The Bugwood Network

Insecticides

A Brief Overview of a Complex Subject


Modes of Entrance into Insect

  • Contact - dermal – through the skin
  • Stomach - oral – through the mouth
  • Respiration - inhalation through the nose or gills
  • Systemic - combination of above

Mode of Toxicity in Insects

  • Physical poison
  • General protoplasmic poison
  • Cellular enzyme poison
  • Nerve poison
  • Growth regulator
  • Disease causing agent
  • Repellant

Toxicity to humans or nontarget organisms

  • Most insecticides have the capacity to affect non-target organisms
  • Same as previously discussed
    • Highly toxic – LD50 0 – 50 mg/kg
    • Moderately toxic - LD50 50 – 500 mg/kg
    • Low toxicity - LD50 500 – 5,000 mg/kg
    • Nontoxic - LD50 <5,000 mg/kg

Toxicity to insects natural enemies

  • Most insecticides have the potential to affect populations of beneficial insects
    • Highly toxic - Pest populations recover much faster than enemy populations in nature
    • Moderately toxic – Pest populations recover somewhat faster than enemy populations in treated environment
    • Low toxicity – Natural enemies are maintained to a degree & quickly attack recovering pest populations
    • Nontoxic – Normal enemy population levels are maintained which quickly attack recovering pest populations

Environmental hazard

  • Environmental hazard of insecticides is generally evaluated as a function of persistence often compared to effectiveness
    • High – Environmental persistence far greater than period of effectiveness (> 5 months and often > a year)
    • Intermediate – Persists beyond effectiveness (3-5 month half-life)
    • Low – Persists about the period of effectiveness (up to about 3 months) and then degrades completely over several months
    • Very low – Persists for short periods (>45 days) and degrades completely

Resistance/Resurgence Hazard

  • The hazard of populations developing resistance and resurging is evaluated for most insecticides
    • High – Strong potential to develop resistance and resurge
    • Intermediate – Moderate potential to develop resistance in treated environments
    • Low – Minimal potential to develop resistance
    • None – No resistance developed, no resurgence after many treatments

IPM Attributes

  • IPM is especially important when discussing the use of insecticides due to the potential for the development of resistance and subsequent resurgence of pest populations repeatedly treated with a single insecticide
  • Repeated treatment with a single pesticide imposes artificial genetic selection on insect populations
  • However, IPM must be effective and so there are several criteria to evaluate
    • Effectiveness in controlling pest populations
    • Cost of treatment
    • Human and nontarget-animal toxicity
    • Environmental persistence

Insecticide groups

  • The following slides present a system in which insecticides are generally catagorized
  • It is not the only system
  • Lumpers and splitters of names have created very different categories, depending on their emphasis

Organochlorines

  • Also called the chlorinated hydrocarbon insecticides
  • Characterized by containing chlorine and carbon atoms
  • Powerful nerve poisons
  • Most affect a broad spectrum of non-target organisms along with the target pests
  • Biochemical mode of action – uncertain
  • Mode of action chemical dependant
  • Long persistence and residual activity
  • Several were used in forestry
    • DDT
    • Lindane
    • Dicofol (Kelthane)
    • Endosulfan (Thiodan)
  • Most have been banned in the U.S
  • Very few still available for our use
    • Endosulfan is sometimes used on ornamentals and in seed orchards
    • Lindane is still registered for Southern Pine Beetle control but no product is available in the marketplace

Organophosphates

  • Also known as the OPs
  • Characterized by containing carbon and phosphorus atoms
  • Chemical and often habitat dependant effect on non-target organisms
  • Mode of action varies by chemical
  • Generally only short term persistence and limited residual activity
  • Unfortunately, often have broad spectrum activity against beneficial
  • Several used in forestry or applied to forests for public health purposes
    • Malthion (Malathion and Cythion)
    • Acephate (Orthene)
    • Methyl parathion (Methyl parathion)
    • Diazinon (Diazinon and Spectracide)
    • Chlorpyrifos (Dursban and Lorsban)
    • Azinphos methyl (Guthion)
  • Most have been lost to forestry due to FQPA (Food Quality Protection Act) review performed by the EPA

Organosulfurs

  • Small group of sulfur containing insecticides
  • Low insect toxicity, but with good miticidal characteristics
  • Have been used in seed orchard work
  • Only a single product relevant to this discussion
    • Propargite (Omite)

Carbamates

  • Insecticides which are derivatives of carbamic acid
  • Non-target toxicity is chemical specific, ranging from low to very high
  • Generally only short term persistence and limited residual activity
  • Often with broad spectrum activity against beneficial insects
  • Very few used in forestry
    • Carbaryl (Sevin)
    • Aldicarb (Temik)
    • Methomyl (Lannate)

