem oil alone is that it is only toxic to spider mites after they eat it. Neem oil has zero effect on spider mite eggs, which adult female spider mites lay in massive quantities.
Spider mite eggs take 5-7 days to hatch. Therefore, neem must be applied several times in seven-day intervals to successfully mitigate an infestation
Avid (Abamectin) used in conjunction with Floramite (Bifenazate) will kill spider mites in every stage of growth, including eggs.
These products are also systemic, which means that they stay in the plant for 8-10 weeks and should NOT be used on flowering plants.
We also recommend dipping new clones in an avid & floramite bath before introducing them into your garden.
Spider Mite Taxonomy
Spider mites may consist of one to two species that are collectively known by six different names and are separated by differences in morphology and ecology. Let's take a closer look at the two that pose the biggest threat to cannabis.
Two-Spotted Spider Mites - Tetranychus Urticae - (Koch 1886)
The two-spotted spider mite thrives in environments cooler than 86 degrees Fahrenheit (30 degrees Celsius) with low humidity
Eggs are spherical, approximately 0.14mm in diameter, translucent to white in color, and turn light yellow just before hatching. Larvae have six legs and two tiny red eyes.
Adults have eight legs and are yellowish-green; often with two large spots on their back. In late fall, the two-spotted spider mite turns bright orange-red.
Two-spotted spider mites generally emerge in spring and feed on plants that are particularly high in chlorophyll.
Females lay eggs on the undersides of plants, or in webs that they spin between the leaf blades. Each female lays approximately 200 eggs in her lifetime.
Carmine Spider Mite - Tetranychus cinnabarinus - (Boisdural 1867)
Eggs and larvae are similar to two-spotted spider mites, however carmine spider mite adults are plum red or brick red with dark markings. Adults have also been known to turn green in cooler climates, making them difficult to distinguish from two-spotted spider mites.
The primary difference between carmine spider mites and two-spotted spider mites is geography.
Carmine spider mites prefer warmer temperatures, typically 95 degrees Fahrenheit (35 degrees Celsius) or above.
For this reason, carmine spider mites are typically found in semi tropical regions.
Differential Diagnosis
Early signs of aphids, fungal wilt, and nutrient deficiencies can easily be confused with spider mite infestation.
That's why it's important to make a positive identification with a 100x microscope before deciding on a course of action.
Early Signs of Spider Mites
Spider mites use their stylets to pierce the undersides of leaves and suck the juices. These puncture wounds appear as tiny brown or yellow spots accompanied by yellowing leaves.
As spider mite populations increase, you'll start to notice tiny webs being spun between leaf blades, starting at the base of the leaf where the gaps between blades are the smallest.
Adult spider mites are visible with the naked eye and can easily be identified by examining the undersides of leaves; often appearing as dark-colored dots.
How to Get Rid of Spider Mites
There are a number of different ways to control spider mites. The following control methods are presented in order from most recommended to least recommended.
Prevention
Cleanliness is the best defense against a spider mite infestation. Spider mites are infiltrate growrooms by attaching to people, pets, clothing, and other plants.
They can even float through the air.
Indoor gardeners should abide by all generally-accepted cleanliness practices, including:
Wearing decontaminated coveralls over clothing and covering hair when working around or near plants.
Dawning clean shoe covers when entering a growroom
Filtering any air that enters the grow space
Greenhouse growers are advised to maintain a three meter wide 'weed-free zone' around their greenhouse at all times.
All indoor and greenhouse growers should decontaminate walls, floors, ceilings, and equipment; with a lime-sulphur spray, fish oil soap, or solution of household bleach (sodium hypochlorite) diluted to 5%.
Biological Control
It's a good idea to practice biological control before spider mite populations explode. If spider mite populations are able to increase unchecked, the late addition of predators will be ineffective.
A mix of the following three biological controls provide excellent protection against spider mites in greenhouses:
Phytoseiulus persimilis
Neoseiulus californicus
Mesoseiulus longipes
Outdoor growers are advised to use a mix of these predatory mites:
Galendromus occidentalis
Galendromus pyri
Neoseiulus cucumeris
Other effective spider mite predators include:
Stethorus picipes (Spider mite destroyers)
Feltiella acarisuga (Midge maggots)
Chrysoperla carnea
Mirid bugs
Pirate bugs
Lygaeid bugs
Predatory thrips
Two fungi, Neozygites floridana & Hirsutella thompsonii, have also been shown to be effective against spider mites. However, each of these fungi requires high humidity.
Regardless of the method you choose, biological control must be achieved before the flowering cycle begins.
Chemical Control
Pesticides should only be used for "spot treatment" during heavy mite infestations. Typically, these types of infestations are more common among plants that are stressed, located near unprotected air vents, or along the windward edges of outdoor fields.
Pesticide should be applied to the underside of leaves and should be reapplied a week later to eliminate any newly-hatched larvae.
If you use pesticide to decontaminate your equipment, make sure to wash it off before introducing a new crop.
Carmine spider mites are particularly susceptible to neem oil.
Cinnamaldehyde kills spider mites at all stages of growth but is also harmful to beneficial mites and other helpful insects.
Pyrethrum works well against spider mites but synthetic pyrethroids (like permethrin) are ineffective at killing mites and can actually induce egg laying.
Imidacloprid and abamectin are both effective chemical controls against spider mites.
Spider Mites on Plants
Bibliography
J.M. McPartland, R.C. Clarke, and D.P. Watson (2000) 'Hemp Diseases and Pests', CABI Publishing, Chapter 4: Insects and Mites, pp. 25-30
J.C. Stitch (2008) 'Marijuana Garden Saver', Edited by Ed Rosenthal, Quick Trading Company, Chapter 2: Pests, pp. 70-73
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