Pesticides
Pests and pesticides
? Pest,ANY UNWANTED organism that directly
or indirectly interfere human activity is called a
pest.
? Pesticides,or biocides,are substances that can
kill organisms that we consider to be
undesirable.
? The most widely used types of pesticides:
– Insecticides
– Herbicides
– Fungicides
– Rodenticides
Reasons for our use of pesticides
? Insects compete with us for food,It is estimated that 30%
of agricultural crops are consumed by insects worldwide.
Swarms of locusts like
this one in Morocco
can completely destroy
crops.
Reasons for our use of pesticides,
Some insects are diseases carriers
Disease Disease carrier
Malaria Anopheles mosquito
Bubonic plague Rat fleas
Typhus Body lice and fleas
Sleeping sickness Tsetse fly
The ideal pesticides
? Kill only the target pest
? Have no short- or long- term health effects
on non-target organisms,including people.
? Be broken down into harmless chemicals in a
fairly short time.
? Prevent the development of genetic
resistance in target organisms.
? Saving money compared with making no
effort to control pest species.
The case for DDT C
H
C
C l C l
C l
C l
C l
D D T
Para-dichlorodiphenyltrichloroethane
DDT,an almost ideal pesticide
? Chemically stable and degrades
slowly
? Low volatility
? Low solubility in water
? Readily penetrates the waxy
coating of insects
? Low toxicity to animals,including
human beings.
Each application is
effective for a long
time
DDT binds to the nerve cells of insects ? hold open the
molecule channels for sodium ions ? uncontrolled firing
of the nerves ? kill the insects.
The minus side of DDT,Bioaccumulation
Example,Accumulation of DDT in the aquatic food chain
Chemical basis for
bioaccumulation,
DDT is more
soluble in fats
than in water.
DDT and mosquito-lizard-cat connection
D D T s p r a y i n g f o r
m a l a r i a e r a d i c a t i o n
K i l l e d m o s q u i t o e s
a s i n t e n d e d
k i l l e d c a t e r - p i l l a r e a t i n g
w a s p s - u n i n t e n d e d
D e a d m o s q u i t o e s
s i c k e n e d
g e c k o l i z a r d s
S i c k l i z a r d s p o i s o n e d
l o c a l c a t s
F e w e r c a t s l e d t o
a n e x p l o s i o n o f
t h e r a t p o p u l a t i o n
R a t s a t e l o c a l c r o p s
a n d t h r e a t e n e d
a n o u t b r e a k o f
b u b o n i c p l a g u e
F e w e r w a s p s l e d t o
a n e x p l o s i o n o f
c a t e r - p i l l a r p o p u l a t i o n
C a t e r - p i l l a r s c o n s u m e d
t h a t c h e d r o o f s
Example,
Accumulation of
DDT in the food
chain on land
The minus side of DDT use
The accumulation of DDT inside birds’ tissue disrupts calcium
control mechanism during egg formation,The egg shells are so
fragile that they crack and do not survive until hatching.
Eagle eggs and DDT
The DDT ban (in 1972) was successful in improving the
survival rate of bald eagles.
DDT was outlawed in Hong Kong and China in 1983.
Insect resistance to DDT
DDE has a distinctly different shape than DDT and no longer
binds strongly to insert nerve cells.
Most pest species can develop genetic resistance to a chemical
poison through natural selection in a short time,(Thanks to the
rapid reproduction rate.)
C
H
C
C l C l
C l
C l
C l
D D T
I n s e c t e n z y m e
D D T - a s e C
C
C l C l
C l C l
D e h y d r o c h l o r i n a t i o n
D D E
Example
Suppose an insecticide is applied to a crop to eradicate fruit
flies,Assume that one out one million fruit flies possesses an
enzyme that breaks down the insecticide into nontoxic
metabolic products,Assume further that the normal fruit
flies die off quickly,the population of the resistant flies
increases geometrically (that is,1,2,4,8,… ),If a new
generation occurs every 23.5 days,in how many days will
the fruit fly population be restored?
The nature of resistance means that no
pesticides can remain effective for long,and
it spurs the drive to develop new insecticides.
DDT analogs:
C
H
C
H 3 C O O C H 3
C l
C l
C l
M e t h o x y c h lo r
The DDT analogs are not as rapidly attacked by resistant
insects’ DDT-ase
Effective component
retained
Make the new molecule
more water-soluble,thus
less persistent in the
environment.
Nonpersistent insecticides
? Break down rapidly into harmless and water
soluble products,once released into the
environment.