Botanicals

  • Chemicals extracted or derived from plants
  • May be present and subsequently extracted from the plant material (a constitutive chemical), or
  • May be activated in the plant as a response to insect activity (inducible chemicals)
  • Limited numbers of extractable chemicals have performed well enough to have been made commercially available
  • Some are chemically modified after extraction to enhance their insecticidal properties
  • Only a few have found a niche in forestry, and generally even these are subsequently replaced by more target-specific, less persistent synthetic chemicals
    • Pyrethrins
    • Resmethrin (Pyosect, Synthhrin)
    • Azadirachtin (Azatin)

Synthetic Pyrethroids

  • Modified esters of chrysanthemate a chemical similar to that which is derived from chrysanthemums
  • Alterations in the acid components yield a reduced degradation rate compared to natural pyrethrins
  • Often with additional modification to enhance synergistic action
  • Rates are often 10% of the rates of OPs
  • Several have been used in forestry, seed orchard or nursery work
    • Permethrin (Pounce, Ambush, Dragnet)
    • Cypermethrin (Ammo)
    • Esfenvalerate (Asana)
    • Lamda cyhalothrin (Karate)

Synergists or activators

  • Chemicals which perform any of a variety of actions which enhance the action of an insecticide
  • Increase the toxicity of the initial chemical above that expected from the combination of the two products
    • Block detoxification of insecticides by insect defensive systems
    • Induce the functioning of otherwise benign chemicals
  • Two primary chemicals used in insecticide formulation
    • Piperonyl butoxide
    • Sesamin

Soaps and Abrasives

  • Produced by rending (cooking) animal fat (lard), fish oil or vegetable oil with an alkali metal such as sodium hydroxide (= hard soap) or potassium hydroxide (= soft soap)
  • Soft soaps from fish oils were the most common insecticidal soaps in the past since they are the most effective insecticidal soaps
  • Soft soaps made from vegetable oils are most common at the present time due to a better smell (not greater efficacy)
  • Soften or wash off the waxy epicuticle covering an insect allowing it to dehydrate
  • Abrasives degrade the epicuticle - same result
  • Two soaps are commonly used
    • Potassium salts of fatty acids (Safer soaps, M-Pede)
    • Boric acid
  • A single abrasive is currently registered as a forestry insecticide
    • Borax

Microbial Pathogens

  • Fungi, bacteria, viruses, etc. which can be used to cause disease in an insect population
  • Relatively narrow spectrum of activity, not broad spectrum insecticides
  • Several have been genetically engineered to kill target insects more rapidly
  • Bacteria in forestry
    • Bacillus thuringiensis var. karstaki (Dipel, Thuricide, Foray, Agrobac, Javelin, Cutlass)
  • Virus in forestry
    • Baculovirus (Nucleopolyhedrosis virus or NPV; Gypchek, TM-Biocontrol-1)
  • Bacteria applied over forests for public health protection
    • Bacillus thuringiensis var. israeliensis
    • Bacillus sphaericus

Microbial Derivatives

  • Generally organic chemicals with a nitrogen component
  • Microbially produced and then extracted and refined
  • Some are toxic to the target organisms at very low doses
  • Only one used in forestry at present
    • Avermectin (derived from Streptomyces avermitilis)
    • Also available are:
      • spinosad (Tracer)
      • pyrroles (Pirate)

Repellants

  • Large group of unrelated chemicals
  • Many experimental chemicals – but to the present no truly effective forest protectant chemical
  • Mostly have found use for people or livestock protection
  • Forestry insect repellants include
    • Verbenone
    • 4-allyl anisole (4AA)
    • Both are anti-aggregant chemicals designed to disrupt pine beetle aggregation and thwart ‘spot’ formation
  • Forester protective repellant
    • Deet (Off, Deep-Woods-Off)

Oils

  • Lightweight petroleum oils mixed with emulsifiers may be used as insecticides in some cases
  • Broadly defined in two groups:
    • Dormant oils are designed to be used to protect dormant plant materials and may have bad effects if used during the growing season
    • Summer oils may be used to protect growing plants
  • Oils kill by suffocation (scales, mealy bugs and aphids)
  • Forestry registered oils include
    • Sunspray
    • Superior oil

Fumigants

  • Primarily used in forest tree nursery beds and greenhouses
  • Fumigants generally contain a halogen (chlorine, bromine, fluorine, etc.) in their molecules
  • Small molecules which vaporize at relatively low temperatures
  • Many are now or will shortly be banned in the US
  • Fumigants which have held forestry or ornamental insecticide registration
    • Methyl bromide (MC33, MC98, Brom-o-Sol, etc.) – NFTA should eliminate this fumigant from the US by 2005
    • Dichloropropene
    • Chloropicrin
    • Metam-sodium (Vapam, Busan, Sectagon)

Transgenic Crops with Insecticidal Properties

  • Plants genetically engineered to enhance insecticidal properties
  • None in forestry as yet

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The University of Georgia - Warnell School of Forestry and Natural Resources and
College of Agricultural and Environmental Sciences - Dept. of Entomology
Last updated on Thursday, November 07, 2002 at 01:20 PM
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