Aloha, a friend gave me your email address as a possible source for garden information. If so, thank you in advance. My greenhouse has contracted a bad case of whitefly. I have been spraying with Dawn soap and alcohol and seem to be making some progress. I will pull all infected plants tomorrow and start again. Any further advice? — Thanks, S.W.
There are more than 1,000 whitefly species in the world. Twenty species occur in Hawaii, 12 of which have been accidentally introduced in the last 20 years. 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. Gardeners have great difficulty in controlling this pest because it is so prolific. To compound this problem, there are certain stages in the lifecycle of this insect, like the egg stage and the nonfeeding pupae that are not susceptible to insecticides as are the other stages. Consequently, eradication of a whitefly population may require as many as four repeat applications of an insecticide. In general, the recommendation is given to spray at five- to seven-day interval. This is to give time for the nonsusceptible stage to develop to the next stage which would then be susceptible to the spray. Above all, it is important that a thorough coverage be applied to the lower leaf surface and that the application is made as soon as whiteflies are detected. Once they have been well established, pulling out the plants may be the only alternative. Also, some whitefly populations have developed resistance to pesticides.
Here are some control measures:
l Treating early is the key. If annual plants like tomatoes and beans are heavily infested, throw them away. If a shrub is heavily infested, severely prune, then deal with any new infestation while the plant is still small. Keep in mind that excessive pruning stimulates much new growth, which is attractive to whitefly.
l 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.
l Yellow sticky cards: These should be placed adjacent or just above the infested plants – one card per two plants. They have good potential for controlling light (or new) infestations. 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.
l 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.
l Spraying: Use insecticidal soaps and horticultural oils including neem products to control whitefly.
l Aluminum foil or reflective plastic mulches have been shown to repel whiteflies away from plants.
l The application of broad spectrum pesticides to control whitefly is generally not recommended, because this type of spray has harmful effects on natural enemies (the good guys).
Here in Hawaii, as in other parts of the world, various natural occurring parasites and predators are doing a good job in controlling this pest. They will not eliminate the whitefly, nor are they an instant control. Yet for various reasons, whitefly can flare up and ruin a good crop. But a healthy population of natural enemies enables gardens to thrive with minimal damage. For more information, visit gardenguyhawaii.com.
Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at askthegardenguy@earthlink.net.
