InsecticidesInsecticides

Photograph by Scott Bauer

Stephen J. Toth, Jr. Wayne G. BuhlerDepartment of Entomology Department of Horticultural ScienceNorth Carolina State University North Carolina State University

Insects and MitesInsects and Mites

• 99% of species are of minor importance

• Beneficial insects and mites are small group which include honey bees, lady beetles, parasitic wasps, and predaceous mites

• Destructive insects and mites represent the smallest but most notable group

Photographs by Steve Bambara and Jack Bacheler.

History of Insecticide UseHistory of Insecticide Use

• Greek philosopher Homer reported the use of sulfur for fumigation and other pest control uses (1000 B.C.)

• Pliny the Elder reported pest control practices from Greek literature (70 A.D.); included use of pepper and tobacco extracts, soapy water, vinegar, turpentine, fish oil, brine, lye, etc.

History of Insecticide UseHistory of Insecticide Use

• As recently as the 1940s, insecticides limited to the arsenicals, petroleum oils, nicotine, pyrethrum, rotenone, sulfur, hydrogen cyanide gas and cryolite

• Synthetic organic insecticides introduced after World War II

National Agriculture Library

The Criteria for JudgingThe Criteria for JudgingInsecticide EffectivenessInsecticide Effectiveness

• Controls insects

• Cost effective

• Environmentally safe

• Can be used safely

Tim McCabe

Mode of Action of InsecticidesMode of Action of Insecticides

• Nerve poisons (axonic and synaptic)

• Metabolic inhibitors

• Muscle poisons

• Alkylating agents

• Physical toxicants

• Cytolytic (cellular) toxins

USDA/ARS

Mode of Action of InsecticidesMode of Action of InsecticidesNerve PoisonsNerve Poisons

• Axonic poisons: effect electronic transmission of nerve impulses along nerve axon; cause repetitive discharges of nerves that eventually results in paralysis; examples include DDT, pyrethrum and synthetic pyrethroids

• Synaptic poisons: effect electronic transmission (chemical) of nerve impulse across junction between nerve cells (synapses); cause repetitive discharges of nerves that results in paralysis; examples include organochlorines, organphosphates, carbamates and nicotine

Mode of Action of InsecticidesMode of Action of Insecticides• Metabolic inhibitors: effect electron transport chain;

examples are rotenone (slows heartbeat, depresses respiration and oxygen consumption, and causes paralysis and death) and arsenicals (inhibit respiratory enzymes)

• Muscle poisons: have a direct action on muscle tissue; examples are ryania and sabadilla which increases oxygen consumption, followed by paralysis and death

• Alkylating agents: react directly with chromosomes and enzymes in cells; examples are fumigants such as methyl bromide and ethylene dibromide

Mode of Action of InsecticidesMode of Action of Insecticides

• Physical toxicants: mechanically blocks a physiological process; examples are oil (blocks respiratory openings in insects) and boric acid and silica gel (effects insect cuticle causing dehydration and death)

• Cytolytic (cellular) toxins: cause cells to rupture and disintegrate; example is Bacillus thuringiensis which is ingested by insects and disrupts cells in the gut (causing paralysis of gut and cessation of feeding)

Routes of Exposure to InsecticidesRoutes of Exposure to Insecticides

• Stomach poisons: insecticide must be ingested by the insect for toxic effect

• Contact poisons: the insect must come into contact with insecticide for toxic effect

Scott Bauer

Classes of Insecticides: InorganicsClasses of Insecticides: Inorganics• Inorganic insecticides do not contain carbon

• Usually white and crystalline, resembling salts

• Stable chemicals (persistent), do not evaporate and are frequently soluble in water

• Sulfur: stomach poison; oldest known insecticide; controls mites, thrips, scale insects and caterpillars

• Arsenicals: stomach poisons; very useful to agriculture from 1930 to 1956; include Paris green, lead arsenate and calcium arsenate

• Others: cryolite (fluorine), boric acid and silica gels

Classes of Insecticides: BotanicalsClasses of Insecticides: Botanicals• Botanical insecticides are toxicants derived from plants

• Flowers, leaves and roots are finely ground and used, or toxic ingredients of plants are extracted and used alone or in mixture; expensive; low toxicity to mammals

• Used for centuries; maximum use in U. S. in the 1960s

• Nicotine: tobacco extracts, nicotine sulfate; nicotine is a nerve poison (mimics acetylcholine at nerve synapse)

