Medical marijuana is a specialized crop grown for human consumption; every effort should be made to grow the crop without the use of chemical insecticides, fungicides or herbicides.
In a modern greenhouse, these plants can be grown year ‘round, at a faster rate, and with less physical damage than crops that contend with weather and wind. However, because the greenhouse also excludes natural pest enemies, the same protective environment that nurtures plant growth tends to make it easier for the pests to become established and to get out of control.
Recognize this is a difficult indoor crop to grow as the plants have not been bred for protected climates. As a result, the plants are usually in some state of stress. Do what you can to minimize this stress by aggressively and proactively humidifying the environment, carefully watering the root systems and ensuring a consistent, and adequate nutrient balance.
Prior to planting
Before you plant, a thorough cleaning with detergent is your number one preventative action and highly recommended. Take care to remove all old plant material, obvious fungal residue and any non-essential apparatus in the greenhouse or growth chamber. As well, wash any previously used tools and clothing to ensure no cross-contamination between crops.
Stressed plants are much more susceptible to fungal pathogens and insect pests. The climate must be appropriate for the growing conditions. Due diligence is needed to ensure that air circulation is efficient and regulated; otherwise, the “humidity umbrella” that the plant normally forms by transpiration can be adversely affected. Pests such as the two-spotted spider mite will thrive on dry plants.
A good preventative strategy is to place a few pots of bush beans throughout the house or room before planting the crop. The beans will attract any pests that were missed in the initial clean up. At that point, a second detergent cleaning may be required.
When you have started planting and the pots are first watered, a soil mite such as Stratiolaelaps scimitus (Ss) or Gaeolaelaps gillespiei should be applied to the soil surface at a rate of 10 mites per square foot. They can be broadcast over the spaced out pots or trays. These soil mites will control fungus gnat larvae in the root zone, leading to a faster growth rate and healthier plants. Both mites also feed on pupating thrip larvae, helping thrip management by breaking the reproductive cycle. If, despite reasonable prevention, the crop ends up with a spider mite problem, apply more Ss to the floor area, focusing on cracks and any other breaks in the floor where the spider mites overwinter. The Ss will feed on the dormant spider mites, reducing their return numbers significantly.
Yellow sticky traps should be applied at least 1 trap for every 500 square feet. Care must be taken to ensure that the height of the traps does not exceed the height of the plants. Traps higher than the plants will not trap a representative sample of thrips.
In a dry environment, spider mites are the most common and serious pest. Spider mites hate high humidity. (We actually manage our spider mite culture on bean plants just by misting them). If you are not battling botrytis or similar molds, try misting the affected areas on a regular schedule for a few days.
Prevention of spider mites is possible by applying Amblyseius fallacis at a rate of 2 mites per square foot. This generalist mite predator evenly establishes itself throughout the crop, preventing spider mite establishment under normal conditions. Hot spot outbreaks should be treated with Phytoseiulus persimilis at a rate of 1 mite per 100 spider mites for control within 1 week. Fallacis and persimilis are compatible and do not interfere with each other. Fallacis does not do well on spider mite webbing while persimilis thrives on it.
Stethorus punctillum is a tiny black beetle that also thrives in low humidity situations. If you are unable to manage the climate effectively and the conditions for spider mite are extreme, Stethorus could save the crop. They should be applied at a rate of 0.1 per square foot in extreme cases, or in moderate cases, at a rate of 0.01 per foot. These beetles find spider mites by smell and quickly move to new infestations, leaving behind their eggs and larvae to finish the job.
Another strategy to control spider mites is to plant bush bean plants amongst the crop. In some crops, such as tomato, this is a very effective strategy for pulling spider mites off the crop. Bean plants are also easy to monitor, as they show spider mite damage within hours. If the beans are moderately successful, apply persimilis to them to create a “banking” system, generating more persimilis that will move into the crop. If the beans are highly successful at attracting the spider mites, carefully remove them along with the spider mites and plant some more beans.
Aphids are attracted to soft new plant growth. Aphids themselves seldom cause permanent damage, but their excrement “honeydew” can lead to sooty mold. Honeydew in turn, attracts ants. If you have aphids, you must eliminate any ants first. Ants “farm” aphids, so they can feed on the honeydew. They will protect the aphids from predation and sometimes actively move aphids around to “greener pastures” within your crop.
Once the ants are gone, aphids can easily be controlled by using Aphidoletes aphidimyza at a rate of 0.01 per square foot, weekly, until the aphids are eliminated. If you have a history of aphids, continue at this rate weekly for the duration of the crop.
Thrips cause damage that is similar to spider mites. Thrips tend to scrape the leaf surface while spider mites pierce the leaf tissue and extract the chlorophyll. Thrips are tiny pests, capable of limited flight. They are attracted to yellow and blue sticky traps, which when used in large enough numbers, can become effective management tools. You can increase the trapping rate by a factor of 10 by attaching a cotton ball to the sticky trap and adding a few drops of vanilla or almond extract to the cotton ball.
There are only a few available biological controls to combat thrips. The predatory mite Amblyseius cucumeris is the best choice. These mites attack the first and second instar larvae. If enough cucumeris are present they are extremely effective, Cucumeris sense the thrip emerging from the leaf, wait for the thrip to stick its head out, and bite it off. Stratos in the soil will ensure that the thrips cannot effectively cycle within your facility by eating the pupae before they emerge.
Whitefly is not a common pest in marijuana crops but because pests tend to adapt, it is important to be on the watch for them. Whitefly is a close relative to the aphid; both of them suck. Both pests rapidly create excessive honeydew that leads to sooty mold. If any whitefly is seen on the yellow sticky cards, begin applying the parasitic wasp Encarsia formosa at a rate of 0.02 per square foot, weekly.
• Keep your plants in humid, healthy environments to minimize stress
• Always start your plants with an application of soil mites
• Spider mite can be prevented with the predatory mite fallacis
• Localized spider mite outbreaks can be treated with persimilis
• Severe spider mite outbreaks in poor climates can use the beetle, Stethorus
• Bush beans can be used to monitor and trap damaging insects
• Thrips can be controlled with cucumeris and sticky traps that have been treated with vanilla or almond extract.