How to control Whiteflies on marijuana plants

What’s the Whitefly?

White-Fly

Whiteflies

Whiteflies are homipteran insects of the Aleyrodidae genus that attack many types of cultivated plants, including cannabis crops. These flies are about 2 mm in length and have whitish appearance, with a pair of wings that serve as displacement method. They?re usually found on the underside of the leaves (where they also lay their eggs, as we?re going to see later) and, like other sucking insects such as Aphids or Mealybugs, they feed on sucking the sap of the plants.

Their body consists of three parts: head, thorax and abdomen. As other insects, they’ve six legs and also a pair of white wings. As already mentioned, they?ve biting-sucking mouth parts thanks to which they can feed on the sap of young leaves and tissues.

Reproduction of the Whitefly

This insect reproduces through eggs, which it lays on the underside of the leaves in a quantity of 180-200 in each egg laying. These eggs are almost microscopic, of oval-pyramidal shape and have a yellowish-white colour. They?ve usually four generations a year (one generation is the duration of the insect?s life-cycle from the egg until it dies like adult) depending on climatic conditions and hygiene, although in the greenhouse they can have up to ten per year, turning so into a serious pest.

Reproduction of the Whitefly, egg laying

Reproduction of the Whitefly, egg laying

We see as it is a pest that likes high temperatures and relatively humid ambient, that?s why the summer is their favourite season and greenhouses and in growing-boxes their favourite habitat. From the egg laying until the birth of the larva they spend approximately 24 hours: then the larva need more less four weeks to become adult, passing through 4 larvarl-nymphal stages in flake form and located on the underside of the leaves.

  • First stage: Approximate size of 0.25mm. The larva feeds on sucking sap from the plant. Only in this stage the larva is capable of moving, the other three are sessile, i.e. the nymph is enclosed in a capsule to protect itself while its structure changes.
  • Second stage: Approximate size of 0.4 mm. The formation of six legs can be seen on the larva.
  • Third stage: Approximate size of 0.5 mm. Transparent appearance.
  • Fourth Stage: Some organs, like the eyes, appear at this stage. Its thickness and size increase. Normally, this nimphal state is called “pupa”; the adult emerges from its protective capsule through a T-shaped slot – normally in the morning – starting to fly immediately.

Symptoms and damages caused by the Whitefly

White-Fly plague

Tremendous Whitefly pest infestation

The first evidence of the attack of these insects are chlorosis, deficiencies (yellowing) in the leaves, which end up drying – usually starting at the edges – and falling. The plant often suffers a slowdown in its development and a general deterioration of its state.

Apart from the symptoms caused directly by its sap sucking action, other  symptoms related to the sugary honeydew secreted by these insects may appear, what favours the appearance of sooty mold, a black fungus that ?dirty? the leaves and weakens the photosynthetic process. It may also appear other diseases, viruses and bacteria.

Keep in mind then – especially when you?re treating with a flying insect which has ease for the displacement – that whiteflies can cause serious damage to a crop, not only by its sap-sucking action, but also by the diverse diseases that can be transmited to the plants, for whiteflies are a vector insect of these.

Prevention and control of the Whitefly

Basil

Basil aroma repels the White-Fly

As in most pests and diseases that may affect the different strains of marijuana, prevention is basic to reduce the chances of possible attacks and infections. We can rotate our plants with others, thus creating an association of beneficial plants: growing Marigolds, Chinese Carnations or Basil will help to prevent the appearance of whiteflies because its smell repels them. We must check the underside of the leaves regularly looking for adults or larvae, and use an organic insecticide like potassium soap or Neem oil every few days. The use of sticky traps, in which adult insects will be sticked, will make things more difficult for them.

If we already have an affected crop we can use different ecological remedies to combat the plague: Rotenone and Pyrethrins (commonly used in organic farming as an alternative to chemicals) work well, and their use can be alternated with other insecticides such as infusions of Tansy or wormwood.

Macrolophus Caliginosus

Macrolophus Caliginosus, natural predator of the White-Fly

If these remedies don?t work – or if we don?t want to use them for any reason – we can combat the White-Fly with different natural predators.  Some of the most effective ones are:

  • Cales Noacki: Small wasp that parasites whitefly larvae. Very effective, although it isn?t commercialized on the market.
  • Encarsia Formosa: especially effective in greenhouses. Small fly of just 1mm of black colour with transparent wings. As the Cales Noacki, it lays its eggs (parasites) in White-fly larvae. It reaches its maximum predatory efficiency at a temperature of 25-27ºC and 50-60% relative humidity, using 10 predators per m2.
  • Macrolophus Caliginosus: useful also to combat other pests like spider mites. They attack Whiteflies in all their stages, preferring eggs and larvae.
  • You can also use other predators, like fungi: Paecilomyces fumosororeus, Beauveria Bassiana, etc.

