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Ecotoxicology - Pesticide Classification - Insecticides

The current proliferation of chemical insecticides dates from World War II, until this time the insecticides available were based on: arsenicals, petroleum oils, sulfur, hydrogen cyanide gas, cryolite and on extracts from plants such as pyrethrum, nicotine and rotenone. Table 2: Classification of Insectides gives a summary of the main chemical classes of insecticide and the main chemicals in each class. The characteristics of the main classes of insecticide: the organochlorines, organophosphates, carbamates and pyrethroids are summarised below.

Also called: chlorinated hydrocarbons
A large and varied group that has a particularly high public profile because of the environmental problems they have caused. They were mostly discovered in 1942-56 and were very important in the early success of synthetic insecticides. They are mostly Insecticides with a very wide range of actions, they can be divided into three main groups:

DDT and related compounds including rhothane (DDD) and methoxychlor.
Widely used during World War II for control of disease vectors (such as mosquitoes) and subsequently much used on agricultural pests such as ectoparasites of farm animals and insect disease vectors and also widely used againstinsects in domestic and industrial premises.

chlorinated cyclodiene insecticides such as aldrin, dieldrin and heptachlor.
most widely used as seed dressings and soil insecticides.

hexachlorocyclohexanes (HCHs), such as lindane.
used against pests and parasites of farm animals, also in insecticidal seed dressings.

Organochlorine insecticides are very stable solids with: limited vapour pressure, very low water solubility and high lipophilicity. They may be very persistent in their original form or as stable metabolites. They tend to be stored in body fats and are particularly hazardous because they are so stable and tend to accumulate in successive organisms in the food chain. DDT and the HCHs a regarded as only moderately toxic to mammals while the chlorinated cyclodienes are highly toxic.

Action: all organochlorine insecticides are nerve poisons but DDT has a different action to the chlorinated cyclodienes and HCHs. DDT acts on the sodium channels in the nervous system so that the passage of an 'action potential' along the nerve is disrupted. It causes uncontrolled repetitive spontaneous discharges along the nerve. Uncoordinated muscle tremors and twitches are characteristic symptoms. The chlorinated cyclodienes and HCHs act on the GABA receptors which function as a channel for Cl - ions through the nerve membranes. They bind to the GABA receptors and reduce the flow of Cl - ions. Typical symptoms include convulsions.

Also called: organic esters of phosphorus acid. Such as bromophos, chlorpyrifos, diazinon, dichlorvos, fenitrothion, malathion, parathion and phorate.
The same basic constituents are combined with many additional chemicals to give a wide range of products with very different properties. Organophosphates were developed during the second world war and have two main uses: as insecticides and as nerve gases (chemical warfare agents).

They are mostly liquids, liphophilic, with some volatility and a few are solids. Generally, they are less stable and more readily broken down than organochlorines and are relatively short-lived in the environment, hence most of their hazard is associated with short-term (acute) toxicity. The water solubility of the various organophosphate compounds is very variable and they are prepared in numerous formulations: as emulsifiable concentrates for spraying and to control ectoparasites of farm animals (particularly sheep dips) and sometimes internal parasites (such as ox warble fly); as seed dressings and as granular formulations particularly used for the most toxic organophosphates (e.g. disyston and phorate) as the active ingredient is effectively 'locked up' in the granule and is safer to handle and only slowly released into the environment. Organophosphates are also used to control vertebrate pests such as Quelea in Africa.

Action: like organochlorines, organophosphates also act as a neurotoxin. They combine with the enzyme acetylcholinesterase and prevent conduction of nerve impulses at junctions in the nervous system where acetylcholine is the natural transmitter. As a result, acetylcholine builds up in the nerve synapse and eventually leads to synaptic block when the acetylcholine can no longer relay signals across the synapse. In neuro-muscle junctions this leads to tetanus, the muscle is in a fixed state, unable to contract or relax in response to nerve stimulation.

E.g. aldicarb, carbaryl, carbofuran, methiocarb, methomyl, pirimicarb and propoxur
Carbamates are a more recent development than organochlorines or organophosphates, they are all derivatives of carbamic acid. The basic carbamate group is combined with different chemicals to produce insecticides with a wide range of properties (in particular they vary greatly in their water solubility) and actions. Carbamates are not only used as insecticides but also molluscicides and nematicides. Carbamates are also used as herbicides and fungicides but these have a different mode of action and are described elsewhere.

