Ent 222: Insecticides in pest management

What are pesticides?

Substances used to mitigate damage from pests; important chemicals used for the well-being of human populations 

→ Regular component in most pest management systems

What is the most widely used category of pesticides?

Herbicides

What are the disadvantages of insecticides?

• Highly effective

• Rapid action

• Reduces pest populations before major damages occur

• Treats infestations while they are happening

• Economical and easy to apply in intensive agriculture

What are the disadvantages of insecticides?

Disadvantages

• Insecticide resistance

• Pest resurgence or replacement

• Harm to non-target organisms

• Health risks for applications and consumers

How are insecticides classified?

• Application

• Nature or source

• Chemical composition

What is a stomach poison?

Poisons that kill insects after ingestion

• Enter the insects body through the digestive system

• Must be eaten by the insect to be effective

• Cause death after ingestion

What are the two kinds of stomach poisons?

1. Systemic insecticides: absorbed and translocated in plants or animals. 

• Effective against piercing-sucking pests of plants and livestock parasites. 

• Not true stomach poisons (often also have contact properties)

EG: Dimethoate

2. Microbial insecticides: produced by microorganisms and toxins are released in insect midgut.

EG: Bt

What are contact poisons?

Poisons that kills insects on direct contact

• Major group of modern insecticides

• Enter insect body through direct contact with treated surfaces

• Absorbed through the body wall

What are fumigants?

Toxic gases that kill insects through respiration

• Become gases at temperatures above 5C

• Applied to enclosures or covered soils

• Usually contain halogens (Cl, Br, F)

• Toxic gases enter the insects tracheal system, circulate and kill all life stages

How is fumigation used for stored grain pests (red flour beetles and rusty grain beetles)?

1. Detection of infestation: check for insects by sieving and probing grain samples; observe samples over heat to detect movement.

2. Prevention: spray empty bins with insecticides or cool treatment (cool grain below 0C)

3. Control (fumigation): they use aluminum phosphide (phostoxin) tablets which they inject evenly, then seal the granary and post warning signs. This is toxic to mammals which means it must be done by trained, registered applicators only.

What are the three sources of insecticides?

1. Inorganic insecticides: lack carbon atoms; sulfur is the oldest effective insecticide which is used against mites, thrips and scale insects.

2. Organic insecticides: contain carbon atoms; most modern insecticides

I.  Natural : refined from plants, bacteria or minerals

• Botanical insecticides

• Bacterial insecticides

• Mineral oils

Ii. Synthetic: chemically manufactured, most widely used today

What are organochlorines?

• First widely used synthetic insecticides; contains DDT

• Oldest major insecticide class

• Contains chlorine, hydrogen and carbon

• Highly effective

• Rarely used today due to environmental and human safety concerns

What is DDT? and its MOA?

Most famous insecticide

It is used medically to control mosquitoes, lice and fleas

Widely used in agriculture with peak US production in 1961

MOA: Disrupts Na+/K+ balance in insect neurons, causing spontaneous firing

→ Use banned in NA in the 1970s and worldwide by the stockholm convention in 2004

But it is still used in some countries for vector control

What are the environmental and health issues with DDT?

• Highly stable and fat-soluble = persists in the ecosystem

• Bioaccumulation and biomagnification = accumulates in the fat of animals and humans

• Top predator decline; ospreys, eagles, seagulls and pelicans

• Human exposure via milk and dairy products

What are cyclodienes and their MOA?

• Introduced after WWII

• Persistent and stable in stable in soil (uv resistant)

• Used for soil insects and termite control

• Most agricultural use banned due to persistency

MOA: Acts on GABA receptors in Insect Neurons; prevents chloride ion entry, causing tremors, convulsions and prostration.

What is an example of a cyclodiene?

Endosulfan used to control whitefly on poinsettia (banned in 2016)

What are organophosphates and their MOA?

• Developed in Germany during WWII by Dr Schrader

• Derived from phosphoric acid

• HIghly toxic to vertebrates

• Unstable in light and breaks down quickly to non-toxic substances

MOA: inhibits cholinesterase = accumulation of acetylcholine at synapses which causes rapid twitching and paralysis.

What are the three kinds of organophosphates?

• Alphiatic

• Phenyl

• Heterocyclic

What are aliphatic OPs? an example?

• OPs that contain straight carbon chains

EG: Malathion: most widely used OP worldwide

• Safe and effective

• Used in agriculture for many pests; grasshoppers, strawberry root weevil, aphids and diamondback moth

What is a phenyl OP? an example?

• An OP that contains a phenyl aromatic ring

• More stable than aliphatic OPs; residues last longer

Examples and use

• Methyl parathion: broad agricultural use; highly toxic (banned)

• Famphur: animal systemic for cattle grubs

• Fonofos: soil insects in field and vegetable crops (banned)

What are heterocyclic OPs? an example?

• Ring structure with O, N or S replacing one or more carbons

• Ring may have 3,5, or 6 atoms

• Most stable and long lasting OPs

• Break down into multiple products; limits use on food crops

EG Chlorpyrifos

Formerly used in a wide variety of plants; a wide variety of pests too!!

