What is a Whitefly?
The most common cannabis pests are those with the physiological capability to bypass plant trichomes with piercing-sucking mouthparts. Such insects include aphids, leafhoppers, and whiteflies . Whiteflies are annoying little pests that are related to aphids (in the order Hemiptera). They feed on the sap of plants, which is a sugar-rich solution transported in the plant phloem.
What Species of Whitefly affects Cannabis?
The most common whiteflies found in indoor grows in California have a wide host range and are mostly classified into a few species: Bemisia tabaci (sweetpotato whitefly), aleurodes vaporariorum (greenhouse whitefly), and silverleaf whitefly (Bemisia argentifolii) . There are many more species of whitefly that may potentially affect Cannabis. However, control methods are largely the same between species.
What is the life cycle of a Whitefly?
Whitefly life cycle
Illustration adapted from M. L. Flint. July 1995. Whiteflies in California: a Resource for Coop. Ext. UC IPM Publ. 19.
The length of time for a whitefly to mature differs between species (32 days from egg to adult for greenhouse whiteflies, 39 days for silverleaf whiteflies ). After hatching, whiteflies must progress through four nymph stages and a pupal stage before becoming adult whiteflies. An adult whitefly can live between one and two months, though different species have different lifespans. Whiteflies tend to lay eggs and spend most of their early development underneath leaves. However, nymphs and adult whiteflies can often be seen in high concentrations on top of Cannabis leaves as well. Whiteflies reproduce most prolifically in warm weather, though I have also had a whitefly outbreak occur in a grow tent that was in the garage during winter. It got quite cold during the nights (50 Farenheit), which for the record, is not ideal for plants either.
What Danger do Whiteflies Pose to my Plants?
Whiteflies can cause direct mechanical damage to your plants. They feed by piercing plant tissue and in high populations, they can cause yellowing (chlorosis) of leaves and even leaf death . Generally damage caused by feeding alone is mild unless populations are very high.
They also cause indirect damage. As whiteflies feed, they exrete excess sugar-rich liquid onto plant leaves known as honeydew. Honeydew can attract other insects such as ants, and can be a substrate for sooty black mold to grow on leaves. Sooty mold and honedew can negatively impact photosynthesis. Furthermore, whiteflies are known to transmit many plant viruses. Though viral Cannabis diseases have not been heavily researched, a 1971 study found that a majority of screened common plant viruses can infect Cannabis . Greenhouse whiteflies have been reported as some of the worst vectors for viruses that can infect Cannabis . Please see my post on Cannabis viruses under pest and disease profiles to learn about what viruses may be transmitted by whiteflies.
What Can I do to Prevent and Treat Whitefly Infestations?
As mentioned, whiteflies do best in warm, humid weather. An indoor grow tent provides perfect conditions for whiteflies to reproduce. Keeping your grow area cool is one way to help, but Cannabis grows best at warm temperatures as well, so you will be stressing your plants and inhibiting growth by doing this. My rule of thumb is to keep your grow area in the low 70s (Farenheit). Humidity should be controlled not only to inhibit the proliferation of insect pests, but also microbial pathogens. My personal preference is to keep the relative humidity of my growing space around 40%, certainly no higher than 50% (for mature plants only, seedlings may require high humidity).
Cultural methods: Keep your grow area clean. Do not bring plants into your grow area that have been kept outside. Sterilize all tools used in the grow area with 70% isopropanol or ethanol. Be sure to clean the inside of your grow tent before introducing new plants, remove all plant debris from the area.
Physical control methods: I like to use reflective mulches in all my grows. If you are doing indoor container gardening, a simple method is to make a carboard cutout that sits inside of your pot, on top of the soil. Make sure to cut a hole in the center for the plant stem, and cut a small strip from the outside of the collar to the center so that you can slide it around the plant stem. Finally, wrap the collar in aluminum foil and slide it around your plant stem. Reflective mulches can repel invading insects and provide more light to the underside of plant leaves which is also beneficial to plant photosynthesis. Furthermore, it helps control other insect pests that utilize the soil to complete their life cycle, and helps maintain soil moisture and temperatures. Collars should be removed for watering and feeding of plants.
Chemical prevention methods: Neem oil is the hydrophobic extract of the seeds of the neem tree . It is typically bought as an extract of 70% neem oil. I advocate using neem oil sprays even before encountering any insect or fungal issues with your plant. I recommend spraying your plants once every two weeks up until flowering is visible. I do not use neem oil after flowering begins because it can affect the taste and the smell of your buds if it gets on flowers. Furthermore, neem oil can cause allergies in some, and has been shown to be a potential toxin to humans when consumed . I would recommend using neem oil only on healthy vegetative plants.
To spray plants, you will need a one-hand pressure sprayer. I like to use 2 Tbsp of neem oil per gallon of water and I add a couple of drops of dish soap to help emulsify and spread the neem oil. Always test one leaf of a new cultivar/strain you are growing before spraying the whole plant by soaking a single leaf in your prepared neem oil solution. Check in a 24-48 hours for any adverse reactions. Spray your plants at night time only, and be sure that there are fans on your foliage to ensure that the solution dries before your lights switch back on. It is important that the solution dries because lights can easily burn your leaves if they are still wet, especially with a neem oil in it. I also like to rinse my leaves 3 days after the last spray. First off, it helps remove extra fats and oils that are just sitting on your leaves, and secondly, it can help rinse off any potential insect eggs. It is a good idea to use a spray bottle that you only use water in to do this. Do not rinse your leaves frequently as this increases risks of foliar diseases developing. Only do this every other week, three days after your neem oil application.
