Leaf-sucking Whiteflies

Whiteflies are small, white-winged, low flying insects that flit erratically when disturbed.  Ten species are common in California.  Whiteflies feed on many crops, weeds, ornamentals, shrubs and trees, including citrus.  As one host plant dries up or is harvested, they move on to another host.  Eggs, laid on the underside of leaves, hatch and the young whiteflies gradually increase in size through four nymphal stages until the winged adult emerges.  All stages of whiteflies, from the barely visible crawler to the winged adult, suck juices from leaves.  A few whiteflies will not significantly damage plants, but large populations of the immature whiteflies can cause plant distortion, discoloration, or silvering of leaves.  Like aphids, whiteflies excrete honeydew, so leaves may become sticky or covered with black sooty mold.  Ants, drawn by the honeydew, fend off bugs that feed on whitefly larvae so controlling ants will aid in minimizing whitefly damage.  

Whiteflies provide the perfect forum for testing Integrated Pest Management.  They are difficult to control, so a multi-faceted, integrated approach will likely be necessary. Pesticides are particularly ineffective for whiteflies since application kills the beneficial insects that keep whiteflies in check and whiteflies quickly develop resistance to insecticides. The best control of whiteflies starts early.  Check the undersides of leaves for tiny oval eggs and barely visible larvae and begin whitefly control measures before you have a major infestation.  Spray the leaves with a stream of water to dislodge adult whiteflies and also the ants that protect the larvae from beneficial insects.  Remove heavily-infested leaves.Support healthy populations of beneficial insects by learning to recognize and protect these garden helpers: lacewings, lady beetles, big-eyed bugs, minute pirate bugs, and Trichogramma wasps (see inserts).  

Other measures that have proven useful include the use of row covers, using squash as a trap crop to protect tomatoes from whiteflies, the use of silver/aluminum mulch, and placing yellow sticky traps where the adults fly.  Yellow traps can be made from squares of quarter-inch-thick Masonite board or plywood, painted yellow and spread with a mixture of 1 part petroleum jelly or mineral oil to 1 part dish detergent.  Whiteflies are not high flyers, so place the trap low, with the sticky side facing the plant.  For more information on identifying and controlling the many species of whitefly, go to Whiteflies Management Guidelines Pestnote.

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