Bug Off!

Controlling common pests part one: Indoors

By: Paul Josephs

 

One aspect of Cannabis cultivation that all growers can empathize with is the sinking feeling that comes with finding unwelcome critters munching on their plants. Some pests are just slightly damaging annoyances, others can kill a plant in fairly short order if not eradicated or controlled.  Thankfully, there are a variety of ways to deal with these malicious intruders that are harmful to them but not to people if performed responsibly.

 

No legal guidelines

What makes this topic tricky is the complete lack of regulation regarding Cannabis cultivation and pesticide use. Since Cannabis products containing THC are illegal under federal law, they have no legal classification as a food, herb, or spice as far as pesticide applications are concerned. Because Cannabis is consumed in a variety of methods by humans, it is realistic to assert that any pesticide not labeled as approved for use on herbs or spices should not be used on Cannabis. It seems reasonable to include pesticides approved for vegetables or fruits, but keep in mind that Cannabis is not easily washed or rinsed prior to use. It is extremely important to read all product labels and make sure the product is approved for food crops, and also to follow directions for application and safety precautions. Products labeled for ornamental plants and not for vegetables, herbs and spices are deemed not safe for human consumption.

 

The bad guys

Spider mites, fungus gnats, thrips, whiteflies, mealybugs, aphids and root aphids  are the worst offenders for most indoor growers.  Let’s examine them one by one and see how they can be managed.

 

Number one offender – spider mites

Spider mites have four pairs of legs like their spider relatives. Spider mites are extremely small and feed on plants by piercing plant tissues and consuming their sap. They have a very rapid reproductive cycle which encourages pesticide resistance and they have been shown to effectively suppress natural plant defenses when they feed. Therefore, they are a formidable pest that cannot be ignored if detected. Spider mites will be evident at first by small white spots that appear on leaf surfaces as a result of the mites feeding. Spider mites prefer the undersides of leaves and depending on the species, may or may not be seen by an unaided eye. Some require up to a 14X magnifier to see – russet mites are an example of the extremely small ones. Large infestations will turn the leaves a sickly grey or reddish color and very small ‘webs’ may be seen across the undersides of leaves. Severely affected leaves will die and fall off the plant. A plant with an infestation this large will produce very little and may even die.

 

Controls for spider mites

Prevention: The very best way to deal with spider mites is avoiding their introduction to the garden in the first place. Cover air intakes to the outside with filter material, some growers use nylon stockings. Remember, mite species are really tiny and they don’t need wings to travel through the air! If you are introducing new cuttings, quarantine them if possible for 10 to 14 days and watch them closely before putting them with the rest of the plants. Be aware that spider mites can also be carried in from outside on clothes and pet hair.

 

Physical removal: If caught early enough, wiping leaf surfaces with a soft damp cloth – especially the undersides – will remove and crush adults and eggs. This becomes less practical as garden sizes increase.

 

Insecticidal soaps: These EPA regulated products are somewhat effective; Safer’s Soap is just one example available on the market today. Frequent applications as directed by the label and physical wiping can knock down populations to manageable levels.

 

Pyrethrum: Some pyrethrum products are registered for use on vegetables and herbs, but not all are suitable for indoor use. Pyrethrum is toxic to fish and other aquatic life and cannot be allowed to enter storm drains or bodies of water. Make sure other pesticides are not also in pyrethrum products. Read and follow all directions and warnings.

 

Oils: Neem oil can suffocate mites. Neem tree extracts containing azadirachtin are effective against mites, but they must be used as labeled, these are pesticides regulated by the EPA. Note: azadirachtin is not found in neem oil, it is an extract concentrate from the neem tree.

 

Light paraffin oils can also be used to smother and kill mites somewhat effectively, depending on how thoroughly they are applied.

 

Essential oils: There have been claims made about emulsified essential oils of rosemary, thyme, cinnamon, garlic and other aromatic botanical products being effective against spider mites. There are many homemade remedies to be found online. Any oil product, including neem oil, should be tested on a small area first to ensure there is no adverse effect on the plants. Some oils may harm plants in very bright and warm conditions.

 

Predators: Using good bugs to eat bad bugs is a very environmentally friendly and safe insect control method. There are some predatory mites and insects that love to eat spider mites and other plant eating bugs. Ladybugs will eat mites voraciously, especially the Stethorus species. Another very effective mite killer is the lacewing, Chrysopa carnea. The larvae of lacewings actively search out and eat spider mites. Predator mites work well against spider mites, but not all of them do well in the warm low-humidity environment of a typical grow room. Two predatory mite species that do well in lower humidity conditions are mesoseiulus longipes and Neoseiulus californicus. A third species, Galendromus occidentalis is borderline for lower humidity environments.  Other predatory insects that eat spider mites are Orius species which are minute pirate bugs – but be warned, these can bite humans sometimes – and Geocoris species, the big-eyed bugs. Predators can be purchased from several internet sources, and much information about them can be found there as well.

