Arthropod Environmental Management 2 Topics for Session 2 Botanical Insecticides Sabadilla,...

Arthropod Environmental Management 2 Topics for Session 2

Botanical Insecticides Sabadilla, Nicotine, Ryanodine, Quassia,

Rotenone, Pyrethrum, Neem Extracts, Essential Oils

Synthetic Insecticides Organochlorides, Pyrethroids, Organophosphates,

Carbamates, Neonicotinoids Growth Regulators

Moulting hormone mimics, Juvenile hormone mimics, Chitin Synthesis Inhibitors

Arthropod Environmental Management 2 Botanical Insecticides

Phytochemicals (plant chemicals) Often Secondary Plant Substances (in

contrast to primary substances) Examples include alkaloids (eg

nicotine), phenolic compounds, and terpenoids

Use dates back ca. 4000 years (continues in native cultures)

Western use since 16th C explorations

Arthropod Environmental Management 2 Botanical Insecticides – Current

Interest Cost of developing new insecticides Small proportion of plants evaluated Previous success with carbamates and

pyrethroids Newer successes with neonicotinoids

and with neem extracts Still used in native cultures

Arthropod Environmental Management 2

(some) Plants Still Used in Native Cultures for Pest Control

Custard Apple, Sweet Flag, Chrysanthemum, Spearmint, Derris, Garlic, Sabadilla, Neem,

Quassia, Chilli, Turmeric

Some authors suggest that ca. 2,400 species are used at the local level

Arthropod Environmental Management 2 Botanical Insecticides – Issues with

Commercial Development Farmers regard botanicals as old-fashioned Not on official lists Often do not get dramatic kills of pests Light and Air may inactivate May be seasonal availability Costs may be high May be of variable toxicity

Arthropod Environmental Management 2 Botanical Insecticides - Examples

Sabadilla Nicotine Ryanodine Quassia Rotenone Pyrethrum Neem Extracts Essential Oils

Arthropod Environmental Management 2 Sabadilla

Powdered seeds used by S. American Indians to protect crops from insect attack (from Sabadilla officinarum – a plant in the Lily family)

Discovered by European explorers Exported to Europe from 16th C to WWII (over

120,000 tonnes to USA in 1940s, use declined because of advent of DDT)

Actives are alkaloids, rapidly inactivated by exposure to sunlight and air (as are many botanicals)

Arthropod Environmental Management 2 Sabadilla

Contact and Stomach poison that is neurotoxic (sodium channels). This is similar to DDT and the pyrethroids

Good for pre-harvest treatments (because it is rapidly inactivated)

Use has largely been phased out because of cheaper alternatives, cannot be synthesised, must be extracted – still used in native cultures

Arthropod Environmental Management 2 Nicotine

Alkaloid nicotine from tobacco plants Discovered by European explorers, in use by

American Indians By 17th/18th Century widely used in Europe AI was named nicotine in 1828, in 1904 it

was chemically synthesised

Arthropod Environmental Management 2 Nicotine

Contact insecticide that works as an agonist for acetylcholine receptors on post-synaptic membranes

Not very selective (high mammalian toxicity in contrast to sabadilla), rapidly inactivated (48 hrs)

Still available for use in Europe in professional situations

More costly than modern synthetics so use largely phased out (originally made from low grade tobacco)

Like sabadilla is still used in native cultures

Arthropod Environmental Management 2 Quassia

From Central American tree Quassia amara

Exported from 18th C to WWII to Europe (from commercial plantations in Venezuela)

Contact and stomach poison from bark and branches. Active ingredient is called quassin. Has not yet been synthesised

Arthropod Environmental Management 2 Quassia

May have some systemic activity (nicotine and sabadilla do not, they are contact and stomach poisons)

Has not been synthesized and exact mode-of-action not clear

Like most botanicals it is too costly in comparison to modern chemicals

Still used in native cultures and there is a revival of interest in quassia in the West.

Arthropod Environmental Management 2 Ryanodine

First botanical to be discovered as a result of an official screening programme (in 1940s in USA)

Active is an alkaloid from a South American shrub in the genus Ryania

It is a contact/stomach poison and is much more stable than other botanicals (up to a week)

Arthropod Environmental Management 2 Ryanodine

Mode-of-action is to bind to calcium channels. Cells then are flooded with calcium molecules and poisoned species die

It has a moderate mammalian toxicity (750mg/kg in contrast to more than 4,000 for sabadilla and to 50 for nicotine)

It cannot be synthesised and so costs associated with its commercial production are prohibitive. Still used in native cultures.

Arthropod Environmental Management 2 Rotenone

Extracts from plants of American & Asian origin (unlike other botanicals)

Used in native cultures as fish poison Used in Western countries from 1840s

but has now declined because of cheaper alternatives

Arthropod Environmental Management 2 Rotenone

Chemical structure was determined in 1930s. At least 10 related chemicals called rotenoids have now been described from over 70 different plant species. Cannot be synthesised.