? Two classes of widely used nonpersistent
insecticides,
– Organophosphates
– Carbamates
? Both are neurotoxins
Organophosphates
General structure
S
P
O R
XO R
X, S R ' o r O R ' g r o u p
O
P
O R
XO Ro r
O
P
O C 2 H 5
OC 2 H 5 O
P a r a o x o n
S
P
O C 2 H 5
OC 2 H 5 O
P a r a t h i o n
S
P
O C H 3
SH 3 C O C H
C
C H 2
O
O C 2 H 5
C
O
O C 2 H 5
M a l a t h i o n
Neurotransmitter,Acetylcholine
? Neurotransmitters are chemicals that transmit
signals from one nerve fiber to another,Once the
nerve impulse has been transmitted,the
neurotransmitter is destroyed.
O
CH 3 C O C H 2 C H 2 N ( C H 3 ) 3
+
Acetylcholine
Acetylcholine is a neurotransmitter molecule responsible for
firing motor nerve cells in higher live forms,
Normal destruction of actylcholine
Enzyme regenerated.
Neuro signal transmission
completed
Inhibition of acetylchlorine destruction enzyme
by organophosphates
Organophosphate mimics
acetylcholine in binding
with the enzyme
Toxicity difference of parathion and malathion
Parathion and marathion possess similar insecticidal activity,but
malathion has much lower mammalian toxicity than parathion
(Malathion has a LD50 for adult male rats about 100 times that of
parathion.)
S
P
O C H 3
SH 3 C O C H
C
C H 2
O
O C 2 H 5
C
O
O C 2 H 5
M a l a t h i o n
c a r b o x y l a s e
H 2 O
S
P
O C H 3
SH 3 C O C H
C
C H 2
O
O H
C
O
O H
+ 2 H O C H 2 C H 2
The enzyme that accomplish malathion hydrolysis are possessed
by mammals,but not by insects.
Carbamates
O
C
O R N H R '
General structure
O C
O
N H C H
3
C a r b a r y l
O
C
O N H C H 3
N
H CC
C H 3
C H 3
H 3 C S
A l d i c a r b
Inhibition of acetylchlorine destruction enzyme
by carbamate insecticides
Broad-spectrum and narrow-spectrum
insecticides
? Broad-spectrum insecticides kill a wide range
of insects,including many that are beneficial.
– Examples,organochlorinated hydrocarbons,most
organophosphates.
? Narrow-spectrum insecticides are toxic to
only a few types of insects.
– Examples,most carbamates.
The case FOR pesticides
? Pesticides save lives.
– In WWII,at least 7 millions of lives were saved because use of
DDT and other pesticides kept insect-transmitted diseases
under control,
? Pesticides increase food supplies and lower food cost.
? Pesticides increase profits for farmers
– For every $1 spent on pesticides,$3-$5 would result from an
increase in crop yield.
? They work faster and better than other alternatives.
– Pesticides control most pests quickly and at a reasonable price.
– Easily shipped and applied.
– Have a relatively long shelf.
– When genetic resistance occurs,pests can still be controlled by
applying stronger doses and switching to other pesticides,
The case FOR pesticides (Continued)
? The health risks of pesticides are insignificant
compared with their health and other benefits,(see
next slide)
– When handled probably,pesticides are safe to use.
? Safer and more effective products are continually
being developed.
The case AGAINST pesticides
? Development of genetic resistance
? Killing of natural pest enemies
? Bioaccumulation
? Short-term threats to human health from
pesticide use and manufacture.
Alternative methods of insect control
? Crop rotation
– Planting a same crop year after year provide
opportunities for pests to multiply to uncontrollable size.
? Intercropping
– Confine insects’ living space
? Planting rows of hedges or trees in and around crop
fields
– Act as barriers to insect invasions
– Provide habitats for insects’ enemies
– Serve as windbreaks to reduce soil erosion
Alternative methods of insect control (Continued)
? Adjusting planting times
– Select the time window so that insect pests either
starve to death before the crop is available or are
consumed by their natural enemies.
? Applying insect sex attractants
– Either lure pests to traps containing toxic chemicals or
attract natural predators into crop fields.
– The sex attractants work on only one species,requires
little amount,do not cause genetic resistance.
– Expensive to produce
– Effective only against adults insects,do not control
juvenile forms.
? Applying insect hormones
Alternative methods of insect control (Continued)
? Biological control
– Introduce predators
– Genetic engineering
? Sterilization
– Breed and sterilize large numbers of the pest insect by
radiation,release the sterilized pests in the target area,
mate but produce no off-springs.
– Require the number of sterilized insects to be sufficient
high.
– The procedure is expensive
Study questions
1,Why does DDT bioaccumulate in the food chain?
Give an example of DDT bioaccumulation.
2,How does DDT kill insects?
3,How do organophosphates and carbamates kill
insects?
4,Why can insects easily develop resistance to
pesticides?
