Tomato yellow leaf curl virus transmitted by silverleaf whitefly.

An explosion of silverleaf whitefly, first seen on poinsettias in west-central Florida, soon became the major pest of tomato and other fruiting vegetables throughout the state. Crops were covered by sooty mold from the insect's sugary excretions, and new disorders appeared, such as irregular ripening of tomato and silver leaf of squash  (source of the name silverleaf whitefly).

A parasitic wasp, Encarsia pergandislla, laying egg in whitefly pupa.

The worst was yet to come with plant virus diseases such as tomato mottle virus and the even more devastating tomato yellow leaf curl virus.To save their crops, growers were obliged to intensify spray programs with increased applications of more powerful insecticide mixtures, resulting in commensurate increases in production costs and possible risk to users, consumers, and the environment.
    In unsprayed weeds and isolated organic vegetables, however, predators and parasitic wasps were seen to control whiteflies to low levels.
 

Bioradonal Insecticides an Alterantive?

University of Florida scientists, led by Phil Stansly, Dave Schuster and T. X. Liu, addressed the following questions: How could biological control be integrated into conventional management systems? And, if biorational insecticides such as oils and soaps compatible with beneficial insects were used, how effective and compatible would they be?
    Spray oil and insecticidal soap were tested in the laboratory and greenhouse againstall whitefly stages,with the following findings:

• Insecticidal soap worked on whitefly nymphs betterthan on adults,but only while wet.
• Household detergent liquids worked at half or less the concentration of soap, were inexpensive, and did not precipitate in hard water. However, these detergents, like soap, caused plant injury at higher concentrations.
• Oil killed all stages of whitefly and had longer residual activity than soap.
• Oil was as effective as the best synthetic insecticide (bifenthrin) but required much better coverage of the lower leaf surface, where whiteflies reside.
• Oil was repellent as well as toxic to whitefly adults.

We then tested detergent and oil in the field with the following results:

• Twice-a-week applications of detergent at concentrations as low as 25 percent worked as well as stronger but less frequent chemical sprays.
• Oil reduced the spread of tomato mottle virus as well as chemical sprays, probably through the combined effects of repellency and toxicity to the whitefly vector.
• Use of air boom sprayers, which gave better coverage of the underside of tomato foliage, also resulted in better whitefly control.

Negative Impact on Some Beneficial Insects

It remained to test soaps and detergents on natural enemies of whiteflies such as ladybird beetles, lacewings, and predaceous wasps. The following results were obtained:

• Fresh oil residues killed as many adult wasps as did bifenthrin, but again, toxicity of oils depended greatly on coverage.
• Soaps were relatively nontoxic to all stages of parasitic wasps but killed young ladybird beetle larvae.
• All stages of lacewing were relatively unaffected by soaps and oils. In contrast, bifenthrin killed all stages except the protected "trash bug larvae."

In Florida, Shift in Insecticides

Although insecticide costs have not decreased, the types of insecticides used with Florida vegetables has changed to those more compatible with biological control. For instance, an average of 6.8 pounds per acre active ingredient of the two most frequently used insecticides (enclodulfan and chlorpyrifos) was applied in 1994. In 1996, 2.3 pounds per acre of these insecticides was applied,a decrease of 300 percent. During the same period, use of the biological insecticide Bacillus thuringiensis, which only affects caterpillars, has increased 91 percent (from 46 percent to 88 percent of tomato acreage).
    Decreased use of broad-spectrum insecticides on vegetables in southwest Florida appears to be even more dramatic: an on-farm chemical supply distributor in Immokalee indicated a 1,700 percent decrease in insecticide sales.The change was due in part to improved whitefly control, but also to changed attitudes about insecticide use. The owner of this particular business said,"Growers that sprayed all the time are no longer with us, whereas those that adapted an integrated pest management approach survived."
    Use of the systemic insecticide imiclacloprid since 1995 and clean fallow periods have largely eliminated the need to spray chemical insecticides on tomatoes for whitefly control in southwest Florida. It would seem that, although soaps, oils, or even hard chemicals are no longer needed to control whiteflies in the Florida system, the lessons learned in the struggle with that pest have aided in the development of more sophisticated biorational pest management systems for vegetables.

Silverleaf whitefly: adult, red-eye pupae nymphs, and eggs

Ladybird beetle, Nephaspis oculatus, a white-fly predator.

Air boom sprayers placed more material than did traditional hydraulic sprayers on the underside of leaves where whiteflies reside.