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Global
cooperation
needed to combat
leafminer fly |
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The
leafminer fly
(Liriomyza huidobrensis),
an insect pest that attacks
potatoes and a range of
other horticultural crops,
is fast becoming a worldwide
cause for concern to farmers,
consumers, and environmentalists |
The tiny leafminer, sometimes
known as the pea leafminer,
emerged from its home in the
Americas sometime in the 1970s,
invading farms in Africa, Asia,
and Europe. Once thought to
have spread to developing countries
from Europe, scientists now
believe that the leafminer traveled
from its place of origin in
Central America on vegetable
and flower exports. It is now
considered a significant threat
to agriculture and the environment.
“Leafminer is nearly as
serious a problem as the whitefly,”
says Pamela Anderson, the International
Potato Center’s Deputy
Director General for Research.
“It’s found throughout
the tropics and is fast becoming
a leading cause of pesticide
abuse.“ Anderson, an entomologist,
was the head of the CGIAR Tropical
Whitefly IPM Project until she
joined the Center in June of
2002. |
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Yellow traps, first
tested in Cañete
Valley, are used to
monitor leafminer
fly populations. |
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CIP
researcher Warsito
Tantowijoyo and farmer-
researcher Gunawan inspect
a field during a project
planning workshop in
Tanah Karo, North Sumatra. |
Test case in Cañete
CIP first began working on the
leafminer fly problem in the
1990s in Peru’s Cañete
Valley, one of the country’s
leading agricultural zones.
This work is well known in agricultural
circles as the first successful
test case of classical integrated
pest management (IPM) for leafminer
fly, says Aziz Lagnaoui, a former
CIP IPM specialist now working
with the World Bank’s
Department of Sustainable Development.
“Cañete was
a convenient place to start,
and the problems we encountered
there were significant. In many
instances farmers’ fields
looked like they had been attacked
by flamethrowers,” Lagnaoui
says.
At the time, the valley’s
potato producers were spraying
pesticides in dangerous chemical
cocktails, mixing compounds
such as methyl parathion with
other toxic insecticides. Virtually
all of the chemicals being used
in Cañete had been classified
by the World Health Organization
as “extremely hazardous.”
The Cañete studies, which
were conducted in cooperation
with the NGO Valle Grande and
local farmers, showed that the
valley’s leafminer fly
explosion was directly linked
to pesticide misuse. Even so,
as the problem intensified,
farmers used increasing amounts
of chemicals, in some cases
spending up to US$800 per hectare.
It was obvious, Lagnaoui says,
that the insect had developed
resistance and that the chemicals
were no longer working.
Research in Cañete Valley
eventually led to a major shift
in strategy. Instead of attacking
the pest during its adult stage,
farmers were shown how to use
parasitoid wasps to kill the
larvae before they could grow
to adulthood and reproduce.
The tiny wasps, which feed only
on leafminers, lay their eggs
on or near the larvae, which
serve as a food source.
Other measures that contributed
to the success included the
use of less toxic pesticides
combined with crop rotation
when possible, practices that
help prevent the pest from developing
resistance, or at least delay
it. In addition, farmers were
taught how to monitor infestation
level before spraying. Valle
Grande officials would later
testify to the results: the
research saved farmers a great
deal of money and helped to
promote a more sustainable way
of controlling the pest.
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Leafminer
damage (above) is
evident in the fields
of
Banjarnegara, Central
java. |
Population dynamics
The progress made in managing
the problem was encouraging,
but by the mid-1990s the pest
was spreading rapidly to other
areas. Leafminers had become
a problem in Southeast Asia,
for example, and were already
a major pest in the highlands
of Indonesia where they attacked
potato and vegetable crops.
In each of the locations where
it was found, the pest had become
increasingly difficult to control
with insecticides, and there
were indications that the natural
enemy complex had been disrupted.
It was agreed that basic studies
on the population dynamics of
the pest and its natural enemies
would be needed to make informed
recommendations for an integrated
management scheme.
In 1997, CIP scientists initiated
the first in a series of population
dynamics studies. By collecting
basic information about the
pest and associated natural
enemies, the studies provided
a better understanding of the
pest’s biology and behavior,
as well as its interactions
in different farming systems
and ecologies. Indonesia was
considered an ideal location
to conduct the studies because
of its tropical environment
and because the leafminer was
a relative newcomer to the area.
