Pests -- White Flies

White flies may be one of the most common greenhouse and indoor pests. This tiny little white insect sucks plant juices for meals and can be seen flying off the plants when they are disturbed. They lay their white eggs on the underside of leaves, and their larva are green and almost invisible to the naked eye. White flies under go several stages of development each called an instar. (Similar to how a caterpillar turns into a butterfly). The larva do damage as they eat the plant too. If white flies are left unchecked indoors they can KILL your plants. I’ve read that some people say white flies can’ kill your plants, don’t believe it. Example, they secrete a substance like aphids which is often called honey dew. (See picture at bottom of post) This shiny sticky substance can be seen and felt on the leaves and is a great environment for molds to grow which also harm your plants. So, if the whiteflies don’t suck your plant to death, they make an environment where mold can harm/kill your plants.
A very important thing to know about white flies is that their life cycle is regulated by temperature.  In general, as the average temperature increases from under 60F to over 85F the number of days in a white flies life (egg to adult) decrease from 100 to 20 days. Below 60 degrees Fahrenheit an adult white fly can live for two months, but over 80F the adult fly will live just over a week. Egg production is also regulated by temperature, at lower temperatures females can lay over 300 eggs in its lifetime. With high temps the number of eggs drops and the average eggs per life cycle is around 30.
If you are into non-chemical cures the interesting thing about white flies is that there are predator wasps which eat the white fly larva. (No, they won’t sting you they are too small). You can find many suppliers if you do a web search for Encarsia formosa, the scientific name of a solitary wasp that preys upon white flies exclusively. Adult wasps eat the eggs and first instars and lay their own eggs in the third instar of whiteflies. This causes the green larva to turn black. In fact whey you buy these predatory wasps they are sold in larval form inside white fly larva that are glued to paper. Encarsia Formosa is also regulated by temperature, perhaps a good example of coevolution. Below 60F degrees it takes almost a month for an egg to reach adulthood. Above 80F it takes 10 days. Below 60F adults live a month above 80 they live just over a week. The number of eggs laid by a female remains at about 30 regardless of temperature, she just lays them faster the hotter it is. Most companies that sell the white flies say you should release them three or more times over a time period. This would allow several generations of white flies to be parasitized. I did not do this, I released them all at once and cleaned out the grow room when the plants were done flowering.

From my experience when I used Encarsia formosa it works to keep the white flies in check, but did not completely get rid of them. For a month, there were almost no white flies. Then they came back, I think because the wasps died of starvation before they found the last few eggs and white fly instars. I could have ordered more wasps but it was the end of the semester so I just harvest/killed all the plants and cleaned out the greenhouse. The ultimate solution to any infestation is you end up killing many plants and hand washing every inch of the plants you keep... A lot of work and not much fun.

Pyrethrum which is sold by HTGSupply.com  works well on white flies, but I am hesitant to use any chemical on plants I consume. If you are growing flowers/plants for show or decoration pyrethrum would be a good choice. You should know the predator wasps are more susceptible to chemical insecticides than white flies so you can’t use them if you have already tried a chemical insecticide. White flies are also attracted to yellow so HTGSupply.com sells yellow sticky tabs that white flies stick to and die. This may help keep down the number of flies but I think is better used for early detection of white flies or to see if they are reoccurring after a treatment.

In my greenhouse the white flies seem to get in every summer since I started working here 6 years ago. I am not sure if some get through the air exchanger/UV filter or if they catch a ride in on cloths. I have seen them flying around my back yard some years so anything is possible. This year I had a new plant and they went for this hibiscus plant almost exclusively. I had to snip off all the leaves and carefully threw them away to get rid of the eggs and larval instars. The bad thing is that a white fly female can fly around and land on any other plant in the room. The other plants I do not eat and are perennial and they will be in the greenhouse every summer. So, I resorted to a systemic chemical insecticide that you water the plants with so it gets into the leaves and keeps insects from completing their life cycle. This is not something you can do with plants you consume. I will never ever reuse the soil in these plants and I will have to wear gloves when I pot up the plants. But, I won’t have to worry about white flies getting into my experimental plants -- I am testing out a LED and 600 W HPS from htgsupply.com.  Also, I am betting next summer will be my first with out a NEW infestation. I’ll let you know.
Good growing,

Dr. E.R. Myers.

If you are interestind in learning more, I have some more information about biological control of insects under my root aphid post

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