As a last resort, and always avoiding their use for the sake of our environment, we can use chemicals to eradicate a Whitefly pest. These treatments will be especially effective against the larvae, which tend to be more sensitive to these substances. It should be noted that Whiteflies have a great capacity to develop defenses against these products, so we should alternate the use of several active principles to get maximum effectiveness. We should always choose insecticides respectful with the natural enemies of the plague that we are going to treat, limiting their use to the most and always as last resource, since a biological crop is always better.

As active substances against the White-fly, and among others, we find Butocarboxim, Buprofezin, Imidalclopid, etc.

We hope to have helped you in the fight against this annoying pest, remember that preseverance in prevention and hygiene are the keys to prevent the emergence of any pest or disease in your plants. We wish you happy and prosperous crops!

Good Vibes!

9 Comments


9 comments on “How to control Whiteflies on marijuana plants

  1. Andi320

    does hydrogen peroxide work as a foliar spray?

    1. Dani Alchimia

      Hi Andi320,

      You have better products to combat whiteflies such as Expeléx or Mittel. Hydrogen peroxide is usually used in the reservoir to eliminate algae or to combat root fungi. In rare cases, it is used via foliar to combat powdery mildew (although other products work better).

      All the best!

  2. shootthatgagmywayace

    i have been fighting whitflies with neems oil and insecticidal soap with poor results. They never completely go away and kill my plants one by one. i noticed when digging into soil that i see that i see a few flies that seem to be just beneath the top layer of soil. the temp is 20.2 celcius and humidity is 47%. Why cant i get rid of these pest? i alternate sprays every couple days and you cant really find them unless i dig around plants. Any help would be appreciated.

    1. Tim Alchimia

      Hi, thanks for your question. I’ve got a couple of ideas that might help, but as I imagine you’ve discovered, you’ll need to take a multi-pronged approach, (IPM or Integrated Pest Managnement) as no single measure will be effective on its own:
      Firstly you can remove them manually, with a water hose, or with the help of a small vacuum cleaner. This is very effective for adults and larvae but the eggs can be more stubborn.
      Try adding some essential oils (a few drops per litre) to your neem/insecticidal soap mix, I’ve had good results against spider mites with Rosemary and Peppermint oils, and Eucalyptus, Rosemary, Lemon and Cinnamon are recommended against whitefly. Remember to spray for at least 4-5 weeks to be sure to catch all the stages of the pests life cycle.
      Another idea is to lower temperatures below 20ºC (without damaging our plants) to create a more hostile environment for the pests.
      Whitefly are attracted to the colour yellow, so hanging some yellow sticky traps can help to control numbers.
      If you’re finding them in the soil then try using a reflective mulch (aluminium or silver polyethylene) to help with that. This is a common measure in large scale commercial operations.
      Companion planting of some beneficial flowers like nasturtiums and marigolds can help to keep whitefly away.
      We can also look at introducing some kind of biological control to deal with the problem, for example these Macrolophus Pygmaeus insect predators are effective against whitefly and many other pests. Predators like Ladybugs and Lacewings are easy enough to find, as well as a wide range of other suitable predators that are widely available to buy these days.

      I hope thats given you a few ideas, we wish you the best of luck getting rid of the problem. Happy growing!

  3. June Hutchison

    Thank you, I will try the cinnamon extract with Castile soap in a spray bottle, but my question is how many times a week?? And should I spray in the evening when the plant’s are not in the direct sun???

    1. Tim Alchimia

      Yes, that’s right June, either in the morning or in the evening, but always spraying when the plants aren’t in direct sunlight. All the best!

  4. Dianne Houde

    So if i soray thr leavse with peppermint oil do i mix it with water?

    1. Tim Alchimia

      Hi Dianne, thanks for your question. Yes, you’ll need to mix the peppermint essential oil with water. Use 1 drop of essential oil per 60ml of water and add a tiny bit of dish soap to help better spread the mixture over the leaves when you spray them. Have a look at our article on Foliar Spray basics for more tips on spraying correctly. Hope that helps, happy growing!

  5. ralshedi@gmail.com

    Small vacuum cleaner will do it under each leaf

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