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The Tomato Magazine
December 2006
Improved Methods of Whitefly Control
By Laura McGinnis
Agricultural Research Service Information Staff
Is there a way to control pest populations
without abusing insecticides? Absolutely,
say Agricultural Research Service
(ARS) scientists in Maricopa, Arizona, and
researchers from the University of Arizona.
The silverleaf whitefl y, a common cotton
pest, damages and kills crops by sucking
their sap, spreading viruses, and excreting a
sticky substance called honeydew. Spraying
is a fairly effective control method, but
some whitefl ies are distressingly resistant,
and many growers use costly chemicals
conservatively. ARS scientists at the U.S.
Arid-Land Agricultural Research Center
are investigating ways to improve whitefl y
population management with minimal insecticide
use.
Predator Identification
Biological control—reducing pest populations
by using natural enemies—is one tactic
that can replace or reduce spraying. Lab
scientists developed a technique to identify
predators by testing their guts for evidence
of whitefl y consumption. Using this method,
scientists James Hagler and Steven Naranjo
identifi ed predation frequency for 18 whitefly predators, many of
them previously unidentified.
Hagler developed the technique in the
early 1990s, when whitefly populations
were surging throughout the United States
and scientists had little
information on their
predators. Using gut
analysis, Hagler could
screen more than 1,000
predators per day. This
helped to quickly identify
which ones should
be conserved for optimal
biological control.
The whitefly-specific
ELISA (enzyme-linked
immunosorbent assay)
rapidly identifies natural
predators. First, the
suspected predator is crushed and placed on
an ELISA plate. Proteins from the insect’s
guts bind to the plate. Next, the scientist
adds a monoclonal antibody that binds only
to whitefly proteins.
The scientist then adds a secondary antibody,
which can only bind
to the plate in the presence
of the whitefly antibody.
A catalyst is added to
show whether binding has
occurred. A color reaction
indicates that it has, meaning
the insect has eaten a
whitefly. This procedure
allows scientists to identify
potential predators without
disturbing the field’s natural
order with cages or other
restrictions.
Predator-Prey Interaction
Understanding pest-predator
interaction helps scientists
develop more effective
management practices.
Hagler also uses ELISA to
study pest and predator dispersal patterns by
marking them with proteins. The insects are
released, recaptured, and analyzed by protein-specific ELISAs. He has
found proteins
more reliable than traditional markers like
paints, dyes, dusts, and trace elements.
In the fi rst open-field study using protein
to mark insects,
Hagler and his colleagues
tracked the dispersal of
Eretmocerus emiratus, a
parasitic wasp. More males were recovered
than females, indicating different dispersal
habits. That’s important information for
farmers who want optimum performance
from biological control agents. Currently
this technique is being adapted to study
dispersal characteristics of
termites, mosquitoes, honey
bees, ants, and many other
insects.
While gut analysis helps
to identify predator species,
it does not measure
their impact. Naranjo and
University of Arizona
researcher Peter Ellsworth
conducted extensive studies
of how whiteflies die. By
tracking individual immature
whitefl ies in the field, they
identifi ed common causes
of death, such as predators,
parasites, and dislodgement.
They also discovered that
whitefl ies were most vulnerable
to predation during the
fourth nymphal stage. This
led them to recommend conserving natural
predators and targeting whitefl ies during
stage four.
Naranjo and Ellsworth found that natural
death rates, though fairly high, are inadequate
to reduce pest populations. According
to their research, death rates of immature
stages must exceed 98 percent to maintain a
stable population, and reducing their ranks
requires an even higher rate. Naranjo concluded
that biological control alone is not
enough to suppress whitefly populations. Moderate
spraying may reduce pest
numbers, but conventional
insecticides can kill
predator and prey alike.
So how do growers
protect fields without
harming the biological
control agents they’ve
enlisted?
“Conservation of natural enemies by
using selective insecticides is a major program
component,” says Naranjo. He and
Hagler recommend complementing biological
control with insect growth regulators.
With Ellsworth, they studied the effects of
growth regulators buprofezin and pyriproxyfen
on 20 common predators. The researchers
found that common insecticides reduced
the population of all predator groups,
whereas the growth regulators only reduced
the densities of eight—and then at a lower
rate.
Even within those reduced groups, the
predator-to-prey ratio was higher with
growth-regulator use, indicating that it
poses a greater threat to whitefl ies than to
their enemies. Further mortality studies by
Naranjo and Ellsworth confi rmed that conservation
through use of selective insecticides
leads to higher predation rates. Hagler
and Naranjo also used the gut analysis to
monitor the sub-lethal effects of insecticides
and growth regulators on predators’ feeding
habits, confi rming that buprofezin and pyriproxyfen
are gentler on most predators than
conventional insecticides.
Research Benefits
ARS and University of Arizona research
has contributed to more effective whitefly
control, benefi ting the growers, the public,and the environment. The scientists
have
developed a successful program for
integrated pest management, giving cotton
growers alternatives to insecticides. “We
have shown that conservation biological
control is possible, to a limited extent, if
you know your key natural enemies and use
selective insecticides,” Hagler says. Naranjo
agrees.
They attribute the program’s success
to pest-avoidance methods like biological
control, careful insecticide use, and predator
conservation. Their work is part of a growing
knowledge base, helping decrease insecticide
use for whiteflies by about 85 percent
since 1995. Naranjo believes the whitefly
management program is responsible for “a
signifi cant insecticide use reduction.”
The research team’s recommendations
for preventive action, selective insecticides,
and biological control have helped growers
respond more effectively to pest invasions.“
The more we can exploit pests’ natural
enemies through conservation biological
control, the less we have to rely on pesticides,” Hagler says.
Editor’s note: The article, “Beyond Insecticides:
Improved Methods of Whitefl y Control,” was
published in the April 2006 issue of Agricultural
Research magazine and was updated on October
25, 2006. This research is part of Crop Protection
and Quarantine, an ARS National Program (#304)
described on the World Wide Web at http://www.nps.ars.usda.gov/. James
Hagler and Steven Naranjo are in the USDA-ARS U.S. Arid-Land
Agricultural Research Center, 21881 N. Cardon
Ln., Maricopa, AZ 85239; phone (520) 316-6300,
fax (520) 316-6330.
© 2006 Columbia
Publishing
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