Ken Hammond

Classes of Insecticides: BotanicalsClasses of Insecticides: Botanicals

• Rotenone: roots of Derris or cube plants; rotenone is both contact and stomach poison; used for control of many insects and fish

• Pyrethrum: extracted from flower of chrysanthemum; nerve poison that paralyzes insects quickly (“knock down” effect); used in household sprays and aerosols, and on many vegetables, fruits and ornamental plants

• Others: Ryania (roots of shrub), Limonene (citrus peels), Sabadilla (seeds of lily) and Neem (oil extracts of neem tree seeds)

Classes of Insecticides: OrganochlorinesClasses of Insecticides: Organochlorines

• Insecticides that contain carbon (organo-), chlorine and hydrogen; also known as chlorinated hydrocarbons

• Highly persistent and bioaccumulate in environment

• DDT: nerve poison (effects axon); more than 4 billion pounds used in agriculture and for public health; very inexpensive to produce (22 cents per pound); DDT’s persistence in the environment resulted in ban of its use in U. S. in 1973

Classes of Insecticides: OrganochlorinesClasses of Insecticides: Organochlorines

• Lindane: gamma isomer of benzenehexachloride; nerve poison that resembles DDT; limited uses remain

• Cyclodienes: includes chlordane, aldrin, dieldrin and heptachlor; extremely persistent insecticides and stable in the soil; used in soil to control termites and soil insect pests of agriculture; uses cancelled (1975-1988)

• Toxaphene: a polychloroterpene; once used on cotton

Classes of Insecticides: OrganophosphatesClasses of Insecticides: Organophosphates

• Insecticides that contain phosphorus

• Distinctive features are their acute toxicity to vertebrate animals and chemical instability (less persistent in the environment than organochlorines); related to nerve gases

• Nerve poisons that inhibit acetylcholinesterase at nerve synapses; cause rapid twitching of muscles and paralysis

• Examples include malathion, chlorpyrifos (Dursban), diazinon and methyl parathion

Classes of Insecticides: CarbamatesClasses of Insecticides: Carbamates

• Insecticides that are derivatives of carbamic acid

• Distinctive features are their low toxicity to mammals (exception is aldicarb) and broad spectrum of insect control (used widely for lawn and garden insects)

• Nerve poisons that inhibit acetylcholinesterase at nerve synapses; cause rapid twitching of muscles and paralysis

• Examples are carbaryl (Sevin) and aldicarb (Temik)

Classes of Insecticides: PyrethroidsClasses of Insecticides: Pyrethroids

• Synthetic insecticides related to pyrethrum (botanical)

• Pyrethroids are much more stable in sunlight (persistent) than pyrethrum and effective against insects at very low application rates (0.1 pound per acre)

• Nerve poisons that effect nerve axon; cause repetitive discharges of nerves which results in eventual paralysis

• Examples are allethrin and permethrin

Classes of Insecticides: BiorationalsClasses of Insecticides: Biorationals

• Natural or synthetic substances specific for target pest(s), but have no adverse effect on humans or environment

• Insect pheromones: sex pheromones used for male trapping, monitoring, detection and mating disruption; example is gossyplure (pink bollworm)

Jack Bacheler

Classes of Insecticides: BiorationalsClasses of Insecticides: Biorationals• Insect growth regulators: disrupt

the growth and development of immature insects into adults; insect mortality is typically slow; examples are methoprene and fenoxycarb (Logic)

• Microbials: bacteria (Bacillus thuringiensis), viruses, fungi, protozoa and nematodes that are isolated, cultured and mass-produced for use as insecticides Scott Bauer

Classes of Insecticides: New ChemicalsClasses of Insecticides: New Chemicals

• Nicotinoids: action similar to nicotine; example is imidacloprid (Gaucho)

• Spinosyns: fermentation product of soil-inhabiting microorganisms; example is spinosid (Tracer)

• Pyrrole: chlorfenapyr (Pirate)

• Phenylpyrazole: fipronil (Frontline)

ReferencesReferences

• Ware, G. W. An Introduction to Insecticides. 3rd edition. Radcliffe’s IPM World Textbook. (http://ipmworld.umn.edu/chapters/ware.htm)

• Ware, G. W. 1994. The Pesticide Book. 4th edition. Thomson Publications, Fresno, California. pp. 41-74.