 

Fungus Gnats

These little eighth of an inch flies are a nuisance when flying around like they own the place, but it’s their larvae that are the bad news for Cannabis plants. The larvae of the fungus gnats will burrow into roots and even stems, slowing nutrient uptake and plant growth considerably if they are present in large numbers. A great way to get a sense of how many larvae exist is to put quarter-inch slices of potato on the surface of the soil. The larvae are attracted to the slices and when turned over you can see the tiny clear larvae wriggling around. The adult flies are attracted to yellow sticky traps available at most garden centers and grower supply stores. Fungus gnat larvae can be controlled with azadirachtin drenches applied to the growing medium. Nematodes found in Nemashield will kill the larvae. Some growers have reported good success with placing mosquito dunks containing Bacillus thuringiensis (israelensis strain) in the water being used to water their plants.

 

Thrips

These tiny one millimeter insects look like really small dark or light brown living pencil leads with even tinier wings. They scrape the surface of leaf cells off and consume the tissue below. At first appearance they cause damage similar to spider mites. The irregular scraped, silvery areas on leaves and lack of any webbing distinguish the damage they cause from spider mites. They also leave very tiny black feces behind. Predators that attack thrips are minute pirate bugs and big-eyed bugs. Thrips can also be controlled with insecticidal soaps, horticultural oils, neem oil and pyrethrum. There is also a product called Botanigard that has fungus spores that will infect and kill thrips and other insect pests, there is also a product that is OMRI approved called Mycotrol O that contains the same spores. OMRI stands for the Organic Materials Research Institute; they approve products used in certified organic growing operations.

 

Whiteflies

Whiteflies are another very small sixteenth-inch pest that damages Cannabis by sucking fluids from the undersides of leaves. They are, as their name implies, white flying insects that will fly around briefly when plants are moved or shaken. In addition to the dehydrating effect of sucking the fluids from leaves, they are known to transmit plant viruses. Affected leaves turn yellow and can fall off. Plants will be stunted and yields are reduced. Whiteflies can be controlled like thrips by applying horticultural oil sprays, insecticidal soaps, and pyrethrum. A good predator for whiteflies is Encarsia formosa. Lacewings, minute pirate bugs and big-eyed bugs are also effective predatory insects for whiteflies.

 

Mealybugs

These are not that common in Cannabis crops, but infestations do occur and are harmful. Mealybugs are related to scale insects and the females can be up to a quarter of an inch long. They produce a waxy white coating and are usually found on the undersides of leaves. They damage plants by sucking fluids from leaves, and they produce a sweet honeydew that can attract sooty molds. Affected leaves yellow and drop off the plant. Mealybugs are not very mobile and can be physically removed by wiping off the undersides of leaves with moistened cotton swabs or Q-tips. They can be controlled by insecticidal soaps which can penetrate their protective waxy coating. Pyrethrum is also effective. Predators include lacewings and the Mealybug Destroyer ladybug, Cryptolaemus montrouzieri.

 

Aphids

There are two types of aphids to be concerned about with Cannabis, the regular ones that affect the leaves and stems and root aphids. Regular aphids are tiny soft-bodied insects that feed on plant fluids. They can be controlled with insecticidal soaps, pyrethrum and azadirachtin products. As always, read the instructions carefully. Predators that will devour aphids are ladybugs and lacewings.

 

Root aphids

These insidious pests are the up-and-coming number one offender for Cannabis crop devastation. This tiny aphid relative known as Phylloxera attacks the roots. Long known as a debilitating vineyard grape pest, this insect laid waste to many French vineyards in the 1860’s. Though they have a winged form very similar to fungus gnats, that form is not always present or is confused with fungus gnats. Phylloxera has been known in California for some time but has in recent years become an increasing problem in Colorado.

 

Root aphids live in the root zone and feed on the sap from the roots. They also inject a toxin into the root which decays the root interior. Growers may think they have a nutrient lockout or major deficiency as the plant leaves rapidly turn yellow from the bottom up and the plant declines rapidly. Phylloxera look like very small aphids and can sometimes be seen washing out of the bottom of pots or if the pots are flooded, they will climb up the stem to escape. Sometimes digging around or stirring up the top few inches of the soil will reveal them moving around, sometimes in alarmingly large numbers. In hydroponics they may be seen on the outside of net pots or on the walls of reservoirs.

 

Control of root aphids can be difficult. The best options are Botanigard WP (wettable powder) as a root drench, or introducing beneficial parasitic nematodes is also effective. NemaSeek has been reported to be the most effective nematode product by some growers. Sadly, many growers have had to resort to a scorched earth approach and cleared out the room, sterilized everything with dilute bleach solutions, and started all over again. Preventing these nasty critters from getting into the garden is by far the best approach. They can come in on cuttings – if at all possible inspect new cuttings very carefully and quarantine them before bringing them into the main growing area.

 

An ounce of prevention

As is the case with many cultivation problems, prevention is much preferred to reaction. When pests do show up, they are best dealt with rapidly before the problem grows into a much bigger one. Whenever possible use the most natural methods available. Taking shortcuts and using products not approved for, or in a manner not specified for spices, herbs, or food crops is illegal and could harm or sicken people.

 

Next month

Now that the outdoors is warming up nicely, June will be a great time to look at the unique pest management challenges faced by outdoor and greenhouse growers.

 

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