Biopesticides - Pesticides: Evaluation of Environmental Pollution - page 598

of helminthic parasitic infections in farm animals as well as for the treatment of river
blindness in humans. It is available for use in various formulations as liquid, paste, tablet,
chewable, feed premix, pour-on liquid, and as subcutaneous injection. It is a mixture of
two avermectins, namely, 22,23-dihydroxy avermectin B1a and 22,23-dihydroavermectin
B1b. The first macrocyclic lactone was developed and introduced commercially in 1981 for
the treatment of helminthic parasitic infections in farm animals as well as for the treat-
ment of river blindness in humans. It is available for use in various formulations as liquid,
paste, tablet, chewable, feed premix, pour-on liquid, and as subcutaneous injection. It is
a mixture of two avermectins, namely, 22,23-dihydroxy avermectin B1a and 22,23-dihy-
droavermectin B1b.
19.9.2 Spinosyns
Spinosyns are a relatively recent example of new broad-spectrum natural secondary metab-
olites derived from the aerobic fermentation of the soil actinomycete Saccharopolyspora spi-
nosa . These are effective against a broad range of lepidopteran insect pests (Thompson and
Hutchins 1999). Successful introduction of spinosad represents an important milestone in
the use of natural products for commercial pest control. These were discovered through
screening of culture broth of Saccharopolyspora spinosa obtained from a Caribbean island in
the 1980s (Hoehn et al. 1991). Subsequent isolation and structural elucidation work led to
the identification of macrocyclic lactones comprising a unique tetracyclic ring system with
two attached monosaccharide units. The major constituents spinosyn-A and spinosyn-D
present in 85:15 ratio together constitute over 90% of the active material. A large number
of synthetic analogs with improved physiochemical properties and field performance and
broader activity spectrum against pests have been developed modifying different por-
tions of the spinosyn molecule (Sparks et al. 2001). Several of the spinosyn-based products
such as Tracer, Spino-Tor, and Success exhibited excellent insect control properties against
a variety of insect pests including thrips, fleas, and pest species of the lepidoptera, diptera,
and hymenoptera. In general, spinosyns exhibit activity at par with some of the synthetic
pyrethroid insecticides.
Spinosyns have been found particularly effective against economic pests on crops such
as cotton and vegetables. They are also effective against several stored grain pests of wheat
(Fang et al. 2002). Extensive field evaluation of the spinosyns has demonstrated their ade-
quate efficacy, stability, and environmental suitability to enable their use without chemical
modification. They exert their effects on insects through hyperexcitation and disruption of
the insect nervous system (Salgado et al. 1997, 1998). Spinosyns are slow to penetrate into
the larvae of H. virescens ; however, once inside the insects, these do not readily metabolize
and therefore, account for high level of activity.
19.9.3 Strobilurins
One of the most promising new groups of fungicides discovered through random screen-
ing is the strobilurins, also referred to as β-methoxyacrylates produced by the fungus
Strobilurus tenacellus , which feeds on decaying plant materials in the soil. Musilek (1969)
was the first to report the isolation of strobilurin-A, originally named as mucidin from
Oudemansiella mucida . Since its first use as an agricultural fungicide, several additional
strobilurins and their related products, namely, oudemansins and myxothiazoles have
been reported. Other strobilurins such as strobilurin-B, -C, -D, -E, -F1, -F2, -G, -H, -X, and
hydroxystrobilurin-D are characterized by the presence of various substituents in the