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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