Managing Greenhouse Whitefly With Encarsia formosa

By Marshall W. Johnson, Ph.D.  Department of Entomology, University of California, Riverside.

As summer heats up, greenhouse crops ranging from vegetables to ornamentals become more and more vulnerable to pest outbreaks that can quickly expand out of control.  Whiteflies are small and commonly go unnoticed unless you’re watching for them.  If you’re not cautious, you can quickly have a major
infestation.  Besides causing visible damage at moderate numbers, they produce cosmetically undesirable sooty mould, reduce overall plant vigor and yields, and can transmit diseases.  Greenhouse whitefly is always a likely suspect, and if you’ve had problems before, it’s always wise to revisit sound preventative guidelines and learn some tips for using natural enemies to add to your integrated pest management toolbox. 

When controlling greenhouse whitefly, or any arthropod pest in a glasshouse facility, always starts with good pest monitoring.  Early detection via yellow sticky traps or trap crops (tobacco plants are especially susceptible and will attract whiteflies quickly), will alert you to an early infestation and allow you to implement control techniques in a timely manner before the population becomes unmanageable.  Maintaining a clean greenhouse will also help reduce pest problems.  Weed control and adding disease/pest-free plants is always a wise decision. 

Effective integrated pest management for any commodity, relies heavily on knowing the economic threshold for the particular crop/pest combination — that point at which control measures must be taken to avoid economic damage.  Additionally, the use of natural enemy releases for control requires early action to ensure that the biocontrol agent can hold the pest in check before pest numbers become damaging.  Contact your county Cooperative Extension to learn what these pest densities might be for a crop susceptible to greenhouse whitefly, and determine your treatment options for a given infestation level.  Many Web sites are available for this information and a few are suggested at the end of this article. 

Encarsia formosa, a small parasitic wasp, is a well-known whitefly natural enemy and is often available through many different commercial insectaries.  It is especially effective against the greenhouse whitefly, Trialeurodes vaporariorum, so your first step is to correctly identify your whitefly species.  Sweetpotato and silverleaf whiteflies are not easily controlled by Encarsia,  so check out the USDA Whitefly Knowledgebase to obtain a correct ID.  Then follow these tips for maximizing greenhouse whitefly control with this effective natural enemy. 

• First and foremost, introduce Encarsia when you first detect greenhouse whitefly, or better yet, anticipate it.  Carefully note the location of the infestation and talk to your commercial Encarsia supplier to determine the appropriate release rate for your population. 
• If possible, avoid using pesticides before releasing Encarsia, especially those with long residual rates, as these pesticides have both lethal and non-lethal effects (e.g., reduced egg-laying ability, reduced searching behavior) on natural enemies. 
• Distribute Encarsia cards throughout the infested greenhouse, but concentrate on spots where whiteflies were detected. 
• Whitefly pupae that have been parasitized turn black, so visual observation of populations with a simple hand lens will give you an idea of Encarsia impact.  Educate greenhouse workers to locate parasitized whitefly and have them avoid deleafing or removing this crop material so the Encarsia can emerge and build up in the greenhouse. 
• Time of the year (and temperature) may influence your release strategy.  The lower light levels and cooler temperatures of winter are not suited to Encarsia.  On the other hand, direct sunlight can affect the parasites too, so releases in the greenhouse during the summer should be made in shadier parts of the plant. 
• Routinely monitor the number of parasitized whitefly pupae and continue releases until 80%-90% of the pupae are black (parasitized).

Encarsia formosa is not the only biological control agent useful against greenhouse whitefly.  Another parasite, Eretmocerus eremicus, may be more effective in warmer climates, and a small black beetle, Delphastus pusillus, is a predator that can work in tandem with the parasites by feeding on the whitefly pupae. 

There is a wealth of information on the biology and use of these natural enemies on the Internet, along with many excellent photos.  Commercial producers will also be happy to explain the use of their products and help you make the right natural enemy selection for your greenhouse pest control needs. 

For more detailed information on using  Encarsia formosa in a pest management program, visit these Web site articles:

1. Greenhouse IPM:  Sustainable Whitefly Control.  National Sustainable Agriculture Information Service.
2. University of California, UC-IPM Guidelines – Whiteflies
3. USDA Whitefly Knowledgebase
4. Management of Whiteflies in Greenhouse Crops.  Ministry of Agriculture Food & Rural Affairs, Ontario.