Pests and pesticides
? Pest,ANY UNWANTED organism that directly
or indirectly interfere human activity is called a
pest.
? Pesticides,or biocides,are substances that can
kill organisms that we consider to be
undesirable.
? The most widely used types of pesticides:
– Insecticides
– Herbicides
– Fungicides
– Rodenticides
Reasons for our use of pesticides
? Insects compete with us for food,It is estimated that 30%
of agricultural crops are consumed by insects worldwide.
Swarms of locusts like
this one in Morocco
can completely destroy
crops.
Reasons for our use of pesticides,
Some insects are diseases carriers
Disease Disease carrier
Malaria Anopheles mosquito
Bubonic plague Rat fleas
Typhus Body lice and fleas
Sleeping sickness Tsetse fly
The ideal pesticides
? Kill only the target pest
? Have no short- or long- term health effects
on non-target organisms,including people.
? Be broken down into harmless chemicals in a
fairly short time.
? Prevent the development of genetic
resistance in target organisms.
? Saving money compared with making no
effort to control pest species.
The case for DDT C
H
C
C l C l
C l
C l
C l
D D T
Para-dichlorodiphenyltrichloroethane
DDT,an almost ideal pesticide
? Chemically stable and degrades
slowly
? Low volatility
? Low solubility in water
? Readily penetrates the waxy
coating of insects
? Low toxicity to animals,including
human beings.
Each application is
effective for a long
time
DDT binds to the nerve cells of insects ? hold open the
molecule channels for sodium ions ? uncontrolled firing
of the nerves ? kill the insects.
The minus side of DDT,Bioaccumulation
Example,Accumulation of DDT in the aquatic food chain
Chemical basis for
bioaccumulation,
DDT is more
soluble in fats
than in water.
DDT and mosquito-lizard-cat connection
D D T s p r a y i n g f o r
m a l a r i a e r a d i c a t i o n
K i l l e d m o s q u i t o e s
a s i n t e n d e d
k i l l e d c a t e r - p i l l a r e a t i n g
w a s p s - u n i n t e n d e d
D e a d m o s q u i t o e s
s i c k e n e d
g e c k o l i z a r d s
S i c k l i z a r d s p o i s o n e d
l o c a l c a t s
F e w e r c a t s l e d t o
a n e x p l o s i o n o f
t h e r a t p o p u l a t i o n
R a t s a t e l o c a l c r o p s
a n d t h r e a t e n e d
a n o u t b r e a k o f
b u b o n i c p l a g u e
F e w e r w a s p s l e d t o
a n e x p l o s i o n o f
c a t e r - p i l l a r p o p u l a t i o n
C a t e r - p i l l a r s c o n s u m e d
t h a t c h e d r o o f s
Example,
Accumulation of
DDT in the food
chain on land
The minus side of DDT use
The accumulation of DDT inside birds’ tissue disrupts calcium
control mechanism during egg formation,The egg shells are so
fragile that they crack and do not survive until hatching.
Eagle eggs and DDT
The DDT ban (in 1972) was successful in improving the
survival rate of bald eagles.
DDT was outlawed in Hong Kong and China in 1983.
Insect resistance to DDT
DDE has a distinctly different shape than DDT and no longer
binds strongly to insert nerve cells.
Most pest species can develop genetic resistance to a chemical
poison through natural selection in a short time,(Thanks to the
rapid reproduction rate.)
C
H
C
C l C l
C l
C l
C l
D D T
I n s e c t e n z y m e
D D T - a s e C
C
C l C l
C l C l
D e h y d r o c h l o r i n a t i o n
D D E
Example
Suppose an insecticide is applied to a crop to eradicate fruit
flies,Assume that one out one million fruit flies possesses an
enzyme that breaks down the insecticide into nontoxic
metabolic products,Assume further that the normal fruit
flies die off quickly,the population of the resistant flies
increases geometrically (that is,1,2,4,8,… ),If a new
generation occurs every 23.5 days,in how many days will
the fruit fly population be restored?
The nature of resistance means that no
pesticides can remain effective for long,and
it spurs the drive to develop new insecticides.
DDT analogs:
C
H
C
H 3 C O O C H 3
C l
C l
C l
M e t h o x y c h lo r
The DDT analogs are not as rapidly attacked by resistant
insects’ DDT-ase
Effective component
retained
Make the new molecule
more water-soluble,thus
less persistent in the
environment.
Nonpersistent insecticides
? Break down rapidly into harmless and water
soluble products,once released into the
environment.
