Pyrethroid Insecticides - Organic Pollutants: An Ecotoxicological Perspective - page 231

12 Pyrethroid Insecticides
12.1 Background
The insecticidal properties of pyrethrum, a product prepared from the dried and
powdered heads of flowers belonging to the genus Chrysanthemum, have long been
recognized. First introduced into Europe in the middle of the 19th century, early
sources were the region of the Caucasus and the Adriatic coast. Subsequently, the
major source of commercial pyrethrum was the species Chrysanthemum cinerari-
aefolium grown in East Africa. In the course of time the insecticidal ingredients
of pyrethrum, the pyrethrins, were chemically characterized. Six pyrethrins were
identified, all of them lipophilic esters (Figure 12.1). They are formed from two
acids—chrysanthemic acid and pyrethric acid—in combination with three bases:
pyrethrolone, cinerolone, and jasmolone.
A serious limitation of pyrethrins as commercial insecticides is their instability.
On the one hand they are photolabile and have only limited life when applied to
surfaces, for example, plant leaves, exposed to direct sunlight. On the other, they
are readily biodegradable and often have only a short “knockdown” effect on tar-
get insects unless they are synergized with compounds such as piperonyl butoxide
that will repress their oxidative metabolism. The important point is that they have
served as models for the development of the synthetic pyrethroids, one of the most
widely used types of insecticide at the present time. The first synthetic pyrethroids,
compounds such as allethrin and bioallethrin, were not sufficiently photostable to
have great commercial potential (Leahey 1985). Subsequently, a series of compounds
were discovered that had greater stability, which came to achieve great commercial
success. Included among these are permethrin, cypermethrin, deltamethrin, fenval-
erate, cyfluthrin, cyhalothrin, and others. Their widespread introduction during the
1970s came on the heels of the environmental problems associated with the persistent
organochlorine insecticides. Although the synthetic pyrethroids have sufficient meta-
bolic stability to be effective insecticides, they are, nevertheless, readily biodegrad-
able by vertebrates and do not tend to be biomagnified in food chains. At the time of
their introduction, they were seen to be environmentally friendly insecticides, which,
for some purposes, were effective alternatives to organochlorine insecticides.
12.2 cHemIcaL ProPertIeS
The structures of some pyrethroid insecticides are shown in Figure 12.1. They are
all lipophilic esters showing some structural resemblance to the natural pyrethrins.
They can all exist in a number of different enantiomeric forms. Permethrin, cyper-
methrin, and deltamethrin, for example, all have three asymmetric carbon atoms
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