Transgenic Cotton for
Insect Control
Peter C. Ellsworth, Ph.D.
IPM Specialist, University of Arizona
Maricopa Agricultural Center
Maricopa, AZ, USA
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Disclosure
• Those engaged in the dialog on biotechnology
should fully disclose their relationships and
opinions “up front” so that audiences can
consider the context.
• Partial support for my research comes from
companies with interests in biotechnology.
• The balance of support comes from state and
federal sources of competitively available public
funds.
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Disclosure (continued)
• Biotechnology and its
products are neither
inherently good nor bad.
• The specific process and
each of its products should
be scientifically and
independently evaluated.
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Transgenic Cotton for Insect
Control
• What is available now & in the
future?
• Origin, identity & development
• Insect target(s) in the U.S.
• Efficacy & utility in the Arizona
system (benefits)
• Safety (risks)
–
–
–
–
Resistance
Impact of gene on plant
Biodiversity
non-target effects
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Products Available for
Cotton Insect Control
• Only 1 ‘trans’-gene has been commercialized
• Based on the crystalline protein produced by
Bacillus thuringiensis (Bt)
• Developed by Monsanto as Bollgard® and
incorporated into commercial varieties by several
cotton seed companies (e.g., Delta Pineland Co. &
Stoneville Pedigreed Seed Co.)
• Sold in the U.S., Australia, Mexico, South Africa,
India, China, Argentina, Indonesia
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Bacillus thuringiensis (Bt)
• Common soil bacterium
• Present in nature in a variety
of forms (species & strains)
• Produces proteins that are
toxic to insects
• Commonly used in garden
sprays & for commercial
agriculture, including organic
farming
• Extremely well-known toxin in
terms of human health &
environmental safety
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Bacillus thuringiensis (Bt)
• Crystalline proteins are
classified according to
structure & have a specific
nomenclature (e.g., Cry1Ac)
• Cotton has been transformed
with Cry1Ac (narrow
spectrum; Lepidoptera only)
• Protein binds with receptors
in the insect gut causing
pores which perforate the
midgut & lead to cell leakage
& insect death
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The Transformation
• The gene of interest is
spliced out of the bacterium
using a vector, like
Agrobacterium tumefasciens,
& transferred to cotton cells
grown in tissue culture
• The cells are grown into a
plant & then, after testing,
plants are back-crossed into
commercial lines to make
new varieties
Coker 312
Recurrent
back-crossing
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Spectrum of Activity for BG
Excellent
Control
Tobacco
No
Budworm, the
Control
Trichoplusia niprincipal pest
Spodoptera exigua
in the South
Heliothis virescens
Spodoptera frugiperda
Spodoptera ornithogalli
Pectinophora gossypiella
Bucculatrix thurberiella
Pink Bollworm
Estigmene
acrea
(PBW),
our
Helicoverpa zea
principal pest
(pre-bloom)
Helicoverpa zea
(post-bloom)
Marmara spp.
Beneficial
Insects
Agrotis &
Feltia spp.
Pseudoplusia includens
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AZ’s Primary Lepidopteran Pest
• Pink Bollworm
• Multiple generations
• Adult lays eggs on bolls
or susceptible squares
(SS)
•Larvae hatch &
penetrate bolls
within 24 hrs
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Alternatives for PBW Control
• Repeated, broad-spectrum
sprays are required to prevent
moths from invading fields
• No effective larvicides or
ovicides
• Biological controls are limited
by the biology of this pest
– Little impact of parasitoid or
predators
• Cultural controls can be very
effective
– Requires early termination &
areawide compliance with
plowdown requirements
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Secondary Lepidopteran Pests
• Occasional pests
Helicoverpa zea
Heliothis virescens
Trichoplusia ni
Estigmene acrea (Arctiidae)
• Induced pests
Spodoptera exigua
Bucculatrix thurberiella
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Bt Cotton Questions
• Efficacy & economic studies
– How effective is the gene?
– Are oversprays required for
lepidopteran control?
– If so, are there new scouting &
threshold considerations?
Ca. 100% for PBW
Not for PBW
Search for large larvae
• Agronomic studies
– Impacts (+/-) on yield & fiber qualities? No unintended effects
• Product integrity & stability studies
– High-dose through life of plant?