? Two classes of widely used nonpersistent
insecticides,
– Organophosphates
– Carbamates
? Both are neurotoxins
Organophosphates
General structure
S
P
O R
XO R
X, S R ' o r O R ' g r o u p
O
P
O R
XO Ro r
O
P
O C 2 H 5
OC 2 H 5 O
P a r a o x o n
S
P
O C 2 H 5
OC 2 H 5 O
P a r a t h i o n
S
P
O C H 3
SH 3 C O C H
C
C H 2
O
O C 2 H 5
C
O
O C 2 H 5
M a l a t h i o n
Neurotransmitter,Acetylcholine
? Neurotransmitters are chemicals that transmit
signals from one nerve fiber to another,Once the
nerve impulse has been transmitted,the
neurotransmitter is destroyed.
O
CH 3 C O C H 2 C H 2 N ( C H 3 ) 3
+
Acetylcholine
Acetylcholine is a neurotransmitter molecule responsible for
firing motor nerve cells in higher live forms,
Normal destruction of actylcholine
Enzyme regenerated.
Neuro signal transmission
completed
Inhibition of acetylchlorine destruction enzyme
by organophosphates
Organophosphate mimics
acetylcholine in binding
with the enzyme
Toxicity difference of parathion and malathion
Parathion and marathion possess similar insecticidal activity,but
malathion has much lower mammalian toxicity than parathion
(Malathion has a LD50 for adult male rats about 100 times that of
parathion.)
S
P
O C H 3
SH 3 C O C H
C
C H 2
O
O C 2 H 5
C
O
O C 2 H 5
M a l a t h i o n
c a r b o x y l a s e
H 2 O
S
P
O C H 3
SH 3 C O C H
C
C H 2
O
O H
C
O
O H
+ 2 H O C H 2 C H 2
The enzyme that accomplish malathion hydrolysis are possessed
by mammals,but not by insects.
Carbamates
O
C
O R N H R '
General structure
O C
O
N H C H
3
C a r b a r y l
O
C
O N H C H 3
N
H CC
C H 3
C H 3
H 3 C S
A l d i c a r b
Inhibition of acetylchlorine destruction enzyme
by carbamate insecticides
Broad-spectrum and narrow-spectrum
insecticides
? Broad-spectrum insecticides kill a wide range
of insects,including many that are beneficial.
– Examples,organochlorinated hydrocarbons,most
organophosphates.
? Narrow-spectrum insecticides are toxic to
only a few types of insects.
– Examples,most carbamates.
The case FOR pesticides
? Pesticides save lives.
– In WWII,at least 7 millions of lives were saved because use of
DDT and other pesticides kept insect-transmitted diseases
under control,
? Pesticides increase food supplies and lower food cost.
? Pesticides increase profits for farmers
– For every $1 spent on pesticides,$3-$5 would result from an
increase in crop yield.
? They work faster and better than other alternatives.
– Pesticides control most pests quickly and at a reasonable price.
– Easily shipped and applied.
– Have a relatively long shelf.
– When genetic resistance occurs,pests can still be controlled by
applying stronger doses and switching to other pesticides,
The case FOR pesticides (Continued)
? The health risks of pesticides are insignificant
compared with their health and other benefits,(see
next slide)
– When handled probably,pesticides are safe to use.
? Safer and more effective products are continually
being developed.
The case AGAINST pesticides
? Development of genetic resistance
? Killing of natural pest enemies
? Bioaccumulation
? Short-term threats to human health from
pesticide use and manufacture.
Alternative methods of insect control
? Crop rotation
– Planting a same crop year after year provide
opportunities for pests to multiply to uncontrollable size.
? Intercropping
– Confine insects’ living space
? Planting rows of hedges or trees in and around crop
fields
– Act as barriers to insect invasions
– Provide habitats for insects’ enemies
– Serve as windbreaks to reduce soil erosion
Alternative methods of insect control (Continued)
? Adjusting planting times
– Select the time window so that insect pests either
starve to death before the crop is available or are
consumed by their natural enemies.
? Applying insect sex attractants
– Either lure pests to traps containing toxic chemicals or
attract natural predators into crop fields.
– The sex attractants work on only one species,requires
little amount,do not cause genetic resistance.
– Expensive to produce
– Effective only against adults insects,do not control
juvenile forms.
? Applying insect hormones
Alternative methods of insect control (Continued)
? Biological control
– Introduce predators
– Genetic engineering
? Sterilization
– Breed and sterilize large numbers of the pest insect by
radiation,release the sterilized pests in the target area,
mate but produce no off-springs.
– Require the number of sterilized insects to be sufficient
high.
– The procedure is expensive
Study questions
1,Why does DDT bioaccumulate in the food chain?
Give an example of DDT bioaccumulation.
2,How does DDT kill insects?
3,How do organophosphates and carbamates kill
insects?
4,Why can insects easily develop resistance to
pesticides?
