Q: I have been gardening for more than 20 years in various cities in California, but I’m about to drop gardening as a hobby. The whiteflies are all over. They killed my zucchini plants and are now all over the bell peppers. I have tried washing them off, spraying with a soap and water spray. Nothing is working. I got a total of three zucchini off of four plants. Help!
Marcia Holmes, Woodland
A: According to UC master gardener Rachel Tooker, whiteflies are tiny, sap-sucking insects that are frequently found in vegetable and ornamental plantings. When infested plants are disturbed, clouds of the winged adult whiteflies fly into the air. Large numbers of whiteflies can cause leaves to turn yellow or appear dry. Like aphids, whiteflies excrete honeydew, so leaves may be sticky or covered with black, sooty mold.
Most of the damage from whiteflies comes from the insect’s nymphal stages (they look small, oval and flattened in appearance), although all stages feed by sucking plant juices from leaves and excreting excess liquid as drops of honeydew.
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Management of heavy infestations is very difficult. Whiteflies are not well controlled with any available insecticides. The best strategy is to prevent problems from developing in your garden to the extent possible. In many situations, natural enemies will provide adequate control of whiteflies. Outbreaks may occur if natural enemies that provide biological control are disrupted by insecticide applications, dusty conditions or interference by ants. Avoid or remove plants that repeatedly host high populations of whiteflies.
In gardens, whitefly populations in the early stages of population development can be held down by a vigilant program of removing infested leaves, vacuuming adult whiteflies or hosing down with water sprays. Look for yellowing, silvering or drying leaves with whitefly nymphs on them and remove these leaves as quickly as possible. Hose off adults or use a hand-held vacuum to suck up flies and larvae (put the vacuum bag in a plastic bag and place in freezer overnight before discarding to kill the flies).
Aluminum-coated construction paper or reflective mulches can repel whiteflies from vegetable gardens. This should be put down at planting time and removed once summertime temperatures rise. Alternatively, yellow sticky traps can be used to monitor or, at high levels, reduce whitefly numbers. These traps won’t eliminate damaging populations but may reduce them as a component of an integrated management program.
Because whiteflies do not travel far, multiple traps (as many as one for every two large plants) may be needed, with the sticky yellow part of the trap level with the whitefly infestation. Traps are commercially available, or can be made out of 1/4-inch plywood or Masonite board, painted bright yellow and mounted on pointed wooden stakes that can be driven into the soil close to the plants.
Stickem or Tanglefoot (both available at nurseries) can be used to coat the traps, or you can try making your own adhesive from one part petroleum jelly or mineral oil and one part household detergent. This material can be cleaned off boards easily with soap and water, whereas a commercial solvent must be used to remove the other adhesives. Periodic cleaning is essential to remove insects and debris from the boards and maintain the sticky surface.
Insecticides have only a limited effect on whiteflies. Most kill only those whiteflies that come in direct contact with them. If you decide to treat, choose products that are least harmful to natural enemies – such as insecticidal soaps and oils including neem oil – and combine their use with the other practices listed above. Avoid using even these pesticides if many natural enemies (lacewings, big-eyed bugs, minute pirate bugs and lady beetles) are present.
In your letter you also mention that the zucchini plants in your garden had poor fruit set. Poor fruit set is unlikely to be caused by whitefly populations. Rather, this problem stems from insufficient pollination of the flowers – often due to low bee activity in cool weather or the presence of too much insecticide.
Zucchini plants, like all cucurbits, bear separate male and female flowers on the same plant. Female flowers have a miniature fruit at the base of the flower and male flowers do not. The pollen is sticky, so wind-blown pollination does not occur. Wild honeybees are rare in some urban neighborhoods, and when bees are absent, fruit set on garden plants in the cucurbit family is very poor and often nonexistent.
To hand pollinate, take freshly opened flowers and use a small artist’s paintbrush or break off the male flower to expose the pollen bearing structure. Then roll the pollen onto the center of the female flower.
For more information, consult UC IPM Pest Management Guidelines for Whiteflies, Publication 7401, and the Whiteflies Quick Tip card, which can be downloaded at http://ipm.ucdavis.edu.
Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to email@example.com. Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:
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Yolo: 530-666-8737; 9-11 a.m. Tuesdays and Fridays, or leave a message and calls will be returned