– High-dose in all varieties?
– Purity?
Yes, actively growing
No, some not marketed
> 98% (?)
• Ecological studies
– Impact on non-target organisms (NTO) No unintended effects
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BG Cotton Efficacy
• Young larvae present regardless of cotton type
• Little difference between Bt & non-Bt (-) varieties
Infested Bolls (%)
100
90
19-Sep 12-Oct
19-Sep 12-Oct
19-Sep 12-Oct
19-Sep 12-Oct
E
80
70
60
4
50
40
3
30
20
2
10
1
0
- BG - BG
- BG - BG
- BG - BG
- BG - BG
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BG Cotton Kills Small Larvae
• PBW larvae must feed in order to be killed.
• Large larvae survive mainly in non-Bt varieties.
Infested Bolls (%)
100
90
19-Sep 12-Oct
19-Sep 12-Oct
19-Sep 12-Oct
19-Sep 12-Oct
E
80
70
60
4
50
40
3
30
20
2
10
1
0
- BG - BG
- BG - BG
- BG - BG
- BG - BG
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Impact on Arizona Cotton
• In 1990, > 6.8 sprays were made against PBW;
still, > 5% yield loss
• Since 1996 when Bt cotton was introduced, it has
never required oversprays for PBW control, AND
• Since 1997, only 0.5 sprays have been made
against PBW over all cotton acreage (Bt and nonBt); i.e., an areawide reduction of PBW has
occurred
• The net reduction in insecticide use has resulted
in huge savings to farmers, and large
improvements to the agroecosystem in terms of
beneficial insect communities & IPM
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Safety - Resistance
• Given time & exposure, insects
have the capacity to overcome
most insecticides. Bt cotton may
be no different, however, there
are safeguards:
• Refugia
• High-Dose Strategy
• Development of additional
proteins
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Refugia
• Objective: provide harborage for susceptible
moth production to reduce the chance of
resistant (R) moths mating with each other
RR
RR
RR
SS
RS
SS
RR
• U.S. growers are required to plant a proportion of
their acreage to non-Bt cotton
– 5% Refuge, if no lepidopteran-active insecticides are
used on it, or else
– 20% Refuge
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High-Dose Strategy,
Depends on:
• The production of a dose high enough to kill:
>99.9% of a susceptible (SS) population, and Yes
>95% of the heterozygous (RS) individuals,
Yes?
• A recessive resistance,
Yes
• Random mating,
Yes, *refuges
• A low initial frequency of the ‘R’ allele.
No (?)
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Development of Additional
Transgenes (Bt’s)
• Bollgard II®
– 2 Bt gene product, original Bollgard (Cry1Ac)
+ Cry2Ab
– Final stages of US-EPA approval
– Limited commercial production in 2003
– Full replacement of BG varieties by 2008?
• Bollgard III
– Little information on this available at this
time; research stages only
• Cry1F
– Under development by Dow Agrosciences in
combination with Cry1Ac
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Impact of Gene on Plant
• Isogenic lines were developed for testing the
impact of the gene(s) on agronomic and efficacy
characteristics of the plant
C312B
DP50
Particle gun
DP50B
DP50II
(Cry1Ac)
(Cry1AC+Cry2Ab)
Lines
1.
Cry1Ac+
Cry2Ab
2.
Cry1Ac
only
3.
Cry2Ab
only
4.
Null
DP50
Cry1Ac
Cry2Ab
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Isoline Studies of BG & BGII
• Replicated studies
• Artificial & natural
PBW infestations
• Sprayed & Unsprayed
conditions
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Warts are often formed
at the site of PBW attack
Dead 1st instar
in Bt cotton
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Pink Bollworm per boll
BGII Results - PBW, 1st Instars
1st live
2nd live
3rd live
4th live
Pupae
1st dead
2nd dead
3rd dead
4th dead
Exits
5
2
4
Dead 1st Instars
3
Live 1st Instars
1
2
1
0
0
50
50B
50BII
50
50B
50BII
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Pink Bollworm per boll
BGII Results - PBW, All Instars
1st live
2nd live
3rd live
4th live
Pupae
1st dead
2nd dead
3rd dead
4th dead
Exits
5
2
4
3
1
2
1
0
0
50
50B
50BII
50
50B
50BII
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BGII Results - B. thurberiella
Bottom
Middle
CLP Mines per leaf
50
40
30
20
10
0
b
c
c
b
DP50BII
b
b
DP50B
Top
• BGII prevented cotton
a
leafperforator
development better
a
a
than BG
• Leaves at top of plant
(younger) express
highest doses of Bt
• Older leaves (bottom)
have reduced doses of
Bt
DP50
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Marmara sp.
