This is why White flies are disastrous for cannabis/marijuana.

by Alex Robles

White flies are tiny pest that could cause major problems for a cannabis plant.  I did battle with them back in 2018 and I learned a few things from a lot of different places.  I went back to my library and read everything I could find, I also went to some trusted online sources and I asked some fellow grower their advice.  Here’s what I think you should know about these little assholes.

What are they?

  • White flies are an Aleyrodidae insect.  There are more than 200 species of this bug and it comes in all shapes, size and color.  The one I’m focusing on today is known as the “Greenhouse White fly”. And they are the 3rd most common pest that cannabis grower get indoors or out.
  • They look like tiny white moths and are easy to spot by shaking the plant.  They’ll fly out from underneath the leaves.
  • They are a sucking insects like spider mites, Aphids or Mealybugs and they feed by sucking the sap from the leaves.
  • The flies are about 2mm in length (approximately 1/16 of an inch)  and have whitish appearance. Their bodies have three parts: head, thorax and abdomen.  They have six legs and adults grow a pair of waxy, white wings that help them fly short distances, like to other plants.  They’re usually found on the underside of the leaves. Their eggs, which can also be found on the underside of the leaf are connected with a small hook, that’s why they’re so hard to remove with a spray from a hose.
  • These little buggers reproduces by laying 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, The oval-pyramidal shape of the egg stack has a yellowish-white color.  You’ll need some kind of magnification if want to see the eggs, I suggest a jewelers loop to get a closer look.
  • They could have four generations a year depending on growing conditions (temperature, humidity), how clean the grow area is.
  • Larva hatches about 4-10 days after the egg was laid.   It’ll go through 4 larval-nymphal stages before it becomes an adult.
  • In about six weeks, depending on temperature, the nymphs will grow, molt and reach their four-winged adult stage. The adults live for one to two months.

Larval-Nymphal Stages

  1. First stage: The larva hatches and starts feeding by sucking the sap from the leaf.  This is the only stage that the larva can move, in the other three stages they’re sessile (fixed in one place; immobile.), i.e. the nymph is enclosed in a type of cocoon for protection while it grows threw the other stages.
  2. Second stage: The formation of six legs can be seen on the larva.
  3. Third stage: Transparent appearance.
  4. 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.
Whitefly lifecycleFound this illustration at “Royal Queen Seeds”

Environment:

  • Since this pest likes high temperatures and relatively low humidity, that’s why summer is their favorite season and greenhouses, indoor grow rooms and growing-boxes/cabinets are their favorite environment.  They usually show up on the weakest plant first
  • White flies thrive in places where there are no (or few) predators around to eat them. That’s why it’s important to make sure that our grow room is clean and clear any other insects that might naturally eat white fly predators (like ants).
  • Try and keep your grow room temperatures around 68-78F (between 20 and 25ºC).
  • If you’ve noticed some white flies in your grow room, you may want to bring temperatures down a little to fend off a bigger infestation (try to bring them down past 68F or 20C, but be careful not to damage your plants).
  • Make sure your grow room is well ventilated, it’s an important step in battling most garden pests.

What does damage look like:

  • The first sign of a White fly is a tiny yellow dead spots on the leaves.  If left unchecked the chlorosis (yellowing) on the leaves will spread and kill the leaf – usually starting at the edges. The plants growth will slow down and look like it’s struggling if the flies aren’t dealt with.  When white fly eat, they punctures the leaves of the plant so they can suck nutrients from them.
  • They’ll usually eat more plant juice than they can digest and poop out the unneeded nutrients as a sweet, sticky liquid known as honeydew.  If your plants are infested with white fly, keep an eye out for honeydew on the leaves of your plants because it will start to grow a black, sooty mold that can slow down our plants ability to photosynthesize.  Honeydew also attracts ants, which tend to scare off White fly predators and could make the infestation worse.
  • We have to remember that because white flies have wings, it’s real easy for them to spread to other plants and possible infect them.  Either by laying eggs, sucking on sap or by spreading disease.

How to fight them.

  • Make it a habit to look at the underside of the leaves to check for pests, not just white flies.  Remove any leaves that have 40-50% damage. You can use organic insecticide like soaps or Neem oil every 5-10 days.  You want to be careful not to go too heavy on the soaps or Neem. They work by dissolving the wax layer or coating the bug has (cuticle) and dry it out.  The plant has a wax layer also, if we go too heavy with our mix or spray too often we could harm the plant.
  • White flies are attracted to the color yellow, that’s why they attack the sickest looking leaves and plants first. That’s also why sticky traps are yellow and work so well.  You can make your own trap by getting a yellow card of some kind and putting some “Tangle Foot” on it.  The flies will stick to that, and when it’s fullish throw it out and put another one down.
  • Also, make sure your grow room temperatures aren’t too high (between 20 and 25ºC or 68 and 77ºF is ideal).
  • If you’ve noticed some white flies in your grow room, you might want to bring the temperature down a little, this will help slow down any infestation that may be brewing.  Try to bring it down past 68F or 20ºC. Careful not to bring it down too low for too long, you don’t want to damage the plants.
  • Make sure your grow room is well ventilated, an important step in fending off most garden pests.
  • Prune any infected leaves. Make sure to get rid of anything you cut off immediately so you don’t  infect other plants.
  • Ecological remedies:: Rotenone and Pyrethrins (commonly used in organic farming as an alternative to chemicals) work well.  I’ve used a pyrethrum spray with success and I know they sell fogger cans. Apply every 5-10 days.  Please be careful when applying any pesticide, it doesn’t matter if it’s organic or synthetic.
  • A natural way to prevent the white flies is with companion planting.  Growing Marigolds, Chinese Carnations or Basil will help to prevent white flies because the smell repels them. When we plant zinnia with our cannabis plants along with the marigolds, it sends a strong signal to the white flies to go somewhere else.
  • Zinnias also help attract natural predators of the white fly into our gardens. They help attract hummingbirds, predatory wasps and flies, all of which love to gobble up white flies. Plants with a minty sent, such as hummingbird bush, pineapple sage and bee balm help to naturally disguise the scent of the surrounding plants that attract white flies.

Natural/Biological predators.  Some of the most effective ones are:

  • Encarsia Formosa: parasitic wasp especially effective in greenhouses. Small fly of just 1mm of black color with transparent wings.  It takes them a while to control the pest population, I don’t recommend only using these for an infestation. It attacks 2nd instar, proactive 8 to 10 weeks covers susceptible stage, low pest density only.  It lays its eggs (parasites) in White-fly larvae. It reaches its maximum predatory efficiency at a temperature of 25-27ºC (77-80F) and 50-60% relative humidity, using 10 predators per m2 or 2 or more per plant.  You’re going to want to do this every 2-4 weeks just to control the white flies.
  • Lady Bugs are also great for pest control, but we have to remember that it will also take some time for them to get established.
  • Macrolophus Caliginosus: useful also to combat other pests like spider mites. They attack White flies in all their stages, preferring eggs and larvae.
  • You can also use other predators, like fungi: Beauveria Bassiana.
  • Also neem oil or garlic oil sprays work.  Remember White flies can develop resistance against some types of sprays so alternate what you use every other application

I hope this helps you understand better what your up against when spot Whiteflies on your cannabis plants.  There are links down below that I think also could help.

Grow Learn Teach

Resource:

Suggestions:

Jewelers loupe

Pyrethrin  fogger

Pyrethrin spray

Tanglefoot

Yellow Sticky Traps

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