(This is the first in a series focused on common pests that affect marijuana and hemp growers.)
Face it, cannabis is an attractive plant for many insects, mites and other arthropods that like to feed on it.
One species of mite that commonly feeds on cannabis is the two-spotted spider mite (Tetranychus urticae), which often is shortened to TSSM.
This pest is found worldwide and is very common on a wide range of crops.
Why are TSSMs such a problem?
Two-spotted spider mites are everywhere.
Considered a cosmopolitan pest, TSSMs have been reported feeding on 3,877 different plant species.
This means they can be living right outside your facility or blow in on the wind from neighboring plant material.
More commonly, they are brought into a grow on young plant material such as clones.
All it takes is one female. Unmated females can lay eggs that will hatch male offspring. Once the offspring are sexually mature, they can mate with their mother.
Once she is mated, she can produce female offspring, and the population just explodes from there.
This is why any new plants brought into a facility should be treated as if they already have two-spotted spider mites on them.
How do you know your plants have TSSMs?
TSSMs are in the spider mite family (Tetranychidae) and are known for their ability to spin webs.
One of the ways to identify this mite is by its distinctive “two spots,” which can be seen on its back – making it one of the easier mites to identify.
They can be tan, green or red in color. The eggs are clear, round and 0.1 millimeters in diameter.
These eggs are often laid in groups on the undersides of leaves or in their webbing.
When they first hatch, the young mites have only six legs; on their next molt, however, they will have the recognizable eight legs.
Adult females (0.4 millimeters) are larger and rounder than males. Mites do not have antennae or wings at any point in their life cycle. They do not fly.
Often, growers see the feeding damage before they see the actual mites.
TSSMs feed by consuming the contents of the epidermal cells, removing the chlorophyll and causing the leaf surface to appear as if it has small white spots (often called stippling).
Once this damage has occurred, the leaves will not turn green again.
As the population grows, the mites will start webbing the leaf surface.
In this matrix, the mites will lay their eggs and be protected from the environment.
Eventually, the webbing can cover plants. If you see this, you know you have a very high population of mites, and action should have been taken a long time ago.
So how do you find them before they become an issue? The answer is a solid scouting program.
Hiring someone who is trained to find pests to scout your facility regularly is essential, before insects such as TSSM become an economic problem.
A scout is the foundation of any integrated pest management (IPM) program.
A good scout:
- Knows how to flip suspect leaves and check them with a hand lens.
- Will carry a white or black plastic board to bang plant foliage over, causing the mites to fall off the plants so they then can easily be seen.
- Will fill out a report listing mite populations, locations, the life stages of the mites observed and whether there are beneficial insects present.
- Keeps track of which cultivars the mites prefer.
With this information, you can make an educated decision on next steps in your IPM program.
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How do you treat for the TSSM?
- Prevent them from coming in by adopting good sanitation practices for plants and employees.
- Start a preventive program in propagation with predatory mites.
- Scout your crop well and create detailed reports to help make educated decisions.
- Spot-treat as needed with biocompatible products to get numbers down before releasing beneficial insects.
- Select quality beneficial insects. Buy directly from insectaries when possible.
- If you are unsure, ask a qualified person. Do not guess.
Historically, conventional pesticides were used to manage this pest.
As these mites developed resistance to traditional chemistries, many ornamental and vegetable growers turned to using predatory mites for management.
Vegetable and ornamental growers have decades of experience using these beneficial insects. Even though those crops are different than cannabis, there are a lot of similarities.
Cannabis growers can learn a lot from those markets.
First, there are no cookie-cutter programs that work in all facilities.
When putting together a pest-management program, there are many facility-specific variables that need to be looked at, including:
- Which life stage the plants are in.
- What has been sprayed on the plants.
- Environmental conditions.
- Pest populations and other pests.
- The presence or lack of beneficials.
Once these have been determined, you can begin making decisions on how to start your program.
Most often, the predatory mite Phytoseiulus persimilis is used to control the TSSM.
This is a mite that is very focused on feeding on all life stages of the TSSM and does not get distracted by other food sources.
P. persimilis females are approximately 0.5 millimeters long and red to orange in color with a pear-shaped body.
They have long legs and can run fast to take down the two-spotted spider mite. They are excellent hunters.
Their eggs are easy to find because they are shaped more like chicken eggs, compared to the TSSM’s round eggs.
There are other options for controlling the TSSM, such as Neoseiulus californicus and Neoseiulus fallacis.
These two mites have their own advantages, which your insectary can explain as you try to decide which is the right choice for your facility.
Other predatory mites can feed on the two-spotted spider mite. But two-spotted spider mites are not the main focus of their diets, so they might not provide the fast control that the P. persimilis does.
If you already have an established population of TSSMs, you will need to do a knockdown spray before releasing predators.
Horticultural oils have proven to be very effective at controlling all life stages of spider mites, and you do not have to worry about building resistance to oils.
Sometimes there are limitations for the use of oils due to state regulations, crop stages or environmental factors.
Insecticidal soaps are another option, however they do not work as well as oils on mites.
For TSSM control, you always want to stay ahead of them.
A good grower will want to start a preventative program early in propagation.
This is very cost-effective and can head off a problem even before they can get started in your facility.
Continue with a solid scouting program, preventive release of predatory mites and target sprays, and you will be able to keep ahead of this pest.
Suzanne Wainwright-Evans is an entomologist who specializes in biological control and pesticides. She is the owner of Buglady Consulting, which has provided independent consulting and training to the green industry for 21 years.
Wainwright-Evans can be reached at firstname.lastname@example.org.