Silverleaf Whitefly Management in California
The silverleaf whitefly, Bemisia argentifolii, previously known as the sweetpotato whitefly strain B, has caused major crop losses in southern California and in
other states over the last several years. The whitefly attacks a broad range of hosts and also has the potential to spread several viruses that can devastate crop plants.
IDENTIFICATION
Adult silverleaf whiteflies have white wings with no dark
markings and a yellow body; their wings are held somewhat vertically tilted like the peaked roof
of a house, instead of flat over their bodies as is the case with greenhouse whitefly. Fourth instar nymphs are transluscent
whitish yellow and have no waxy filaments around their edges;
there is little wax in colonies, unlike most other whitefly species. Nymphs on hairless leaves are oval in shape, and on very
hairy leaves they commonly have distorted borders.
HISTORY
Bemisia tabaci, the sweetpotato whitefly, has been a pest in cotton, melons, lettuce, and
certain other crops in California since at least 1925. However, it did not become a serious
problem in the state until 1981 when it caused millions of dollars of losses in the Imperial and Palo
Verde valleys largely due to transmission of lettuce infectious yellow viruses in cucurbits and
lettuce and squash leaf curl virus. Virus transmission continued
to be a concern with the whitefly in these crops through the 1980s.
In 1990 and 1991,whiteflies reached very high population levels and caused near total crop losses
in cucurbits and other vegetables in southern California, Texas,
and Arizona, primarily due to direct impact of the whiteflies rather than virus transmission. The whitefly also caused economic losses in
crops that had not suffered damage in the past. As a result, researchers and growers began to
suspect that a new strain of the whitefly had evolved. This
strain was commonly called "strain B" to differentiate it from the strain ("strain
A") of the sweetpotato whitefly that had been present for many years without causing such
severe economic losses. In 1994, researchers at University of California Riverside published a
paper designating it a new species of whitefly, the silverleaf
whitefly, B. argentifolii. For practical purposes, the new species is difficult to distinguish in the field from the sweetpotato
whitefly. Furthermore, the sweetpotato whitefly is thought to have been completely displaced by
this new pest and not present in California any longer.
DISTRIBUTION AND PHENOLOGY
The silverleaf whitefly is a heat loving species and has caused
the most serious economic losses in the southern desert valleys of California. It has also been
recorded in high populations in the southern San Joaquin Valley
and is present in some fields and landscapes throughout the San Joaquin and Sacramento valleys,
although not commonly at damaging levels. It is also present and sometimes damaging on
landscapes and crops in other parts of southern California. The pest causes economic damage in
greenhouses throughout the U.S.
Field populations of whiteflies build up gradually in spring and
summer, usually peaking in early fall and dropping dramatically at the onset of cold weather,
which also coincides with the loss of many host plant species. The developmental thresholds for sweetpotato whitefly are
50'F base, 90'F upper and 582 DD per generation. Based on 30 year temperature averages,
anywhere from 7 to 14 generations can be expected per year. Silverleaf whitefly may have a
higher upper threshold and more generations yearly in warmer areas. Typical silverleaf whitefly
populations build up over the summer, peaking in early fall.
HOSTS
The whitefly has a wide range of plant hosts, including many agricultural crops and ornamental plants. In
the Imperial Valley, extensive surveys have indicated a typical cultivated host sequence of cucurbits in the spring, cotton in the summer, and alfalfa and cole
crops in the late fall and winter. Major weed hosts include velvetleaf and sowthistle in the spring,
and sunflower and groundcherry in the early fall.
DAMAGE
Silverleaf whitefly feeding can reduce yield directly due to its removal of plant sap. Plants are
commonly damaged by excretion of sticky honeydew, which
supports the growth of black sooty mold. Feeding can
additionally cause discoloration of certain crop plants--including silverleaf in squash, irregular
ripening in tomato, white stalk in broccoli and cauliflower,
and white petiole in sugar beets. The whitefly also has the potential to cause serious losses
through the transmission of viruses as discussed below.
VIRUS TRANSMISSION
The piercing-sucking mouthparts of whiteflies provide an
excellent mode for transmitting disease-causing viruses from
one plant to another. In the 1980s, the sweetpotato whitefly was associated with serious losses in
lettuce and cucurbits through the transmission of lettuce
infectious yellows and squash leaf curl viruses. The currently
prevalent silverleaf whitefly is not as effective in transmitting these viruses but has been associated
with transmission of the geminiviruses, which can be extremely
virulent. Geminiviruses have already been identified damaging tomato, pepper and squash in the
southern United States. Affected plants may show vein clearing or yellowing, golden or yellow
mosaic patterns, crumpling and distortion of leaves, and stunted or distorted plant growth.
