The Use of Plants in Vector Control - Biological and Environmental Control of Disease Vectors - page 53

side-ef ects to non-target species, humans and
the ecological balance of the ecosystem (Ault,
1994). Environmental management practices
should be specifi cally tailored to the ecological
requirements of local vector populations they
are designed to control (Konradsen et al ., 2004).
However, despite its potential signifi cance in
disease control, there is still a lack of basic
knowledge of the behaviour and ecological
requirements of most vector species (Ferguson et
al ., 2010). The importance of obtaining
accurate information on the distribution of
breeding sites is becoming increasingly
recognized (Takken et al ., 1990; Walker and
Lynch, 2007), and remote sensing technologies
and geographical information systems (GIS) are
being used to create spatial maps of larval
habitats in relation to local conditions (Thomas
and Connor, 2000; De Castro et al ., 2004; Cecchi
et al ., 2008), so environmental control practices
may be improved and more widely used in the
future. This section describes some studies that
have taken advantage of the association between
plants and the life cycle of disease vectors, often
as part of integrated vector control programmes,
in order to reduce or eliminate their habitats.

extreme light-blocking ef ects and suppresses the
growth of other weeds (Krock et al ., 1991).
Thus, mosquito oviposition and larval
development of Anopheles and Culex mosquitoes
can be inhibited by extensive growth and
complete coverage of water bodies by this
aquatic fern as several laboratory studies in Asia
(Amerasinghe and Kulasooriya, 1985; Mogi et
al ., 1986; Bao-Lin, 1987; Rajendraan and
Reuben, 1988) and fi eld studies in India (Ansari
and Sharma, 1991; Rajendraan and Reuben,
1991) and in Tanzania (Mwingira et al ., 2009)
have shown. In addition, azolla forms a highly
productive nitrogen-fi xing symbiosis with the
blue-green algae Anabaena azollae . This complex
is of commercial value in South-east Asia as a
biological fertilizer for rice, and would therefore
encourage the use of azolla in irrigated rice
farming, making it a socially acceptable vector
control method (Lumpkin and Plucknett, 1980;
Bao-Lin, 1988; Rajendraan and Reuben, 1991).
Sites infested with duck weed ( Lemna minor ),
another aquatic plant with larvicidal properties,
are avoided by ovipositing Cx . pipiens pipiens
mosquitoes (Eid et al ., 1992). However, the
introduction of such weeds may have signifi cant
impact on non-target organisms (Lacey and Orr,
1994). Therefore, its spatial and temporal
introduction would have to be carefully planned
and monitored, so it does not spread beyond the
target sites and so that it coincides with the
period before mosquito breeding season in order
to be as ef ective as possible (Mwingira et al .,
2009).

4.2.1 Vegetation management

Shading

Some disease vectors require sunny conditions
for larval development (Gimnig et al ., 2001),
and thus planting trees to shade prominent
breeding sites has been used as a means to
reduce the abundance of sun-loving malaria
vectors such as An . gambiae , An . minimus , An .
funestus and An . sundaicus (Rafatjah, 1988).
The control of An . maculatus mosquitoes, which
transmit malaria in Asia, by the use of coconut
husks to cover streams and to create shady
environments was an important step in
reducing the disease burden in the more
mountainous areas of Malaysia (Hackett et al .,
1938; Konradsen et al ., 2004).
Rather than indiscriminately planting new
trees, which may or may not be of commercial
or social value to the local people and economy,
the free-fl oating fern azolla has been investigated
for use against mosquitoes. Azolla, which grows
in a thick mat on the surface of water bodies, has

Vegetation removal
While some mosquitoes require sunny con-
ditions to complete their life cycles, other species,
such as An . umbrosus and An . dirus in South-east
Asia, prefer shaded breeding sites (Rafatjah,
1988). Forest clearance has been an ef ective
measure against these malaria vectors in
Malaysia, but indiscriminate removal of jungle
can lead to the emergence of other disease
vectors, for example the malaria vectors An .
maculatus , An . minimus and An . fl uviatilis , which
favour breeding at the edges of streams exposed
to sunlight and contribute to new epidemic
outbreaks (WHO, 1987; Konradsen et al ., 2004).
In addition to forest clearance, removal of
aquatic vegetation has also been part of