AIs disrupt mitochondria reducing oxygen uptake so poisoned species suffocate. Is of moderate mammalian toxicity

Rapidly inactivated by exposure to light and air In 2000 could still buy amateur and

professional formulations in the UK. Still used in native cultures

Arthropod Environmental Management 2 Pyrethrum

Best known botanical. From Chryanthemum. Extracts from flower heads

Use since start of 19th C to present. Still used today. Kenya and Tanzania grow most of the plants that are used commercially

Very unstable, good pre-harvest and safe to use in domestic situations

Arthropod Environmental Management 2 Pyrethrum

Natural pyrethrins are a mixture of AIs. All are contact insecticides that act on sodium channels. Mammalian toxicity is 1,600 mg/kg.

Synthetic versions of natural pyrethrins have now been produced (first in 1949/1950 was an AI called Allethrin)

Arthropod Environmental Management 2 Neem Extracts

Extracts from Asian neem tree. Main active is thought to be azadirachtin

These trees are now grown throughout the world including the USA (plantations in California and in Florida)

Best extracts from seed but all plant parts have been used

Azadirachtin cannot be synthesised. Full chemical structure published in 1985

Arthropod Environmental Management 2 Neem Extracts

Main AI is azadirachtin but further AIs are continually being discovered, e.g. nimbin and sallanin. Crude extracts always more potent than azadirachtin.

Since 1990s has been increasing interest in these extracts and a number of formulations now exist

Works by disrupting development of larvae (hormonal interference) and as an antifeedant (has systemic activity). Very high mammalian toxicity (> 5,000 mg/kg)

Inactivated by sunlight. Interest in neem formulations continues to grow

Arthropod Environmental Management 2 Essential Oils

Many are recorded as having insecticidal activity, primarily as repellents rather than as toxicants

Most well known are eucalyptus, peppermint and citronella

Most are highly volatile and are extracted in such small amounts that they are uneconomic to develop on a large scale

Still used in native cultures

Arthropod Environmental Management 2 Botanicals - Summary

Most success with Pyrethrum, neem is of current interest

All contact/stomach poisons except neem and quassia extracts (systemic)

Most act of nervous system, some on respiration (rotenone), some on development (neem)

Most are unstable (except ryanodine) and difficult to synthesise (except pyrethroids and nicotine)


Botanicals – Questions?

Arthropod Enviornmental Management 2

Chemical Insecticides Organochlorides Pyrethroids Organophosphates Carbamates Neonicotinoids Growth Regulators


Organochloride Insecticides First extensively used group of

insecticides Comprised of three groups: DDT

group, HCH group, and cyclodienes

CH C lC l

C

C l

C lC l


Organochloride Insecticides - DDT DDT (DichloroDiphenylTrichloroethane) First prepared in 1874, insecticidal

properties discovered in 1939 Used during war to control vectors of

malaria and typhus (mosquitoes and fleas) 1948 – Paul Muller was awarded Nobel prize

for medicine for his discovery of DDT Cheap and stable molecule Now banned throughout Europe and other

Western countries


Organochloride Insecticides - DDT Fat soluble (lipophilic) so biomagnifies

or bioconcentrates

Species Group DDT Concentration Concentration Factor Water 0.000003ppm not applicable Zooplankton 0.04 ppm ca. 13,000 Small Fish 0.5 ppm ca. 17,000 Large Fish 2 ppm ca. 700,000 Fish-Eating Birds 25 ppm ca. 8 million


Organochloride Insecticides - DDT Toxicity of DDT

DDT appears to disrupt calcium metabolism in birds leading to egg-shell thinning

This caused particular problem for predatory birds (e.g. eagles)

Main mode-of-action is as neurotoxin Interferes with sodium channels on nerve axons Is a contact/feeding poison with no systemic

activity Still used in Africa and Asia for mosquito control


Organochloride Insecticides - Summary Extremely stable group, half lives measured

in years Chemically composed of C, H, and CL

groups 10 – 20 actives developed are marketed No longer used in Western countries, most

used in developing countries They are cheap, stable and effective,

especially for control of vectors of disease


Pyrethroid Insecticides Synthetic pyrethroids First was marketed in 1950 (allethrin),

since then ca. 30 AIs have been produced. Examples include permethrin, deltamethrin, cypermethrin

Synthetic development has focussed on stabilising the natural version (pyrethrum) of the active molecule


Pyrethroid Insecticides Comparisons between pyrethum and

synthetic versions

C C

CH

CH3

3

CO O

CH CHCH CHCH 22

O

CHH

H

H

3

CHCH

H H

H

3 3

H



synthetic versions

C CC

O

F

CO

F

C L

H

CHCH

H H

H

3 3

F

F

F

F

F

CH2

3CH



synthetic versions Much more photostable, sites of auto-oxidative

attack removed. Natural pyrethrum open to hydrolytic, esteratic and oxidative attack

Synthetic chemicals are cheaper to produce Much more toxic than natural pyrethrum, can

be applied at far lower application rates


Pyrethroid Insecticides – Mode of Action Contact & stomach poisons, no systemic activity Interfere with sodium channels on nerve axons

