Except for the ,pyrethrins, they were displaced in the 1940's and
1950's by synthetic organic or "second generation"
insecticides which provided nearly complete control
at reduced cost due to high potency or persistence or
both these properties. There are four principal
classes of second generation insecticides, i.e., organophosphorus
compounds, methylcarbamates,
chlorinated hydrocarbons, and pyrethroids; all act as
nerve poisons. The first two classes inhibit acetylcholinesterase
and thereby disrupt synaptic transmission.
The others probably act at nerve membranes
principally by altering sodium conductance
mechanisms. Several of the chlorinated hydrocarbons
(e.g., DDT, aldrin, and dieldrin) have been
restricted or banned because of unacceptable persistence,
effects on wildlife, and evidence of possible
carcinogenic activity. Some of the important organophosphorus
and methylcarbamate insecticides
are very hazardous to manufacture, formulate and
apply due to their high acute toxicity when ingested,
inhaled or absorbed through the skin. Most of the
pyrethroids appear at present to have generally
favorable persistence and toxicological characteristics.