Another good leaf rinse besides water is a citric-acid based rinse such as Nuke Em by Flying Skull. Doing a weekly or biweekly leaf rinse with such a product can help prevent insect outbreaks as well as powdery mildew outbreaks. I cannot recommend this enough. You could even use this rinse instead of the water rinse after your neem application.
Because indoor grow tents tend to be isolated, natural predators of pests are not present to help control pest populations. Releasing lacewing larvae and/or ladybugs in your grow tent can be a good idea soon after planting to help reduce any potential pest populations before they get out of control. You must be aware that you may need to alter your spray program if you are introducing beneficials as many pesticides will also kill your beneficial insects.
The most important thing after prevention is monitoring. If you have taken proper preventative measures and you still notice a growing population of whiteflies in your grow area, it is time to be more aggressive. At first sight of whiteflies, prune and remove infested leaves from your growing area.
Chemical Control Methods
Insecticidal Soap 
At this point, it is a good idea to spray down your plants with insecticidal soap. However, if your plant is in flower, do not spray your buds with insecticidal soap. Though safe, the soaps may affect the taste of your buds. Insecticidal soaps will also kill beneficial insects, so it is not recommended to use insecticidal soaps if you are utilizing beneficials. Always test the sprays on a small portion of the plant before spraying the entire plant.
Neem Products 
If an infestation is in progress, I personally like to swap out neem oil for an azadirachtin product such as Azamax. If you continue to use neem oil, begin applying on a weekly basis as opposed to a biweekly basis. Azadirachtin is the most active chemical found in neem oil, and I find it to be more effective than just neem oil. Azadirachtin is OMRI certified. It can become systemic in the plant and has low toxicity. However, there have been negative effects reported from neem products in some people, and so I would recommend not to use any neem products on your plants during flower, and certainly not during the last 4 weeks before harvest. If you are growing cannabis to sell it or give to others, I also do not recommend using neem products as some people may have allergies. I have found this pesticide to be very reliable in getting rid of whitefly infestations (as well as certain spider mites). This product will also kill most whitefly predators, with the possible exception of ladybugs, so it is not recommended to use this product along with beneficials.
Citric Acid Products 
My chemical of choice would have to be a citric acid based pesticide such as Nuke Em by Flying Skull. Citric acid pesticides are OMRI certified and very effective at killing whiteflies. They are nontoxic, and can be sprayed directly on buds without worry of altering the flavor or smoke of your bud. In my personal experience, this should be your go-to chemical product if you are worried about using neem products at all. I would recommend reapplication every 5 days until the infestation clears up. It would be useful to have a one-hand pressure sprayer for application.
Pyrethrin Insecticides 
Pyrethrin insecticides can be quite effective, but I would not recommend using them at any point during the flowering cycle of your plant as they are toxic to humans as well as insects. I would recommend to stick to neem and citric acid products.
Spinosad Insecticides 
Spinosad is composed of two bacterial-derived chemicals that are toxic to a wide range of insects. It can be quite toxic to beneficial insects as well as bees, and so I recommend spraying at night and only utilizing it if necessary (especially if you are venting to outside). It should not negatively affect beneficials if they are released after the spinosad completely dries. However, it is nontoxic to humans, the label claims that it can be used safely up to the day before harvest (though I would not spray this on flowering plants as according to the UC IPM website for whiteflies ) and can be useful in a microgrowery IPM program if you are not using beneficials. Spinosad can be quite effective at killing whiteflies.
Beneficial insects can be very effective at controlling whiteflies. Often, whitefly populations only get large due to the lack of natural predators present in greenhouse and grow room operations. If you are growing inside and do not want a lot of insects flying around, this may not be the best option. However, if you want to avoid using chemical control methods, this can be an effective replacement. If you are using chemical control, beneficial insects can be used in conjunction with certain sprays to maximize your pest suppression.
Green lacewings (Chrysoperla rufilabris) are particularly effective at controlling populations of soft-bodied insect pests. Adults are not predatory, but their larvae will feed on whitefly eggs and nymphs . If you are not using sprays in your IPM program, it is a good idea to release lacewing adults early in the vegetative stages. You must provide the adults with flowers so that they can feed on nectar and pollen (spring-flowering plants for vegetative growth, fall-flowering plants for your flowering cycle) . They will lay eggs and newly hatched larvae will help control pest populations. Alternatively, if there are already pests present in your grow room, you can release green lacewing larvae directly to address the infestation. If you want to maintain your lacewings, you must add flowers for them to feed on when they mature.
Encarsia formosa (for greenhouse whitefly) and Eretmocerus eremicus are two commonly used parasitoids of whiteflies that lay their eggs in whitefly nymphs, eventually causing the death of their hosts . They are selective and highly effective at addressing large populations of whiteflies and should be released in the case of a heavy infestation.
Combining Chemical and Insect Control Methods
The only chemicals I would recommend combining with insect releases are neem oil and citric acid-based sprays. At the first sign of an infestation, begin with a wash of insecticidal soap. Following this, apply a neem or azadirachtin spray to your plants. 3 days following this, rinse your plants with water followed by a citric acid-based pesticide. After your plants dry completely, it is safe to release your beneficial insects. Do not apply any other sprays for at least 2 weeks after releasing your beneficials. If you are still experiencing an infestation (which is highly unlikely), I would recommend trying a spinosad insecticide.
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