Control with Arthropods - Biological and Environmental Control of Disease Vectors - page 16

negative correlation was found between the
presence of the Toxorhynchites predator and the
St . aegypti prey, showing that in the wild, these
predatory mosquitoes can control vector mos-
quito numbers ef ectively. In 70% of the Tx .
splendens- positive larval habitats, no St . aegypti
larvae were found (Yasuno and Tonn, 1970). In
Tanzania, Tx . brevipalpis were found to naturally
co-occur with St . aegypti mosquitoes living in
discarded tyres and tins. A negative correlation
was found between St . aegypti numbers and Tx .
brevipalpis presence (Trpis, 1973). In South
America, the Tx . guadeloupensis predator has
been reported to naturally co-exist with St .
aegypti , albeit in a particularly shady garden in
Brazil (Honorio et al ., 2007).
Nevertheless, there are many areas where
the local Toxorhynchites species do not overlap
with the targeted disease vectors. In northern
Argentina, Toxorhynchites were only found in
bromeliad leaf axils, whilst St . aegypti were
found in man-made containers (Stein et al .,
2011). Similarly, in Puerto Rico, St . aegypti were
predominant in urban areas whereas Tx .
portoricensis were found in rural forested areas
(Cox et al ., 2007). Furthermore, even though a
predator species may overlap with the prey
species in some larval habitats, they do not
always co-occur in all habitats. For instance, in
Bangkok Tx . splendens were found in water jars
but never in ant traps or fl ower pots (Yasuno and
Tonn, 1970), whereas St . aegypti regularly
breeds in all of these habitats. Thus, Tx . splendens
would only be able to be used for St . aegypti
control in water jars, which would never crash
the dengue vector population while alternative
habitats are still available. Additionally, the
native Toxorhynchites species may not overlap
with all disease vectors in a given area. In Puerto
Rico, both St . aegypti and St . mediovittatus are
dengue vectors, but only St . mediovittatus
coexists in the same habitat as the native Tx .
portoricensis (Cox et al ., 2007).
Successful disease vector control will rely
on this ovipositional overlap of the predator and
prey mosquitoes within a given habitat. Studies
were conducted specifi cally looking at
oviposition of Tx . moctezuma (previously Tx .
theobaldi ) in Mexico. In a graveyard, Tx .
moctezuma often oviposited in typical St . aegypti
breeding habitats, with a positive linear cor-
relation between the surface area of the water

body and the number of predator eggs
laid (Arredondo-Bernal and Reyes-Villanueva,
1989). Previous work had also shown that
larger water containers were more frequently
found with eggs when compared to smaller
containers (Reyes-Villanueva et al ., 1987).
Recently, Tx . moctezuma were found to be fi ve
times more likely to oviposit in a habitat
containing prey St . aegypti larvae than in a
control habitat (Macias-Duarte et al ., 2009).
Such high specifi city of the predator for its prey
is a useful asset in biological control.
The deployment of predatory Toxorhynchites
mosquitoes for dengue vector control has been
ongoing for several decades in many dif erent
countries. In 1980, a study was set up in New
Orleans, USA, that was designed to estimate the
ef ect of low levels of Tx . rutilus rutilus on adult
St . aegypti and Cx . quinquefasciatus emergence.
Every 10 days, zero, one or two Tx . r . rutilus
larvae were put into tyres, paint cans and
buckets distributed between three residential
blocks. By the end of the 76-day study period,
one Tx . r . rutilus larva per container reduced
prey emergence by an average of 65% (compared
to no predator larva), whereas two predator
larvae resulted in an 82% reduction (Focks et al .,
1982). Thus, even low levels of predators can
have signifi cant impacts on vector mosquito
populations. When adult Tx . r . rutilus were
released to examine their natural oviposition
behaviour, little dispersal from the release site
and a lack of ovipositional overlap between Tx . r .
rutilus and St . aegypti was found, leading the
authors to conclude that the species 'has little
value as a biocontrol agent of Ae . aegypti and Cx .
quinquefasciatus in the urban environment'
(Focks et al ., 1983). Adult Tx . amboinensis were
also released into 16 residential blocks in New
Orleans. In general, around 40% of containers
and 90% of tyres were found to be positive for
these predators, indicating good oviposition
rates or less frequent loss of predator eggs (the
eggs, being hydrophobic, are readily splashed
out of some containers with rainfall). When
present in a container, Tx . amboinensis were able
to reduce prey productivity by 81% (Focks et al .,
1985), but as with Tx . r . rutilus , there was
limited movement of the predatory mosquito
outside of the study area.
Toxorhynchites splendens have been suc-
cessfully used in Thailand, leading to a more