Whiteflies Are Not Flies
But they can be a big problem. New research shows the best control methods.
No, whiteflies are not flies. Flies like the typical housefly
belong to the order Diptera, which has only two wings. Whiteflies are in the
order Homoptera and have four wings. The order Homoptera also include cicadas,
planthoppers, treehoppers, leafhoppers, sharpshooters, spittlebugs, psyllids,
scale insects, mealybugs and aphids.
There are over 1,200 species of whiteflies identified
worldwide; however, only a few of them are classified as pest species. Newly
introduced species of whitefly can quickly become pests and may adapt to new
host plants and environmental conditions. For example, one year after the
silverleaf whitefly reached greenhouses in California, it replaced the
greenhouse whitefly as the dominant species on poinsettias.
Monitoring for Whiteflies
Any whitefly management program
will require the use of monitoring practices to be effective. Accurate records
of pest numbers, crop damage and changes in whitefly numbers will allow the
development of treatment thresholds for specific crops and can help to time
pesticide applications. Yellow sticky cards are an effective method for
monitoring adult whiteflies. One trap per 1,000 sq. ft. of growing area is
recommended. Place traps a few inches above the plant canopy, and move them up
as plants grow. Check the traps weekly, and keep a good record of trap counts.
Recognizing hotspots and treating them early may reduce the overall need for
pesticide applications. Because traps only capture winged adults, it is
important to also monitor plants. For more information about monitoring, see
Since whitefly infestations can
easily go unnoticed until they reach high numbers, it is important to minimize
potential infestations by employing all possible control tactics prior to
chemical control. Begin the cropping cycle with pest-free plants, and exclude
the pest, if possible, with exclusion screens. Many weeds are good hosts for
whiteflies. Therefore it is important to keep the growing area clean and free
of weeds. Biologicals are also a good control method for whiteflies; see the
sidebar at the right for
information about biologicals.
All whitefly stages typically
occupy the undersides of leaves of infested plants. The more susceptible stages
are the younger nymphs. The later stages, such as the fourth instar and redeye
stage, are more difficult to control. Therefore, the best control is achieved
when a maximum effort is made to contact the undersides of leaves with repeated
spray applications (about every six days) targeting the earlier, susceptible
stages for a period of about three weeks. Heavier infestations may require more
applications for a more extended period of time.
An effort should be made to hold
whitefly populations in check early in the cropping cycle. Contact
insecticides, including soaps and oils, may be more effective early in the
growth cycle when the foliage is less dense and contact with young nymphs is
more likely. Later in the cropping cycle, when dense foliage is present, a
systemically acting material may be more efficient in reaching the insects.
Figures 1-4, pages 58-62, are
summaries of recent whitefly trials conducted at the University of
California-Riverside. Some of Á the pesticides are registered for use on
the intended target; others are experimental. We do not always use labeled rates
in our trials because these trials are for experimental purposes — to
increase our knowledge about the products and their capabilities. Labels
constantly change; therefore, it is always the pesticide applicator’s
responsibility to follow all label directions. No endorsement is intended for
products mentioned, nor is criticism meant for products mistakenly omitted.
There are a relatively large
number of insecticides available for chemical control of whiteflies, providing
many options for rotation (see Table 1, page 59). In addition to Marathon
(imidacloprid), several new chloronicotinoids like acetamiprid (Aventis), Forte
(a new formulation of imidacloprid, Bayer), thiamethoxam (Syngenta) and
thiacloprid (Bayer), have been registered recently or are in the development
phase. All of these materials are very effective against whiteflies. In
addition, a new Novaluron from Crompton/Uniroyal (Pedestal) looks very
promising (see Figures 3-4, pages 60-62).
Whiteflies typically have
several generations each year, especially in a greenhouse environment. They
take about 21-25 days to develop to an adult, depending on temperature. Because
the nymphs are on the plants for a long time and passing through many molts,
whiteflies are susceptible to pesticides that act as IGRs like Distance
(pyriproxifen, Valent USA) and Pedistal (novaluron, Crompton/Uni-royal). For
optimal control on a long-term crop, pesticide class should be rotated every
generation or two, or about every 4-6 weeks.
Although many effective Á
strategies are available to manage most whitefly pests, the relatively recent
introduction of several new whitefly pest species and whitefly-transmitted
pathogens emphasizes the need to constantly be on the alert for the spread of
these pests or the establishment of new exotic species.