However, all these symptoms may be caused by other plant viruses and some may be associated
with insect feeding or physiological disorders. Diagnosis of gemiviruses must be done by a
trained plant pathologist. Only two geminiviruses transmitted by Bemisia spp. currently
occur in California -- squash leaf curl geminivirus and cotton leaf crumple geminivirus -- both only
in southern desert areas. However, neither currently is causing major economic loss.
MANAGEMENT ON AGRICULTURAL CROPS
Management guidelines for the whitefly will vary from crop to crop. Consult the University of
California IPM Pest Management Guidelines available from
your county UC Cooperative Extension office for materials and practices recommended in your
crop. However, some general comments can be made.
Monitoring
Growers and PCAs growing susceptible crops in any areas threatened by the whitefly should keep
a vigilant watch for the pest. Yellow sticky traps can indicate
when adults are moving into a field or area, especially when large numbers of traps are monitored
in a regional program. Once the whitefly is established in a field, regular counts of adults and nymphs on leaves provide a good record of whitefly activity and
can help in determining need for treatments.
Transplants
Transplants are a major vehicle for the spread of the whitefly into new areas. An even greater
concern is the whitefly-vectored geminiviruses. Growers using transplants should check their source
of plants to determine where they were grown and what control practices were used against the
whitefly and geminiviruses. Growers should also inspect transplants carefully upon arrival and
over the first three weeks for signs of whiteflies and geminiviruses. Bring any suspected virus symptoms or whiteflies to your county agricultural
commissioner's office for identification.
Cultural Controls
Imperial Valley growers found that relying on a combination of cultural practices, together with
insecticide treatments when needed, has been essential in
managing the silverleaf whitefly. Fields relying primarily on cultural rather than chemical controls
are likely to get the most help from natural enemies. Reliance
on cultural controls may also help slow development of pesticide resistance. To get maximum benefit from cultural
controls, every grower in the area must cooperate to bring down valley-wide populations of the
whitefly. The following practices are major components of the cultural control program:
- Disc under susceptible crops immediately after harvest to kill
all remaining nymphs and prevent population buildups
- Control volunteer host plants and weeds that come up after
discing fields.
- Alter planting times to avoid high whitefly populations. For
instance, growers in Texas are planting early in the spring to
avoid high infestations later in the summer. It may also be
possible to avoid problems by delaying planting of fall crops.
- Avoid planting susceptible host crops next to crops infested
with whitefly or downwind from whitefly sources.
- Control vegetation along field edges, ditch banks, roadsides,
and other noncrop areas that may be overwintering sites.
- For small acreage, high value crops, consider floating row
covers where the whitefly is a serious problem.
Although natural enemies alone will not currently control
damaging populations of the silverleaf whitefly, fields without pesticide sprays or which are sprayed with less persistent materials
such as oils or soaps can get some benefit from natural enemies. Whitefly nymphs are preyed upon by bigeyed bugs, lacewing larvae, and other general
predators. Several parasitic wasps, including species in the
Encarsia and
Eretmocerus genera
attack whiteflies. A ladybeetle,
Delphastus pusillus, is being introduced into
southern California to assist in biological control. Research into finding other potential natural
enemies is ongoing at the University of California, USDA, and in other parts of the country.
Chemical Control
Do not spray whitefly infested plants unless it is necessary to prevent economic losses. Insecticides are not always effective against the pest, and frequent
spraying may make the problem worse by hastening development of resistant populations.
Judicious use of insecticides will also help to preserve populations of natural enemies of whiteflies and other pests. If you start to see
economic losses, alternating sprays of insecticides with different modes of action may help delay
the development of resistance. Economic thresholds for
whitefly damage have not been developed for many crops. As a general guideline, however, the
economic threshold used by cotton growers -- 10 whitefly adults per leaf -- may be appropriate.
If sprays must be applied, good underleaf coverage is
essential.
Specific Crop Guidelines
Available pesticides and specific management guidelines vary
significantly from crop to crop. Obtain a copy of the UC IPM Pest Management Guidelines for
your crop from your UC Cooperative Extension office for specific guidelines.
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Parts of this material may be reproduced for educational use. Please credit "United States
Department
of Agriculture, WHITEFLY KNOWLEDGEBASE"