High mammalian toxicity

Non-target toxicity to bees and fish

Cancelled as of December 2023

What are carbamates? their MOA?

• Broad spectrum insecticides

• Widely used in agriculture

• Break down quickly in the environment

• Derived from carbamic acid

MOA: inhibits acetylcholine; effects are irreversible

What is the most common carbamate?

Carbaryl: first successful carbamate in 1956

What is the impact and usage of Carbamates?

• Highly toxic to hymenoptera

• Low toxicity to humans

• Used in home lawn and garden products

• Used in fruit production; insecticide and fruit thinning 

• ECO Bran registered for grasshopper control

What are pyrethroids? their MOA?

• Synthetic insecticides developed to mimic pyrethrum

• Highly toxic to insects at very low rates

• Fast knock-down effect

MOA: Axonic poisons; keep Na+ channels open in neuronal membranes, which causes repetitive nerve firing and eventual paralysis

What are the advantages of Pyrethroids?

• Safe application when used properly

• Economical

• Developed in several generations of improvement

What is the difference between third and fourth gen pyrethroids?

Third Generation : Permethrin (pounce)

• First pyrethroid stable for leaf feeding insects

• Used on crops: cotton, soybean and corn

• Effective against many above ground pests

• Low application rates

• Stable from 4-7 days

Fourth Generation : Cypermethrin (Cymbush, ripcord)

• Most potent pyrethroid

• Used on many crops; broad spectrum

• Low mammalian toxicity but toxic to bees and fish

• Very low application rates

• Stable up to 10 days

What are neonicotinoids? their MOA?

• Synthetic analogs of nicotine

• Systemic and contact insecticides

• Widely used as seed treatments in many crops

MOA: Block nicotinic acetylcholine receptors in insect CNS; mimics acetylcholine causing irreversible nerve damage

What is an example of a Noenicitinoid?

 Imidacloprid

• Registered in NA in 1992

• Low mammalian toxicity

• Different MOA; useful for managing insecticide resistance

What are the environmental concerns with neonicitinoids?

• Linked to colony collapse disorder in honey bees

• Some uses banned or under review by PMRA

What are sulfoximines? their MOA?

A new class of insecticides

MOA: Nicotinic acetylcholine receptor modulator

What is an example of a sulfonimine?

Eg: Sulfoxaflor

• Registered in 2013

• Effective against sap-feeding insects

What are the advantages of sulfoximines?

• Fast knock-down effect

• Good residual activity

• Compatible with natural enemies

• Different interactions than neonicotinoids → little cross resistance

What are botanicals? why are they popular? what are their downfalls?

Insecticides derived from plant or plant products

• Used longer than any other insecticide type

• Popular due to natural status; but can be as toxic as synthetic

• Expensive to extract; limiting large-scale agricultural use

What is pyrethrum?

Extracted from chrysanthemum flowers; inspired the development of pyrethroids

• Contain 4 active compounds

• Broad spectrum insecticide

• Rapid knock-down effect

• Axonic poison

What are the uses of Pyrethrum? how is this limited?

• Household pest control

• Fruit trees, ornamentals, vegetables, flower

Limited use as it breaks down quickly in UV light

What is Azadirachtin?

A botanical Extracted from neem tree seeds

It is highly effective and safe

Wide spectrum insect growth regulator

Interferes with feeding, oviposition, growth and reproduction

Affects PTTH synthesis in insect brain cells

Breaks down quickly in UV light

What is rotenone? its uses?

Second most common botanical insecticide

Extracted from the roots of legumes

Used against chewing and sucking insects

Rapid knockdown effect

Used on garden and fruit crops

Historically used as a fish poison and for warble fly control

How is nicotine used as a botanical?

• Extracted from tobacco leaves

• Highly toxic to insects and humans

• Acts on central nervous system

• Mimics acetylcholine in CNS ganglia
  

Uses:

• Formerly sold as sprays and dusts

What are the two main examples of bacterial insecticides?

1. Bacillus thuringiensis

• Microbial insecticides

• Produces toxins in the insect midgut
  

2. Spinosyns: Spinosad, spinetoram

• Derived from bacterium: Saccharopolyspora spinosa

• Active by contact and ingestion

• Broad-spectrum insecticide

What is the MOA of Spinosyns?

MOA: Bind to nicotinic acetylcholine receptors, disrupt acetylcholine neurotransmission, secondary effect on GABA receptors, cause hyperexcitation of insect nervous system

What is an example of a muscle poison insecticide?

Anthranilic diamides

EG Chlorantraniliprole

• Used as a seed treatment

• Controls a variety of pests

• Targets ryanodine receptors in muscles; caused released of stored calcium which disrupts muscle contraction

What is a physical toxicant insecticide? examples?

• Blocks insect metabolism by physical rather than chemical action

Examples:

• Oil ; clogs insect spiracles

• Abrasive cuts; absorb wax from cuticle which causes water loss, desiccation and death