How Often Do I Spray Tomato Plants to Get Rid of Whiteflies?

Whiteflies on Tomatoes

• The greenhouse whitefly (Trialeurodes vaporariorum) infests tomatoes and many other types of plants, including poinsettia, begonia, nicotiana, aster, calendula, cucumber, lantana, grape, ageratum, bean and hibiscus. The silverleaf whitefly and bandedwinged whitefly species may also infest tomato plants. The damage caused by whiteflies is two-fold: The flies feed directly on the plants and leave feeding holes that wilt and yellow leaves. They also secrete honeydew as they move across the plant, a sugar-rich excrement that creates ideal growth conditions for black, sooty mold, a fungus that can also threaten the health of the plant.

Applying Pesticides

• The frequency of pesticide applications for whiteflies depends on the type of pesticide you are using. One difficulty facing gardeners that treat whitefly infestations with pesticides is that the chemicals often kill adult and nymph whiteflies while leaving egg and pupal whiteflies intact, which often requires repeated applications. Systemic pesticides, such as those containing imidacloprid, are usually only applied once prior to the growing season while contact insecticides with pyrethrins or pyrethroid compounds may need to be applied numerous times until the infestation is controlled.

Other Application Tips

• It is of the utmost importance that gardeners always follow the pesticide manufacturer's application instructions. Individual products have their own guidelines as to how often they should be applied, guidelines that are customized for the unique blend of ingredients in that product. The labeling also includes important guidelines, such as to how much pesticide should be used in a single application and how best to apply it in order to avoid unintended harm to other plants.

Remarks on Pesticides and Tomatoes

• There is no shortage of chemical pesticides available to consumers for whitefly control on tomato plants, but experts at the University of California and Washington State University caution against using pesticides at all for whitefly controls. Whiteflies have a high tolerance for pesticides, and over-reliance on chemical treatments is often to blame for a whitefly infestation. Manage whiteflies with cultural and natural measures first and use pesticides only as a last resort.