They are not only a menace to our gardens, but to commercial agriculture as well. There are more than 1,000 whitefly species in the world. Twenty species occur in Hawai`i; 12 of which have been accidentally introduced in the last 20 years. They attack citrus, avocado, plumeria, poinsettia, anthurium and a whole host of vegetables and other ornamentals. As you approach heavily infested plants, clouds of adult whiteflies fill the air.
The adult whiteflies, which are not true flies at all, look more like tiny, white moths. They are sapsuckers, continually drawing off water and nutrients from plants, causing leaves to yellow, shrivel and drop prematurely. Eventual death of the plant can occur. Whiteflies also transmit several viruses. In addition, they cause an indirect decline of the plant by their heavy excretion of a liquid, rich in amino acids and sugars, called ‘honeydew.’ This liquid is constantly dripping to coat the leaf surface below, which is soon colonized by a common airborne fungus, called sooty mold. Although the fungus itself does not invade the leaf tissue, the leaf quickly becomes black and dirty looking, restricting respiration and photosynthesis in the plant.
Gardeners have great difficulty in controlling whiteflies, because they are so prolific. Another reason is that the different stages of growth of this pest have varying degrees of susceptibility to pesticides. This means that after you spray, only a portion of the whitefly population died. Therefore, repeated spray applications are needed. Also, some whitefly populations have developed resistance to pesticides.
The application of broad spectrum pesticides to control whitefly is generally not recommended, because this kind of spray has harmful effects on natural enemies (the good guys). Broad spectrum pesticides are those that affect a wide range of insects as opposed to those which affect the target species only.
So what do we do? First of all, if your tomato or bean plants are heavily infested, throw them away. If a shrub is heavily infested, severely prune, and deal with the new population while it is still small. Here are some other tips:
· Yellow sticky cards – These should be placed adjacent or just above the infested plants – one card per 2 plants. They have good potential for controlling light (or new) infestation. The insects are attracted to yellow and will stick to the card when they land. Once the population has built up, however, the cards are not very effective.
· Syringing - Apply a strong stream of water at the underside of infested leaves as often as possible. This will wash off many of the whiteflies, as well as the honeydew. University of California studies have shown this method performs as well or better than chemical treatments.
· Pruning – Prune away severely infested portions of the plant. Place material in sealed plastic bags and dispose of properly. Keep in mind that excessive pruning stimulates much new growth, which is attractive to whitefly.
· Ants – Ants are a detriment to biological control and must be eliminated. The ants are bigger than many whitefly parasites and, by their aggressive behavior, will discourage the good guys from the scene.
· If you spray, use insecticidal soaps and horticultural oils; this minimizes the adverse effects of harsh pesticides on the natural predators and parasites.
· Aluminum foil or reflective plastic mulches have been shown to repel whiteflies, especially away from small plants.
Many parasites and predators of the various whiteflies have now been identified. Some of these have been imported from other parts of the world. Here in Hawai`i, as well as in other states, parasites and predators are doing a good job in controlling this pest. They will not eliminate the whitefly, nor are they an instant control. And, for various reasons, whitefly can flare up and ruin a good crop. But a healthy population of natural enemies enables us to garden with little or no apparent damage to our plants. Controlling the ants and keeping dust to a minimum is important in encouraging the natural enemy population.