The Use of Plants in Vector Control - Biological and Environmental Control of Disease Vectors - page 52

the dif erences in habitat preferences. In Brazil, a
15-20-day anaerobic fermentation made of
fresh and mature Panicum maximum leaves was
the optimum infusion for trapping ovipositing
St . aegypti and St . albopicta mosquitoes (Sant'ana
et al ., 2006).
Millar et al (1992) isolated and identifi ed
fi ve chemical compounds from a fermented
Bermuda grass infusion, which attracted and
stimulated oviposition of gravid Cx . quin-
quefasciatus females. The single most attractive
of those compounds was 3-methylindole (or
skatole), not just to Cx . quinquefasciatus in the
laboratory (Millar et al ., 1992) but also to other
species such as Cx . tarsalis , Cx . stigmatosoma and
Cx . cinereus in the fi eld (Beehler et al ., 1994;
Mboera et al ., 1999, 2000a,b). Odorant
receptors of Cx . quinquefasciatus are highly
sensitive to skatole and indole (Leal et al ., 2010;
Pelletier et al ., 2010). Another plant-derived
compound, which has been isolated from the
summer cypress plant ( Kochia scoparia ) seed oil,
is (Z)-5-hexadecenoic acid, which gets converted
to (5R,6S)-6-acetoxy-5-hexadecanolide (also
known as plant-derived pheromone, PDP)
(Olagbemiro et al ., 1999). Laboratory and fi eld
assays show that PDP was as ef ective in
stimulating egg-laying of gravid Cx .
quinquefasciatus as a synthetic oviposition
pheromone (Olagbemiro et al ., 2004). PDP can
be produced at a cost of approximately US$3
g −1 , US$12 g −1 cheaper than the conventional
synthetic methods (Logan and Birkett, 2007).
Botanical infusions made from crude plant
extracts have been found to be impossible to
standardize due to variation in fermentation
times, seasonal changes and batch-to-batch
dif erences among plants (Beehler et al ., 1994;
Mboera et al ., 1999; Sant'ana et al ., 2006;
Ponnusamy et al ., 2010b). Therefore, specifi c
habitat-derived volatiles such as skatole and
PDP, which can be produced commercially, may
be useful tools in future mosquito surveillance
and control programmes.

leaves have an oviposition deterrent and repel-
lent ef ect of 90-100% against An . arabiensis
and Cx . quinquefasciatus in Sudan (Elimam et al .,
2009). The infusion was particularly powerful
when mosquitoes were given a choice between
untreated control and the botanical extract.
When there was no choice, gravid females laid
most of their eggs into cups containing the
lowest extract concentration (Elimam et al .,
2009).
Neem tree extracts can inhibit oviposition
in mosquitoes (Mulla and Su, 1999). Gravid St .
aegypti were deterred from egg-laying by crude
neem leaf extract concentrations of 0.005%,
0.05% and 0.5% compared to ethanol, although
egg hatching rates were not af ected (Fuk,
2004). Oral administration of neem extracts
suppressed blood-feeding and oviposition of Cx .
tarsalis and Cx . quinquefasciatus females (Su and
Mulla, 1999a). Anopheles stephensi and An .
culicifacies adult females exposed to neem
volatiles exhibited oviposition suppression and,
after continuous exposure for 7 days, irreversible
inhibition of the gonotrophic cycle (Dhar et al .,
1996). Female An . stephensi treated orally with
the commercial neem formulation NeemAzal ®
(NA), consisting of limonoids, particularly 34%
azadirachtin A, laid fewer eggs (Lucantoni et al .,
2006). The number of eggs per microlitre
produced by An . stephensi decreased from nine
eggs in the control mosquitoes to fi ve eggs at a
concentration of 100 ppm and to zero at a
concentration of 1000 ppm NA. The oviposition
behaviour of An . gambiae , on the other hand,
was not af ected by crude neem tree extracts in
the laboratory (Howard et al ., 2011). Overall,
the evidence so far gathered for plant-derived
oviposition repellents is scarce and contradictory.
Before botanical compounds can be used for this
purpose, more practical research is needed.

4.2 Environmental Control of Disease
Vectors with Plants

Many disease vectors rely heavily on key
resources such as suitable aquatic breeding and
larval sites to complete their life cycles. The aim
of environmental control is to modify or
eliminate these aquatic habitats in order to
reduce the number of vectors and disrupt disease
transmission whilst causing minimal adverse

4.1.5 Oviposition repellents

The use of botanical compounds to repel
mosquitoes from potential oviposition sites has
not been extensively explored. Crude aqueous
extracts from castor oil plant Ricinus communis