Introduction

Pyrethrum is one of the oldest and most widely used botanical insecticides. Its insecticidal properties have been known for more than 150 years; although the earliest mention of the Chrysanthemum flowers from which it originates comes from early Chinese history, where it is believed that the flower passed into Europe along the silk roads.1 The term "pyrethrum" refers to the dried and powdered flower heads of a white-flowered, daisy-like plant belonging to the Chrysanthemum genus. Pyrethrum's insecticidal properties were recognized in the middle of the 19th century, when an American named Jumticoff discovered that many Caucuses tribes used it for the control of body lice.1 The earliest cultivation of pyrethrum, also called "Persian pyrethrum'' or ''Persian powders'', was in the region of the Caucuses extending into Northern Persia.2 The first Persian powders that were processed and commercialized in Europe in the 1820s were most likely prepared from a mixture of C. roseum and C. corneum. During and after 1876, these preparations were introduced into the USA, Japan, Africa and South America.3,4 The superior insecticidal properties of C. cinerariaefolium were first discovered around 1845 and these species subsequently supplanted previously cultivated species. Chrysanthemum cinerariaefolium is currently cultivated in the USA, Japan, Kenya, Brazil, the Democratic Republic of the Congo, Uganda and India.2,3

RSC Green Chemistry No. 11 Green Trends in Insect Control

Edited by Oscar Lopez and Jose G. Fernandez-Bolaiios © Royal Society of Chemistry 2011

Published by the Royal Society of Chemistry, www.rsc.org

In 1917, the U.S. military made the first pyrethrum extracts by percolating the ground flower heads with kerosene, which were then incorporated into space sprays for use against house flies and mosquitoes.1 Since pyrethrins are derived from plants, however, the supply has always been highly variable. A shortage during World War II hastened the search for synthetic insecticides like dichloro-diphenyltrichloroethane (DDT), which could be consistently produced and which was subsequently used by the Allies to manage insect vectors of human pathogens. The introduction of synthetic insecticides like organochlorines, organophosphates and carbamates represented a revolution in insect control because of their high insecticidal toxicity and consistent supply, however, they have been, or are being, phased out of use due to biomagnification, high non-target toxicity, or both.

The commercial limitations of pyrethrum extracts, which are collectively known as pyrethrins and are a mixture of six lipophilic esters, have long been recognized because of their high rate of photodegradation and a short "knockdown" (rapid paralysis) effect. After the discovery of the constituents of pyrethrins, researchers searched for derivatives of pyrethrins that had a higher resistance to photodegradation. This search directly led to the synthesis of pyrethroids. The advantages of pyrethrins and pyrethroids are that they are highly lipophilic, have a short half-life in the environment, have low toxicity to terrestrial vertebrates and do not biomagnify like older chemical classes, such as organochlorines (see Tables 3.1 and 3.2). In her book Silent Spring, Rachel Carson recognized that insecticides like pyrethrins offered alternatives to many of the insecticides that were used during the 1940s to 1970s.

Pyrethroids, the synthetic derivatives of pyrethrins, have changed structurally over the past several decades. However, the basic components of pyre-thrins, a chrysanthemic acid linked to an aromatic alcohol through an ester linkage, have been conserved (see Figures 3.1 and 3.2). The widespread use of pyrethroids began in the 1970s after the development of photostable pyre-throids like permethrin and fenvalerate. Pyrethroid use has increased substantially throughout the world over the past few decades as organophosphate, carbamate and organochlorine insecticides are being phased out.5 7 Pyrethrins and pyrethroids are estimated at 23% of the insecticide world market, with more than 3500 registered formulations, and are widely used in agriculture, residential areas, public health and food preparation.8,9 Permethrin and cypermethrin are the most widely used pyrethroids in the USA, with about

Table 3.1 Bioconcentration factors (BCF) for type I and II pyrethroids and DDT for rainbow trout (Oncorhynchus mykiss) from Muir et al?21

Compound

BCF

Cypermethrin

832

Permethrin

1940

Deltamethrin

502

Fenvalerate

403

DDT

72500

Table 3.2 LC50 values of pyrethrins, type I (allethrins, permethrin and resmethrin), II (cypermethrin and deltamethrin) and pseudopyre-throid (etofenprox) for mallard duck (Anas platyrhynchos), rat (Rattus norvegicus) and rainbow trout (Oncorhynchus mykiss).

Compound

Mallard Ducka

Rata

Rainbow Troutb

Source

Pyrethrins

>5620

700

5.1

USEPA118

Allethrins

>2000

720

9.7

WHO222

Permethrin

>10000

8900

6.43

USEPA10; Kumaraguru

and Beamish223

Resmethrin

>5000

4639

0.28

USEPA68

Cypermethrin

>2634

247

0.39

USEPA10

Deltamethrin

>4640

128

1.97

WHO224

Etofenprox

> 2000

> 5000

13

USEPA25

"Acute oral LC50 b96-h LC50 (mgl-

(mgkg-1).

Alcohol Moiety

Acid Moiety

Esters of chrysanthemic acid_Esters of pyrethric acid_

R1 R2 R1 R2

Pyrethrin I CH3 CHCH2 Pyrethrin II CH3OC(O) CHCH2 Cinerin I CH3 CH3 Cinerin II CH3OC(O) CH3 Jasmolin I CH3 CH2CH3_Jasmolin II CH3OC(O) CH2CH3

Acid Moiety

Esters of chrysanthemic acid_Esters of pyrethric acid_

R1 R2 R1 R2

Pyrethrin I CH3 CHCH2 Pyrethrin II CH3OC(O) CHCH2 Cinerin I CH3 CH3 Cinerin II CH3OC(O) CH3 Jasmolin I CH3 CH2CH3_Jasmolin II CH3OC(O) CH2CH3

Figure 3.1 The chemical structure of the six constituents of pyrethrum extracts which are collectively known as pyrethrins.

910 tonnes of permethrin and 455 tonnes of cypermethrin applied annually.10,11 Pyrethroids are also used extensively in urban areas, accounting for about 70% of the total usage in California.6

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