of natural origin

Chapter 40 Pesticides of natural origin



ACARICIDES 533

INSECTICIDES 533

RODENTICIDES 535

MOLLUSCICIDES 535

Pesticides may be classified according to the type of organism against which they are effective, namely, fungicides, herbicides, insecticides, molluscicides, nematocides, rodenticides. The origin of the use of natural products in these respects is lost in antiquity (see Further Reading) and a large number of such materials, of local use, still remain to be chemically investigated and evaluated. Although the majority of pesticides used in modern agriculture are synthetic, plant products still contribute to the insecticides and rodenticides. Phytochemicals can also serve as lead compounds from which others, exhibiting, for example, a greater toxicity towards the pest, a wide spectrum of activity such as the inclusion of mites, a lowered mammalian toxicity and a decrease in photodecomposition, can be developed.



ACARICIDES


Mites and ticks are small arachnids of the order Acarina (Acari). Specific mites infest crude drugs and food (Chapter 13) and the house-dust mite, Dermatophagoides pteronyssinus, is well known for its possible association with asthma. Ticks are the largest members of the order and economically the most important. They are all blood-sucking parasites responsible for microbial infections, e.g the spirochaete infection causing Lyme disease, and protozoal diseases in animals.


The control of mites by plant products has centred largely on essential oils. In a report (Pharm. J., 1998, 261, 406) on the laboratory testing of three oils by I. Burgess and colleagues, tea tree oil was the most effective, giving 100% immobilization of house-dust mites at 30 min, and 100% mortality at 2 h; for the same exposure times lavender oil gave figures of 87% and 87% and lemon oil 63% and 80% respectively. Australian workers have demonstrated that for laundering purposes several essential oils are effective acaricides when emulsified in low concentrations of the laboratory detergent Tween and that a simple washing procedure with eucalyptus oil, without the use of very hot water, controlled house-dust mites and their allergens in clothing and bedding (E. R. Tovey and L. G. McDonald, J. Allergy Clin. Immunol., 1997, 100, 464).


For Third World countries where synthetic acaricides are relatively expensive the exploitation of suitable local plants is important. The essential oils of some members of the Capparidaceae have been shown to be effective antitick agents and the situation is outlined by W. Lwande et al. (Phytochemistry, 1999, 50, 40) in their studies on the tick-repellent properties of the essential oil of Gynandropsis gynandron. This East African annual species has been proposed as an anti-tick pasture plant as it disrupts the free-living stages of Rhipicephalus appendiculatus, the vector of the pathogen causing East Coast fever in animals. Twenty-eight compounds were identified in the essential oil, carvacrol, phytol and linalool being the major constituents, although greatest repellency towards the tick was shown by a number of minor constituents. Methyl isothiocyanate was also identified in the oil (2.1%) and could contribute towards the activity. It may be noted here that G. gynandron is also employed in traditional medicine for a number of conditions and its essential oil is used as a repellant for head-lice.



INSECTICIDES



PYRETHRUM FLOWER


Pyrethrum flowers (Insect flowers, Dalmatian insect flowers) are the dried flower-heads of Chrysanthemum cinerariifolium (Trev.) Vis. [Tanacetum cinerariifolium (Trev.) Sch. Bip., Pyrethrum cinerariifolium Trev.] (Compositae). The plant is perennial, about 1 m high, and indigenous to the Balkans. Principal cultivated sources are Kenya, Tasmania, Tanzania and Rwanda. Smaller amounts are grown in Japan, Eastern Europe, Brazil and India.



History


The insecticidal properties of Persian or Caucasian insect flowers (Chrysanthemum coccineum Willd. and C. marshallii Aschers) have long been known in their country of origin, but the use of the Yugoslavian species dates from the middle of the last century. Persian insect flowers are now rarely seen in British commerce, and Kenya, the largest exporter, produces flowers of the Dalmatian type, the original Dalmatian seed being introduced by Gilbert Walker in 1928.