• Citrus Peel Miner is an incidental
lepidopteran that mines the main
stem and boll surfaces
• Cry2Ab alone (‘X’) is more effective
than Cry1Ac (‘B’)
Citrus Peel Miner Presence (per 10 plants)
10
a
a
a
NL S
8
6
b
b
4
2
b
c
c
50X
50BII
b
0
(-)
(-)
B
BII
50
50B
50B
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Spectrum of Activity for BG
(Cry1Ac)
Excellent
Control
No
Control
Pectinophora gossypiella
Heliothis virescens
Bucculatrix thurberiella
Marmara spp.
Helicoverpa zea
Estigmene acrea
Trichoplusia ni
Beneficial
Insects
Agrotis &
Feltia spp.
Pseudoplusia includens
Spodoptera exigua
Spodoptera frugiperda
Spodoptera ornithogalli
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Spectrum of Activity for BGII
(Cry1Ac + Cry2Ab)
Excellent
Control
Pectinophora gossypiella
Heliothis virescens
Bucculatrix thurberiella
Marmara spp.
Helicoverpa zea
Estigmene acrea
Trichoplusia ni
Pseudoplusia includens
Spodoptera exigua
Spodoptera frugiperda
Spodoptera ornithogalli
No
Control
Beneficial
Insects
Agrotis &
Feltia spp.
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High Dose and % Efficacy?
• Throughout our early work with BG cotton, we
often would find low levels of “survivors” from
our field plots
Infested Bolls (%)
100 12-Aug 31-Aug 11-Oct
90
80
70
60
50
40
30
20
10
0
- BG
Exits
4th
3rd
2nd
1st
- BG - BG
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Source of Survivors
• Low expression of Bt in
plants?
• Low levels of non-Bt
contaminants?
– In the seedbag
– From volunteer seed
• Resistance?
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Before plants
are tested for
presence of Bt
After PBW from
non-Bt plants
are discarded
% Efficacy Against PBW
Cry1Ac Cry2Ab Variety
+
DP50B
+
985X
+
+
985BX
+
DP33B
+
DP448B
+
DP458BR
+
+
DP33BX
+
SG215BR
+
SG125X
+
+
SG125BX
Raw* Adjusted
100
100
99.591 99.591
100
99.324
100
100
100
99.788
100
99.536
100
100
100
100
99.256 99.758
100
100
Cry1Ac
100%
Cry2Ab
99.67%
Both Genes
100%
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Biodiversity / NTO Studies
• The reports of Bt
effects on Monarch
butterflies have fueled
much emotional
debate on the use of
biotech crops.
• Monarch Butterfly,
symbol of nature and
“wildness” in North
America.
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Non-Target Organisms (NTO)
• Over 370 arthropod species have been tracked in
2 years of field studies using a variety of
methodologies.
• So far, no major or functional differences have
been found in Arizona between BG, BGII, and
conventional cotton communities…
• Except where harsh PBW sprays are needed in
conventional cottons.
• Thus, Bt cotton ecosystems are not only safe, but
safer than conventional cotton ecosystems where
insecticidal inputs are higher.
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Conclusions
• The use of Bt cottons in Arizona has provided the
first larvicidal and selective approach to controlling
PBW.
• The control provided by Bt cottons approaches
immunity. No survivors have been found in field
studies.
• Bt cotton has revolutionized our ability to
implement IPM in AZ cotton & reduced our
insecticide inputs by over 60%.
• Future transgenic products for insect control in
cotton should be independently & scientifically
tested.
• Other than new Bt genes/events, there are few, if
any, development plans for insect contol products.
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Information
• All University of Arizona
crop production & crop
protection information is
available on our web site,
A
C
I
S
• Arizona Crop Information
Site (ACIS), at
• http://ag.arizona.edu/crops
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