There are over 1,200 species of spider mite in the world. The two-spotted spider mite, or Tetranychus urticae Koch, may be the most common pest in Cannabis production.
Two-spotted spider mites are easily identified by the two ‘spots’ on their back. These spots are actually food waste that is visible through the body of the mites. The mites use their delicate mouthparts to feed upon individual cells on the undersides of leaves, causing the classic stippling damage that indicates their presence.
The webbing commonly associated with spider mite infestations has many uses besides protection. The mites will sometimes use the silk as a ‘sail’ catching the wind to travel.
What to look for:
Leaf stippling and webbing
Spider mites thrive in high heat and low humidity
Life Cycle of Spider Mites
Two-spotted spider mites have 5 life stages. They go through a ‘resting’ phase after each molting.
Egg- Spider mite eggs are spherical in shape, creamy in color, and are often laid in groups.
Larvae- The larvae have only 6 legs, and are a light yellow color.
Nymphs- There are two nymphal stages: Proto- and deuto-. The nymphs also have 8 legs and are yellowish green in color.
Adult- Females are larger than males. They have 8 legs, though both sexes have the two ‘spots’ visible through their body wall.
• Twos-potted spider mites can develop between 54 and 104˚F.
• They thrive in hot, dry conditions.
• At the optimum temperatures, ranging between 86˚ to 90˚F, they can go from egg to adult in 6 days.
• The egg laying habits of females depends on both temperature and food availability, though on average a female spider mite will lay 10 eggs per day at 77˚F.
• Diapause in two-spotted spider mites is initiated by a combination of short day length (>13hrs), lack of food, and/or low temperatures. Adult mites undergoing diapause may appear orange or crimson in color. A fixed chilling period is necessary to terminate diapause.
I see spider mites, now what?
There are many forms of bio-controls for Two-spoted spider mites, but the most effective predators that we have seen in the presence of spider mite infestations are Persimilisand Californius. Since persimilis STRICTLY feeds on two-spotted spider mites, it is a great hot-spot, or curative, treatment. In some cases, 2-3 applications, approximately 5-7 days apart, will be necessary to get mites under control.
I don't see any, but I'd like to act preventatively....
While Persimilis is the best option for established infestations, there are a few other species that will feed on two-spotted spider mites and are packaged in sachets for preventative control. Sachets usually allow predators to continuously reproduce and emerge from the sachets onto the canopy for 3-5 weeks. Californicus and Andersoni are good options, though factors such as the presence, or potential presence, of other pests in the garden and environmental conditions, like temperature and humidity, will dictate the predator of choice. Please read the brief descriptions below and see what is going to be the best option for your operation.
* Thrips can feed on spider mite eggs, but we don’t recommend relying on them as a method of control!
Two-Spotted Spider Mite Predators
Phytoseiulus persimilis is a predatory mite that feeds only on two-spotted spider mite adults, nymphs, and eggs. Because spider mites are their only food source, Persimilis are best introduced when spider mites are first discovered, rather than preventatively. Once they exhaust their food source they will disperse or die of starvation. Persimilis do not go into diapause and are capable of complete eradication of spider mite populations if climate conditions are suitable.
Amblyseius californicus is a generalist predatory mite that primarily attacks spider mites, but will also feed on many other leaf inhabiting mites (even some microscopic species, like broad mites), other small insects and pollen. Californicus is tolerant of various temperatures and lower humidity, but works best under warm to hot conditions.
Amblyseius andersoni feeds on russet mites, broad mites and two spot spider mites. They will also feed on pollen and thrips larvae, allowing the population to survive when pest mite populations decrease. Active and effective in both high and low temperatures (43˚-104˚F), andersoni is an easy choice for preventative mite control in your commercial garden.
Stratiolaelaps scimitis(formerly H. Miles)
Stratiolaelaps scimitus (formerly called Hypoaspis miles) is a soil dwelling, generalist mite that feeds on fungus gnat larvae, pupating thrips, pathogenic nematodes, immature root aphids, and small insects in your substrate.
Preventative applications will protect against many of the most common pests that feed on your roots or have a life stage in the soil. They can also be released onto the floors and corners of room and greenhouses.