Collection


Conditions for pyrethrum cultivation are particularly favourable in Kenya; the producing areas have an altitude of 1900–2700 m and an annual rainfall of 76–180 cm. The altitude is important, giving a low night temperature (5–15°C), which stimulates maximum bud production. Collection takes place for about 9 months of the year. As about 90% of the insecticidal activity of the plant is present in the flowers, only these are collected. Before drying they are not toxic to insects. In Kenya all the flowers are delivered to the Pyrethrum Marketing Board at Nakuru. Here all samples are analysed and the growers paid on the pyrethrin content of their deliveries. The thousands of African smallholders are organized on cooperative lines and all the profits of the Board are returned to the growers. At the factory of the Board some of the flowers are baled for export but the majority are made either into powder or into standardized liquid extract. Current developments may be followed in the biannual journal Pyrethrum Post.



Characters


The closed flower-heads are about 6–9 mm in diameter and the open ones about 9–12 mm in diameter. They bear a short peduncle which is striated longitudinally. The involucre consists of two or three rows of yellowish or greenish-yellow, lanceolate, hairy bracts. The receptable is nearly flat and devoid of palae. It bears numerous, yellow tubular florets and a single row of cream or straw-coloured ligulate florets. The ligulate corollas are 10–20 mm in length and have about 17 veins and three rounded teeth, the central one very small (distinction from ox-eye daisies, C. leucanthemum, in which the ligulate corollas have seven veins and three teeth, the centre one being the largest). The achenes are five-ribbed (achenes of Persian flowers usually 10-ribbed). The flowers have a slightly aromatic odour and a bitter, acrid taste.



Characters of powders


The species used as insecticides are C. cinerariifolium, C. coccineum and C. marshallii, the powders from which show the following elements: parenchyma often containing aggregate crystals, T-shaped hairs, numerous spherical pollen grains, sclerenchymatous cells (particularly from Persian flowers), tracheids and epidermal cells having a striated, papillose cuticle. Kenya flowers are guaranteed to contain not less than 1.3% of pyrethrin; Japanese usually contain 0.9–1.0% and Dalmatian about 0.7–0.8%.



Constituents


Pyrethrum owes its insecticidal properties to esters which are reportedly produced by a number of different cell types (oil glands, resin ducts and mesophyll cells). Pyrethrin I, jasmolin I and cinerin I are esters of chrysanthemic acid (chrysanthemum monocarboxylic acid), while pyrethrin II, jasmolin II and cinerin II are esters of pyrethric acid (monomethyl ester of chrysanthemum dicarboxylic acid). The alcohol component of the pyrethrins is the keto-alcohol pyrethrolone and of the cinerins the keto-alcohol cinerolone. Pyrethrum flowers also contain sesquiterpene lactones and the triterpenoid pyrethrol. The biosynthesis of pyrethrin I in seedlings of C. cinerariifolium has been studied using [1-13C]-D-glucose as a precursor; the acid portion of the molecule is derived from D-glucose and the alcohol moiety possibly from linoleic acid (K. Matsuda et al., Phytochemistry, 2005, 66, 1529).


Pyrethrum Extract of the BP (Vet.) contains 24.5–25.5% of pyrethrins; it may be prepared extemporaneously from the flower-heads and is used for the preparation of the BP (Vet.) dusting powder and spray. The dusting powder (pyrethrum extract, diatomite, talc) has a pyrethrin content of 0.36–0.44%, of which not less than half consists of pyrethrin I. It is assayed by titrimetry for both pyrethrin I and II. Extracts containing 50% more active material compared with commercial extracts can be obtained by extraction of the plant material with liquified carbon dioxide (100 bar). The extract is usually diluted on farms with kerosene to a pyrethrin strength of about 0.2%. For work on Pyrethrum hybrids, see Chapter 14.


The popularity of pyrethrum derived from its rapid knock-down action (largely due to pyrethrin II), lethality to insects (pyrethrin I) and low mammalian toxicity. However, synthetic analogues of natural pyrethrins with higher insecticidal activity (over 1000 times that of pyrethrin I), more photostability and a similar low toxicity have virtually displaced pyrethrin from the market, particularly in the area of domestic insecticidal sprays. There continues, however, to be a market for natural pyrethrins in special areas such as food processing plants and insecticidal spraying of edible fruits and vegetables shortly before harvest.



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Jul 18, 2016 | Posted by in PHARMACY | Comments Off on of natural origin

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