Tomato is a “favorite food” of whitefly and this past growing season proved this fact. While whitefly infestation of greenhouse-grown tomatoes is common, it is much less so on outdoor crops. A number of Missouri growers reported whitefly problems with their field tomatoes this past growing season. This article is written to address the problem next year.
There are two species of whitefly which most often are responsible for infestation of tomatoes grown in Missouri. They are the greenhouse whitefly (Trialeurodes vaporariorum) and the sweet potato whitefly (Bemisia tabaci). A second strain of the latter has been identified and given the name of silverleaf whitefly.
Whitefly is a member of the insect order Hemiptera and family Aleyrodidae. It goes through a life cycle that begins with an egg and ends with an adult. In between, there are four stages of nymphs. The fourth is called a pupa, although this technically is incorrect. Under field conditions, the life cycle takes between 25 and 50 day, depending upon temperature.
Other than the egg and pupa, all stages of whitefly feed on plants by inserting their piercing mouthpart into the phloem and extracting sugary sap. In the process they secrete a toxin in their saliva which decreases the turgor pressure of the cell. Heavy whitefly infestation not only weakens a plant, but also leads to the development of sooty mold which grows on honeydew secreted by the whiteflies. Sweetpotato whitefly is especially damaging because it also causes tomato fruits to ripen unevenly.
Additionally, whiteflies are known to be vectors of plant diseases caused by viruses. For obvious reasons, control of this insect is important.
Control of whitefly in field plantings is best accomplished following an integrated approach. Whitefly cannot overwinter outdoors in Missouri. Therefore, growers start each year with a “clean slate”. Outdoor infestations primarily are the result of using contaminated transplants, or the introduction of whitefly populations from southern states. Although they are poor flyers, whitefly can travel hundreds of miles on wind currents.
Starting with whitefly-free transplants is the first step in their control. When growing your own transplants, practice good whitefly management practices in the greenhouse and monitor populations diligently. Transplants purchasing from someone else should be carefully inspected before planting.
Field scouting of outdoor plantings is necessary to determine when a significant population has developed. Visual inspection of upper leaves for adults and lower leaves for nymphs should be done on a weekly basis. Concentrate scouting at field margins since these areas often are infested first.
If whitefly populations build to the point that chemical control measures are warranted, there are several approaches that can be taken. Imidacloprid (Admire PRO) can be effective as a means of “cleaning up” transplants or for early season infestations. Given that imidacloprid has a 21 day P.H.I., it should not be used later in the season.
Insecticides with shorter P.H.I.s are needed later in the life of the crop, especially if harvest has begun. The table below lists chemicals with relatively short P.H.I.s recommended for whitefly control by the Midwest Vegetable Growers Guide. Rotation between modes of action will help to delay development of pesticide resistance.
|Trade Name||Active Ingredient||P.H.I.
|Mode of Actio Group|
|M-pede||K salts of fatty acids||0||not listed|
Whatever the control measure chosen, early intervention cannot be overstressed. Each female whitefly has the ability to produce 400 eggs in her lifetime, which leads to rapid population development. The warmer the temperature, the more rapid the infestation. Whitefly control is very challenging in heavily infested plantings.
REVISED: November 23, 2015