Why whiteflies must be stopped

One of the toughest professional challenges I encountered happened about 25 years ago when whitefly problems in greenhouses were so severe that crops were beyond rescue. Available controls weren’t working well and as whitefly populations increased, plant inventories declined drastically.

Entomologists working with growers found they could successfully manage infestations but treatments had to be carefully selected and timed for just the right stages in the life cycle, requiring more effort to get it ‘right.’

That experience taught the following:

• Whiteflies are capable of serious economic damage.
• Early detection and regular monitoring are invaluable.
• Effective, alternative controls are critical. 

Insecticide resistance and failure occur when too much emphasis is placed on insecticides and not enough attention is paid to other strategies. Today, more attention is given to all aspects of managing whiteflies, helping assure that newer insecticides will continue to perform as expected.

Why whiteflies matter

Whiteflies are objectionable for several reasons:

• High numbers of whiteflies on plants can cause leaves to yellow and drop. They leave deposits of honeydew, followed by sooty mold, that can spoil the appearance of produce and make harvest difficult.
• All whiteflies can transport broad mites from plant to plant, introducing another damaging pest to new areas.
• One specific whitefly, the Sweetpotato whitefly, is associated with virus transmission in tomatoes, a potentially serious problem.
  

The (two) usual suspects

Two kinds of whiteflies are common in greenhouses: greenhouse whitefly (GHWF), Trialeurodes vaporariorum, and sweetpotato (sometimes referred to as silverleaf) whitefly (SPWF), Bemisa tabaci.

Although GHWF had been trouble enough, the arrival of SPWF in greenhouses in the mid-1980s was responsible for the situation I encountered. It was a serious pest in outdoor crops such as lettuce and melons, resulting in temporary produce shortages. To make matters worse, some SPWF strains or ‘biotypes’ later appeared with resistance to more insecticides that once used to work. This is one reason it is important to know what whitefly is present. Another reason is that biological control for whiteflies can work very well but only if the natural enemy is matched to the kind of whitefly (GHWF or SPWF) present.

It isn’t difficult to distinguish the two kinds of whiteflies; the pupal stage is most distinctive and usually present in any whitefly population. Some growers mistake the pupae for ‘eggs,’ but they are actually the final stage from which the adult whitefly emerges. (The eggs themselves are almost invisible to the naked eye.) Pupae are about 1/16-in. in diameter, somewhat raised, and found under leaves. GHWF pupae are pale, oval, and almost white, with a kind of cake-shape and a fringe of hairs. SPWF are more distinctly yellow, rounded or mound-like, without hairs. The adults differ as well, and are easily recognized with some experience.

Once they invade greenhouse and high-tunnel vegetable production areas, whitefly populations are largely protected from weather and natural enemies that can keep populations under control, and they can thrive with well-fed host plants growing at nearly ideal temperature and humidity. And because vegetable crops grow over a period of many months, there’s plenty of time and opportunity for multiple generations of whiteflies.

With almost nothing restricting development, population growth is exponential. The problem is compounded in successional plantings where there are no breaks between crops. Controlling whiteflies is also much more difficult when plants are large, and getting good spray coverage is nearly impossible, to say nothing of worker exposure concerns. Under these conditions growers need to have a very low tolerance for early production infestation.

Steps toward protection

The following practices are effective in protecting crops and effectively dealing with whiteflies in particular:

• Start with a clean environment. Whiteflies persist between crops on weeds or left behind plants. Sanitation should extend at least to the perimeter of the greenhouse, leaving no sources of infestation immediately nearby.
• Beware of ‘Trojan horses.’ Whiteflies can be inadvertently introduced on plant material. Old stock plants kept for propagation and ‘pet plants’ have been common sources. Other serious pests such as thrips, aphids, and spider mites, and pathogens such as viruses also can be brought in this way.
• Train detectives on staff. Your work force is in the crop every day and can actively help with monitoring. Show them what to look for and where to find whiteflies. Turn over lower leaves to look for adult and immature stages. Watch for the adults flying between plants. Install yellow sticky tape at crop canopy level and regularly check for trapped whiteflies. Place some around areas of air movement, in vents, and anywhere whiteflies might be traveling.
• Consider screening. In some areas where invasions of whiteflies from the outdoors are annual problems, insect-exclusion screening may be just tyhe answer. The screening retrofit and maintenance costs might very well ouweigh the labor expense for insecticides.
• Intervene early. When some whiteflies are detected, try to find the source(s) and treat promptly, particularly if the crop is at an early stage. If harvest is almost complete and populations have been very low control may not be needed.
• Consider biological control. For whiteflies results have often been very good, providing the beneficials have been released early, in sufficient numbers, and in a favorable environment. For GHWF Encarsia formosa is used; for SPWF Eretmocerus is more effective. Check with suppliers or your Extension specialist on release rates, monitoring viability after shipment and performance in the crop, and handling.
• Choose insecticides thoughtfully. Products don’t all work the same way, are not equally effective, and have different crop and application restrictions.
  

For very dense whitefly infestations I like to lead with M-Pede Insecticidal Soap, or a horticultural oil (SuffOil-X, Sunspray Ultra-Fine, JMS Stylet Oil). Some products can be used for whiteflies only on one or two crops (tomatoes: TriStar, Talus; cucumber: Beleaf; both crops: Admire Pro/generics; some or all fruiting vegetables: Distance; cucumber and fruiting vegetables: Akari). Others have broader crop uses (hort. oils, M-Pede, Pyganic, Pyronyl, Pyrenone, BotaniGard, Mycotrol, Grandevo, Azatin, Ornazin, Neemazad, Neemix, Aza-Direct, AzaGuard, Molt-X). We found tomatoes were sensitive to the BotaniGard ES formulation but the WP product is fine. Some products have labeling for use on vegetable transplants and may not have crop uses. Therefore, reading labels carefully is crtical.

Thanks to the lessons learned so many years ago, greenhouse growers today, who practice the steps for guarding against and battling whiteflies outlined in this article, will avoid having to deal with what once was one of the greatest pest-control challenges ever.

Daniel Gilrein is an extension entomologist at Cornell Cooperative Extension of Suffolk County and a regular contributor to Produce Grower and Greenhouse Management.