Carbamates are mainly used to control insect pests in agriculture and horticulture, they have abroad spectrum of activity and usually act by contact or stomach action although a few possess systemic activity (aldicarb, carbofuran).

Action: basically the same as organophosphates, inhibiting the action of acetylcholine at the nerve synapses. Doses of carbamates are not accumulative and carbamate poisoning is more easy to reverse than that caused by organophosphates. They are generally regarded as representing a short-term hazard.

Such as cypermethrin, deltamethrin, permethrin, phenothrin, resmethrin.
Pyrethrin insecticides were developed from naturally occurring chemicals found in the flower heads of Chrysanthenum sp. and these provided the model for the production of synthetic pyrethroid insecticides. Pyrethroids are generally more stables than natural pyrethrins. The development of pyrethroids can be traced over 4 main phases (Ware 1991). The first generation allethrin was a synthetic duplicate of a natural pyrethrum, cinerin I. The second generation included bioallethrin, phenothrin, resmethrin and bioresmethrin. These were marginally more effective than natural pyrethrums but were neither effective enough nor photostable enough to be used extensively in agriculture. However, they are still used in pest control formulations for the home. The third generation of pyrethroids included fenvalerate and permethrin which were stable in sunlight and only slightly volatile and could be used successfully in agriculture. Finally, the fourth and current generation of pyrethroids can be used at much lower concentrations (one-fifth to one-tenth) that those in generation 3 and are all photostable.

Overall, most pyrethroids are not sufficiently soluble in water to be used a systemic insecticides. They are mainly formulated as emulsifiable concentrates for spraying. They control a wide range of agricultural and horticultural insect pests and are used extensively to control insect vectors of disease (e.g. tsetse fly in Africa)

Action: pyrethroids are generally solids with very low water solubility and they act as neruotoxins in a very similar way to DDT. They are readily biodegradable but can bind to particles in soils and sediments and can be persistent in these locations. They are particularly toxic to insects as opposed to mammals and birds and the main environmental concerns are over their effects on fish and non-target invertebrates.

Table 2: Classification of Insecticides
Data from: Whitehead, R (1995) The UK Pesticide Guide. CAB International & BCPC.

Chemical group Compound Action Notes
  amitraz   also ACARICIDE
  azadirachtin insect growth regulator extracted from Neem
  nicotine contact, non-persistent general purpose, extracted from tobacco
  pyrethrin contact, non-persistent extracted from Pyrethrum
  rotenone contact extracted from Derris and Lonchocarpus
  aldicarb systemic also NEMATICIDE
  bendiocarb contact & ingested  
  carbaryl contact also WORM KILLER, FRUIT THINNER
  carbofuran systemic also NEMATICIDE
  methiocarb stomach acting also MOLLUSCICIDE
  methomyl fly bait  
  pirimicarb contact & fumigant aphids only  
  propoxur fumigant, maimainly in glasshouses  
  thiocarb pelleted bait also MOLLUSCICIDE
diphenyl aliphatic derivatives DDT    
  rhtohane (DDD)    
benzene derivatives lindane ? gamma HCH contact, ingested & fumigant  
cyclodiene derivatives aldrin persistent UK revoked 1989
  dieldrin persistent UK revoked 1989
  endosulfan contact & ingested also ACARICIDE
aliphatic derivatives dichlorvos contact, fumigant  
  dimethoate contact, systemic also ACARICIDE
  disulfoton systemic, granules  
  malathion contact also ACARICIDE
  phorate systemic  
phenyl derivatives fenitrothion contact, broad spectrum  
heterocyclic derivatives chlorpyrifos contact & ingested also ACARICIDE
  diazinon contact  
  fenbutatin-oxide   ONLY ACARICIDE
generation 1 allethrin    
generation 2 bioresmethrin contact, residual also ACARICIDE
  phenothrin contact & ingested  
  resmethrin contact  
  tetramethrin contact  
generation 3 fenvalerate contact  
  permethrin contact & ingested, broad spectrum  
generation 4 bifenthrin contact, residual also ACARICIDE
  cypermethrin contact & ingested  
  fenpropathrin contact & ingested also ACARICIDE