The Use of Plants in Vector Control - Biological and Environmental Control of Disease Vectors - page 47

activity, low mammalian toxicity and limited soil
persistence, were achieved in the period between
1968 and 1974 by Elliott's research team at
Rothamsted Research Station, Harpenden, UK
(Elliott et al ., 1978). The fi rst of these compounds
was permethrin, which was soon followed by
cypermethrin and deltamethrin. These com-
pounds have low mammalian toxicity and
persist for several months. They have proven to
be the mainstay of vector-borne disease control,
especially used on insecticide-treated bed nets
since the late 1980s (WHO, 1989).
Since the introduction of DDT (dichloro-
diphenyltrichloroethane) for mosquito control
in 1946, DDT resistance at various levels has
been reported from over 50 species of anopheline
mosquitoes (Hemingway and Ranson, 2000).
Cross resistance between DDT and synthetic
pyrethroids has been observed, as both
compounds target the voltage-gated sodium
channel proteins found in arthropod nerve cell
membranes (Hemingway et al ., 2004).

alternative insecticide to pyrethroid for indoor
vector control applications where insecticide
resistance cannot be managed by other means.
In concentrations below those necessary
to produce a knockdown ef ect, pyrethrins
ef ectively protect individuals from host-seeking
mosquitoes. The presence of small amounts of
molecules in the air is enough to interfere with
the blood-feeding process, which is highly
complex and requires great precision and
coordination by the mosquito (Haynes, 1988).
Pyrethrins have been used for centuries in Asia
and are still popular for personal protection
against nuisance mosquitoes in the form of
mosquito coils (Hewitt et al ., 1996; Konradsen et
al ., 1997; Snehalatha et al ., 2003). A mosquito
coil is usually shaped into a spiral, and
traditionally made from a dried paste of
pyrethrum powder. Pyrethrin-treated coils
(0.5%) tested in a 25 m 3 room after burning for
30 minutes presented KD 95 of 16.1 and 19.7
min for An . stephensi and St . aegypti , respectively
(Chadwick, 1975) and showed 84% and 93%
deterrency against An . gambiae s.l. and Mansonia
uniformis , respectively, as well as good repellency,
feeding-inhibition and mortality (Hudson and
Esozed, 1971). Another novel application of
pyrethrins was in topical insect repellents, with
one study reporting similar ei cacy between the
hexane extract of the pyrethrum daisy and DEET
(N,N-diethyl-meta-toluamide) at equivalent
con centrations of up to 50% (Hadis et al ., 2003).

Knock-
down resistance ( kdr ) is a mechanism of
resistance in mosquitoes, which is caused by
point mutations at the sodium channel pre-
venting the binding of the insecticide. Thus, the
large-scale use of synthetic pyrethroids has
inevitably led to the development of insecticide
resistance in many areas of Africa and Asia
(Ranson et al ., 2011). This situation has ordered
a demand on new strategies to fi ght resistant
strains. Organic chemistry is dominated by a
remarkably small number of molecular scaf-
folds with 0.25% of the molecular frameworks
found in 50% of the known compounds, and
looking closely at plant toxins may of er novel
alternatives.
Looking back at older plant-based in-
secticides may also be helpful in the management
of resistance. Duchon et al . (2009) compared
the ei cacy of pyrethroid- and pyrethrin-treated
nets against pyrethroid-resistant strains of An .
gambiae s.s. in the laboratory. Pyrethrin was
shown to be less af ected by the presence of the
kdr mutation in the mosquitoes, still inducing
95% knockdown ef ect (KD 95 ), repellency and
blood-feeding inhibition against susceptible
An . gambiae for up to 9 months after application
under laboratory conditions. Resistance man-
agement strategies in the future might consider
using pyrethrin or formulating pyrethrins with
wash resistance and UV protection as an

Rotenone
Rotenone is an insecticidal isofl avonoid
extracted from the roots or rhizomes of more
than 60 species of the family Leguminosae , in
particular Derris , Lonchocarpus and Tephrosia .
Insect mortality only occurs after ingestion of
the active compound, which af ects energy
production by blocking the dehydrogenase
enzyme at mitochondrial level. Rotenone is
highly toxic to insects while being relatively
harmless to plants, making it a commonly used
pest control agent in organic agriculture.
However, its environmental and end-consumer
safety is questionable because of its high toxicity
to fi sh (Teixeira et al ., 1984; Cheng and Farrell,
2007). Rotenone has recently been linked to the
onset of Parkinson's disease in humans, leading
to most manufacturers voluntarily withdrawing