There is no doubt that the cannabis industry is a growing one. Man has cultivated this crop for many years, yet only in the last few has commercial production come into the spotlight. It seems that with all our years of experience, humans would know how to grow this crop without issues, but going from growing a few plants to commercial production changes things. One issue that many growers are finding is that cannabis has many pest problems. Root aphids, foliar aphids, thrips, and fungus gnats are just a few in a growing list. The most dreaded problems are the pest mites. Growing operations are regularly losing crops to these pests, and understanding them is the key to controlling the problem.
Proper Identification Is Critical to Successful Control
Saying you have mites is a broad statement. There are many kinds of mites in commercial production, and cannabis is susceptible to a few different species. It is important to identify correctly which mite you have so you can use the right control option. You cannot guess; you must have 100% positive identification. Your pest consultant can help in making the identification if you are not sure.
For control, many growers are opting for the use of biological control agents. Due to the concerns about pesticide residues on a consumable crop, state regulations, and resistance management issues, the biocontrol option is a perfect fit. The key is to start early and with quality products.
Mites that are commonly found in cannabis crops can be broken down into three families: Tetranychidae, which are the spider mites, Tarsonemidae, the thread-footed mites, and Eriophyidae, the gall mites. This list may expand over time, as there are new host records.
The Twospotted Spider Mites
When someone talks about spider mites, they most often are referring to the twospotted spider mite (Tetranychus urticae). Keep in mind that spider mites are a broad family of mites. There are many spider mite species, but only one is the twospotted spider mite. This is what is commonly found in cannabis. Twospotted spider mites are also found in many other ornamental and vegetable crops, making this pest challenging to manage because it is ubiquitous.
The adult females are around 0.4 mm long, with males slightly smaller. Often, they are recognized by the webbing they spin on the leaf surfaces. In this web, the females will deposit their eggs (up to several hundred), which are perfectly round.
These mites thrive in hot, dry conditions commonly found in greenhouses. It seems like populations explode overnight, but often they have been there building, unnoticed all along. When living on the leaves, twospotted spider mites feed by inserting their mouthparts into the plant cells and feeding on the contents. If they are controlled early, the plant can recover potentially without damage to the foliage. If plants go untreated, leaves can become yellow and develop necrotic spots. The mites can also move into the flowers, becoming an issue while the plants are drying at harvest.
Broad Mites (Polyphagotarsonemus latus)
Broad mites are another mite with a wide range of host plants and worldwide distribution. They are a much smaller mite than the twospotted spider mite (at least 20x magnification is needed to see them). The adult females are 0.2 mm long while the males are slightly smaller. The easiest way to identify them is from their eggs. The eggs are elliptical with white tufts on them. They almost appear to have white dots on them.
Broad mites’ presence can be hard to detect until the damage occurs. That is often how growers find out they have them. The mites have a toxic salve that causes new leaves to grow in a distorted and thickened manner. Even if treated, these leaves will not recover from this damage. New leaves that emerge (that are free of mites) will be normal.
Hemp Russet Mites (Aculops cannabicola)
This mite has been a challenge for growers in 2017. It has spread like wildfire due to production practices and poor sanitation. This mite is different from the previous two mites in that it is host-specific to cannabis. There has been some confusion thinking it is the same russet mite species found in tomato crops, but that is a different mite (Tomato russet mite, Aculops lycopersici).
This mite is very small, and you need high magnification to see it. The small size makes it easy to hitch rides completely unnoticed on growers’ clothing and tools. Most growers do not know they have it until they see the damage, and by then the mites are at very high levels. When the mites feed on the crop, it causes bronzing, leaf curl, and in some cases, blistering. Once there is a severe infestation, it is hard to get this pest cleaned up.
Again, this is why starting a control program early to prevent a buildup is essential.
Prevention Starts With Good Sanitation Practices
One thing each of these mites has in common is that you can greatly reduce your chances of getting them by using sound sanitation practices. Just a few simple, low-cost steps might stop an outbreak. Treat your growing areas as you would a hospital operating room.
• Limit visitors and staff: If someone (including you) has been at another growing operation, do not let them into your production area without clean coveralls or a change of clothing. Even then, it is best not to let anyone in unless it is his or her first stop of the day. When you brush up against infested plants, you may pick mites up on your clothing. If you then rub that clothing up on other plants, you can transmit pests.
• Tools: Regularly clean tools with disinfectant when moving between plants and crop areas.
• Clones or cuttings: This is how many operations are unknowingly infecting themselves. Pests are arriving directly on the introduced plant material. When cuttings are brought into an operation, there should be a standard operating procedure on how to treat them to ensure a clean start. Remember, you most likely cannot see the problem with the naked eye at this stage. A dip in horticultural oil or insecticidal soap can greatly reduce the risk of bringing in new mite pests. When these cuttings are stuck, do not have them in your main growing area with your other crops. Keep them isolated to make sure no pests were missed in the dip.
• Pet Plants: Do not be tempted to use the grow facility to overwinter staff houseplants or other pet plants. Many cross-host pests would be happy to jump over to your crops.
• Start now, don’t wait: Once cuttings are stuck, start them on a predatory mite program immediately (Table 1.). Even ornamental growers, whose individual plant value is lower than cannabis, have started this practice of keeping the crops clean from the start. Do not wait until you have a problem.
Some states have provided approved lists of pesticides that can be used in cannabis production. Many of these products are considered Minimum Risk Pesticide Products. This means they are exempt from the Federal Insecticide, Fungicide, and Rodenticide Act requirements. These products have not gone through the rigorous testing that EPA-registered products have.
Most of the time with mites, horticultural oils can provide excellent control, but spray coverage is essential. If mites are missed, their numbers can ramp up quickly. Also, once most oils dry, beneficials can be released.
Biological Control Options
Early proactive treatment is essential, especially when using biological control agents. As a cannabis crop matures, trichomes will develop. Once this happens, the plant will become too sticky for the predators to move easily around on the plant. Treat before this happens, when the beneficials can freely move around.