(like DDT) Have a rapid knock-down effect suggesting that

the peripheral nervous system is affected immediately


Organophosphate Insecticides Developed from wartime research on potential

nerve agents (sarin and tabun) First insecticide was schradan (now replaced), 80 –

100 AIs developed All tri-esters of phosphoric acid

P P

O

OH

OH

HO

O

O

S o r

O X

CH C H2o r3 5

CH

C H2

o r3

5

O


Organophosphate Insecticides Break down in days – weeks Some are systemic Examples of OPs include malathion,

parathion, chlorpyrifos Very widely used group of insecticides

worldwide


Organophosphate Insecticides Mode of Action

Neurotoxins – interfere with synaptic transmission of nerve impulses

Block the enzyme that breaks down acetylcholine Acetylcholinesterase binds OPs and the

neurotransmitter builds up in synapses In human poisoning the acetylcholine inhibitor

atropine is administered


Carbamate Insecticides First (carbaryl) marketed in 1956 Developed from knowledge that a natural

carbamate (physostigmine) had anti-cholinesterase activity

Ca. 30 AIs developed, all with an N-methyl carbamate group

OC

O

NH

CH3


Carbamate Insecticides Physostigmine is very hydrophilic, synthetic

carbamates are much more lipophilic Examples include pirimicarb and aldicarb,

the most toxic insecticide marketed to date Overall, their use is declining but they are

still widely available.


Carbamate Insecticides – Mode of Action Like OPs they block the enzyme

acetylcholinesterase The process is called carbamylation (rather

than phosphorylation) Like OPs atropine is used in cases of

poisoning


Carbamate Insecticides – Bhopal 1984 – factory in India producing carbaryl released a

cloud of Methyl Isocyanate Of the 800,000 people living in Bhopal, 2,000 died

immediately, at least 10,000 have died subsequently and at least 300,000 people were affected

This remains the world’s worst industrial accident where human lives are concerned

In 1989 a final settlement was supposed to have been reached ($470 million)

Many people think that more still needs to be done – Bhopal is still contaminated


Neonicotinoids Newest group of synthetic insecticides Imidacloprid was marketed in 1991, now 4

others available All have contact and systemic activity Based on the alkaloid nicotine

N

N

CH3

H

H

HH

HH

H

H

H

H H


Neonicotinoids Like nicotine they work as agonists to

acetylcholine This leads to a build up of acetylcholine which

causes death Now one of the most extensively used group of

insecticides worldwide


Synthetic Insecticides - SummaryEnvironmental Exposure NumberStability and Toxicity of A.I.’s

Organochlorides Months to Contact & ca. 10 - 20

Years Feeding Poisons.Organophosphates Weeks Contact, Feeding ca. 80 - 100

& SystemicPoisons

Pyrethroids Weeks Contact & ca. 30 - 40Feeding Poisons

Carbamates Weeks Contact, Feeding ca. 30 - 40& SystemicPoisons

Neonicontinoids Weeks Contact, Feeding ca. 5-7& SystemicPoisons


Insect Growth Regulators Chemicals that affect growth and development Usually applied through conventional application equipment Mostly effective against juvenile stages (timing important) Three groups: juvenile hormones, moulting hormones, chitin

synthesis inhibitors


Insect Growth & Development

All arthropods have exoskeletons (support, movement & prevent dehydration) These must be shed to grow, a process controlled by hormones Moulting hormone: determines whether a moult occurs Juvenile hormone: determines type of moult Studies on hormones for back 40 years (not new)


Juvenile Hormone (JH) Mimics Studies on plant extracts (firs & ferns) indicated compounds with JH activity Led to production of farnesol (first synthetic hormone) Keeps insects in juvenile stage (limited use), does not penetrate cuticles

well Some commercial successes: kinoprene, methoprene


Juvenile Hormone (JH) Mimics

Most uses against pests of public and animal health (i.e. fleas) Break down fairly rapidly on exposure to light and air

Compounds with Anti-JH activity have been developed but none sufficiently active


Moulting Hormone (MH) Mimics MH causes a moult to occur Can be used against juvenile stages Examples include: tebufenozide, halofenozide They cause a precocious moult to occur in treated pest species Used against a range of pest species


Chitin Synthesis Inhibitors Most successfully developed group of IGRs > 10 formulations marketed All cause a reduction in chitin synthesis (structural polysaccharide) Disrupt the enzyme chitin synthetase Insects die from dehydration, predation, etc. Very successfully used in locust control operations in Africa

Arthropod Environmental Management 2 Insect Growth Regulators – Summary

Most success with CSIs (only effective against juveniles, adults do not moult!)

Some other successes with MH and JH mimics, particularly for control of public health pests

All highly specific with zero impact on vertebrates Overall, still a relatively small section of pest control market

Arthropod Environmental Management 2 Topics for Session 2 Botanical Insecticides Sabadilla,...

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