“Population dynamics
studies can be tedious and even
back-breaking research, but
without them you’re working
in the dark,” says Anderson.
“The natural tendency
is to say that leafminer is
a problem, so let’s do
something about it. Until you
have a basic understanding of
how the insect relates to the
environment and to the farmers’
production system, however,
it’s unlikely that you’re
going to make progress towards
sustainable management.”
Making way for natural enemies
Working with funds provided
by the Australian Centre for
International Agricultural Research,
CIP scientists studied the leafminer
and its natural enemies in close
collaboration with local farm
groups, national research institutes,
and a variety of nongovernmental
organizations. The trials, which
are on-going, continue to provide
important new information on
the biology and ecology of the
pest and its natural enemies.
One key finding has been that
in potato plots where no insecticides
were applied over three cropping
cycles, leafminer populations
gradually decreased, along with
damage to the crop. The studies
also showed that parasitoid
species emerging from leafminer
pupae had become more diverse,
increasing the chances of controlling
the pest without insecticides.
In North Sumatra, where the
pest was previously abundant,
the studies showed that the
insects had all but disappeared.
Moreover, when farmers who had
stopped potato production resumed
growing the crop, the leafminer’s
natural enemies that had re-established
themselves continued to suppress
the pest. Equally encouraging
was the fact that participating
farmers began to recognize the
negative link between pesticides
and leafminer infestation.
Global project
“We’ve also learned
from the studies that the best
way to tackle leafminer fly
is globally and collaboratively,”
Anderson adds. The problem is
simply too big, too widespread,
and too important for any one
organization to handle it on
its own.
For example, taxonomists estimate
that there are approximately
3,000 different species of leafminers,
although only four species cause
serious damage to food crops.
Those four, however, can be
hard to distinguish. “To
move forward, we will need the
services of molecular taxonomists
who can identify with certainty
the existence of the pest at
specific locations,” says
Anderson. Similar services would
be required for leafminer predators
and parasitoids, which are also
difficult to identify.
“It’s our hope that
in the future leafminer research
will be conducted globally so
that we can attack the problem
from a systems perspective,”
Anderson says. At the moment,
CIP research is limited to the
species that attack potatoes.
A global project would address
all major species across a range
of cropping systems and ecologies.
It would also involve researchers
from many different national
and international organizations,
drawing in financial resources
from multiple sources and eliminating
duplication of effort.
“What we would like to
see is a program that pulls
together the best research currently
available, fills in the gaps
in our knowledge, and eventually
provides us with a comprehensive
overview of the problem,”
Anderson says. Critical to that
effort would be the use of modern
molecular techniques, geographic
information systems, and computer
modeling. The project, Anderson
estimates, would cost approximately
US$7.5 million over a five-year
period.
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Hope
for a leafminer-resistant
potato
The long-term
goal of a population
dynamics study
is to set the
stage for the
development of
recommendations
for an integrated
pest management
(IPM) program
that farmers can
use to effectively
and sustainably
manage pest infestations.
IPM “packages”
typically include
traps, biological
control agents,
selective insecticides,
and, when available,
resistant varieties.
At the present
time, there are
no commercial
potato varieties
with high levels
of resistance
to leafminers.
Recent trials,
however, have
identified five
advanced lines—two
of them with high
levels of resistance—that
may prove useful
in managing the
pest.
“The resistance
was discovered
by chance when
a series of CIP
breeding lines
entered standard
evaluation trials
in 2002,”
says Meredith
Bonierbale, Head
of CIP’s
Crop Improvement
and Genetic Resources
Department. “These
particular lines
were developed
for other purposes,
such as for resistance
to late blight
or production
from true potato
seed,” she
says. “Fortunately,
the broad genetic
base of CIP’s
breeding populations
and their exposure
to diverse stress
conditions during
selection enables
us to identify
varieties that
will be robust
in countering
various pest and
disease problems
beyond our primary
targets.”
Experiments conducted
with farmers in
Peru’s Cañete
Valley will help
to test whether
or not the new
lines can be used
to produce a successful
crop without resorting
to pesticides.
If the trials
are successful,
it should then
be possible to
boost the effectiveness
of future IPM
schemes by providing
farmers with the
option of planting
resistant varieties.
The availability
of a leafminer-resistant
potato, entomologists
say, could reduce
the number of
times a farmer
needs to apply
insecticides by
70 or 80 percent. |
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