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Pyrethroids - wide range of insecticides PDF Print E-mail

A group of synthetic insecticides based on the structure of pyrethrins.

1. First generation 
Allethrin, first developed in 1949, is slightly more stable and so more persistent than pyrethrum.

Health effects
First generation pyrethroids are considered to be of low toxicity to people and other mammals because they are rapidly broken down in the body. They may irritate the mucous membranes, particularly in asthmatics.

Environmental effects
First generation pyrethroids decompose quickly in sunlight and air and thus pose little risk in the environment but all pyrethroids are toxic to aquatic animals.

2. Second generation 
Tetramethrin, resmethrin, bioresmethrin and phenothrin.
These pyrethroids are used in domestic aerosols as flysprays and for mothproofing. Bioresmethrin is effective against insects in stored products such as grain and flour, especially where resistance to organophosphate insecticides has built up. Phenothrin has been used in airport quarantine pest control. These pyrethroids are generally more effective than natural pyrethrins but act in the same way.

Health effects
Second generation pyrethroids are not acutely toxic to people or other mammals.

Bioresmethin, tested on rats, has an LD50 of 8,600 mg/kg (oral) and 10,000 mg/kg (dermal). However, chronic effects have not been exhaustively tested and exposure should be avoided.

Environmental effects
These pyrethroids decompose rapidly in sunlight. They thus pose little threat to the environment, but for the same reason they are not suitable for agricultural use.

3. Third generation 
Permethrin 25/75 and fenvalerate were developed to fulfil demands for safe and effective agricultural pest control. Third generation pyrethroids do not decompose in sunlight and contain some of the most powerful insecticides known.

Permethrin is used in domestic aerosols mainly as a surface spray and remains active for several weeks after application. It is also used as an insecticidal dust and wettable powder in the pest control industry. Permethrin can also be used in the garden as it is absorbed to some extent by the waxy layer (cuticle) on plants. Permethrin and fenvalerate are being used increasingly in agriculture, particularly in integrated pest management (see Introduction).

These pyrethroids are nerve poisons and produce hyperactivity, convulsions, paralysis and death in the affected insects. Fenvalerate may cause lethargy rather than excitation.
Health effects
Third generation pyrethoids are not highly toxic to people or other mammals mainly because they decompose rapidly in the body.

Environmental effects
Agricultural use of this group requires responsible application to avoid polluting waterways and to minimise danger to beneficial non-target species, such as bees.

These pyrethroids are toxic to fish.

4. Fourth generation 
Bifenthrin, cypermethrin; deltamethrin/decamethrin; fenpropathrin; flucythrinate; fluvalinate.

Developed mainly for agricultural and pest control use, fourth generation pyrethroids offer the most resistance to exposure to sunlight and air and, therefore, are more persistent. Their effectiveness is not significantly improved by pyrethrin synergists.

These pyrethroids are used in the pest control industry when a long-lasting residual insecticide is required to control ants, cockroaches and household pests. The stability of deltamethrin allows it to be injected into soil on a localised scale to control nesting ants. Deltamethrin has been used in the field where doses as low as one gram per hectare can last longer than other pyrethroids and many established organophosphate and carbamate insecticides.

These pyrethroids are nerve poisons and affect the central and peripheral nervous systems of insects. There are no symptoms of excitation in the affected insects. It is suggested that these pyrethroids interfere with cellular membrane transport. They are some of the most effective insecticides known.

Health effects
This group is more toxic to people than other pyrethroids and therefore requires more care in use.

Environmental effects
Because they are more stable in the environment, fourth generation pyrethroids may be carried by water or air to areas where they may endanger non-target species such as birds, reptiles, fish and plankton. The effectiveness of these compounds, however, means that very small amounts (10 times smaller than with third generation pyrethroids) can be applied to specific areas.

Pyrethroids are themselves good alternatives to most other pesticides which are generally more toxic. Allethrin and permethrin are the only ones recommended for household use. For the control of garden pests, an integrated approach is best, for example the careful use of a spray combined with cultural practices such as crop rotation and picking up fallen fruit.

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