BiocontrolNews and Information 2000 Vol. 21 No. 4 105N – 116N
The biology of Toxorhynchites mosquitoes and
their potential as biocontrol agents
Larissa E. Collins1and Alison Blackwell2
Department of Biological Sciences, University of Dundee, UK
Abstract
Toxorhynchites spp. mosquitoes are recognised as potential biological control agents of pest
and vector species of mosquito. There have been many attempts to use them for this purpose
since the beginning of the twentieth century, although with relatively low levels of success,
which has been attributed to a lack of knowledge of the general biology of Toxorhynchites
mosquitoes. Increasing resistance of vector mosquitoes to traditional chemical pesticides and
the expansion of the ranges of these vector mosquitoes have made the search for alternative
methods of mosquito control imperative. This review draws together the current knowledge
of both the taxonomy and the general biology of Toxorhynchites mosquitoes and details
previous attempts to use this group as biocontrol agents and inintegrated control programmes.
In addition, it makes recommendations for further study of this group in order to facilitate their
successful utilization against vector mosquitoes.
Introduction
Mosquitoes(Dipt., Culicidae)are responsible for the transmission
of the pathogens causing some of the most life-threatening and
debilitating diseases of man, including malaria, yellow fever,
dengue fever and filariasis. In many areas the incidence and
geographical distribution of these diseases have expanded, largely
as a result of decreasedefficacy of vector-control programmes and
subsequent increases in vector mosquito populations.
Mosquitoes are also becoming increasingly resistant to traditional
chemical pesticides and there is growing concern about the
potential health and environmental risks surrounding these
products. Environmental protection agencies have banned or
placed severe restrictions on the use of many pesticides which
were formerly used in mosquito control programmes and there are
now fewer adulticides available than there have been for the last
20 years (Rathburn, 1990). Furthermore, manufacturers
themselves have withdrawn some insecticides due to the high cost
of carrying outthe additional tests now required by governments,
in addition to the fact that the production of crop pesticides for the
agricultural market is much more lucrative (Rathburn, 1990). It is
likely, therefore, that mosquitoes will very quickly develop high
levels of resistance to the remaining available adulticides, leading
to concern among operational mosquito control personnel that
effective insecticides may not be available in the near future
(Kline, 1994). Hence, it is imperative that novel mosquito control
methods are developed and put into general use as soon as
possible.
One potential alternative approach to the use of chemical
pesticides is the use of Toxorhynchites spp. mosquitoes as
biological control agents of pest mosquitoes. This was suggested
as early as 1911 by W. R. Colledge in an address to the Royal
Society of Queensland and since this time there have been many
attempts, some successful and some not. Toxorhynchites
mosquitoes have an unusual life cycle in that they are not capable
of blood feeding and, therefore, are not pests or vectors. In
addition, their larvae are predatory on other mosquito larvae. In
the development of any biological control strategy it is imperative
that the biology and taxonomy of both the target species and the
potential biological control agent are understood fully, whichhas
not been the case for most Toxorhynchites species. Due to their
lack of importance as pest species, their general biology and
taxonomy have been largely neglected. Exceptions have been
many isolated descriptions of particular aspects of the biology of
some Toxorhynchites spp. mosquitoes, in addition to a small
number of taxonomic studies.
This review draws together the current knowledge of the general
biology and taxonomy of Toxorhynchites spp. mosquitoes. It also
describes previous attempts to use these mosquitoes as biological
control agents of vector mosquitoes and makes recommendations
for further studies of this group.
Review Article
1Present address: Dr L.E. Collins, Central Science Laboratory, Ministry of
Agriculture, Fisheries and Food, Sand Hutton, York YO41 1LZ, UK.
2Present and correspondence address: DrA. Blackwell,
Centre for Tropical Veterinary Medicine, Royal (Dick) School of Veterinary
Studies, University of Edinburgh, Easter Bush Veterinary Centre,
Roslin, Midlothian, EH25 9RG, UK.
106N BiocontrolNews and Information 2000 Vol. 21 No. 4
General Biology of Toxorhynchites Mosquitoes
The biology of Toxorhynchites species has been reviewed by
several authors since Colledge’s address on the biology of
Toxorhynchites speciosus Skuse (Colledge, 1911). These reviews
have dealt with specific groups of Toxorhynchites mosquitoes
(MacDonald, 1958; Horsfall, 1972), the wider biology of the
Toxorhynchites group (Steffan & Evenhuis, 1981), in addition to
their potential as biological control agents (Focks, 1982; Gerberg,
1985). A small number of species with potential as biological
control agents has been considered more fully (Table 1).
Eggs
The eggs of Toxorhynchites spp. mosquitoes are white or yellow,
oval and waterrepellent andare found either floatingon top of the
water surface or just below it. The incubation period is 40-60
hours and is temperature dependent. Egg viability is 57-100% and
decreases with the age of the female in some species (Steffan &
Evenhuis, 1981). A description of the general structure of
Toxorhynchites eggs is included in Sahlen (1996).
Larvae
All instars of Toxorhynchites spp. larvae are predatory. They are
traditionally thoughtto feed on mosquito larvae;most commonly
approximately the same size as themselves, although it is thought
that they will also feed on larvae up to twice their size. They will
also take almost any typeof moving prey and in the absence of live
prey,theywillfeedondetritus(Steffan&Evenhuis,1981).
The feeding rate and total prey consumption during larval
development depend on a number of factors, including container
size, prey size, prey type, water temperature and light level.
During its development, one Toxorhynchites spp. larva requires
approximately 5000 first-instar prey larvae and 300 fourth-instar
prey larvae (Steffan & Evenhuis, 1981; Focks, 1982). The larvae
of a number of Toxorhynchites species have been described and
their predatory behaviour studied. In particular, descriptions of the
searching behaviour and population densities of Toxorhynchites
spp. larvae in relation to prey larva populations have been
published for a number of species (Table 2).
Prepupal killing behaviour has been observed in the larvae oftwo
Toxorhynchites species; Toxorhynchites brevipalpis Theobald
Table 1. Studies of the general biology of some Toxorhynchites species.
Species Reference
T. amboinensis Steffan et al. 1980
T. brevipalpis Muspratt 1951; Corbet 1963;
Corbet & Griffiths 1963; Lounibos 1979
T. kaimosa Corbet & Griffiths 1963
T. rutilus rutilus Jenkins & Carpenter 1946; Focks et al.1977
T. rutilus septentrionalis Jenkins & Carpenter 1946; Williams et al. 1961; Dodge 1964;
Crans & Slaff 1977
T. splendens Newkirk 1947; Breland 1949; Chan 1968;
Furuzimo & Rudnick 1978
T. theobaldi Rubio et al. 1980; Rubio & Ayesta 1984
Table 2. Studies of larvalbehaviour and population densities of Toxorhynchites species.
Species Reference
T. amboinensis Focks et al. 1981; Robert et al. 1983; Barber & Hirsch 1984; Russo 1986;
Linley & Duzak 1989; Linley 1990
T. brevipalpis Corbet & Griffiths 1963; Goma 1964; Sempala 1970, 1983; Trpis 1972, 1973;
Lounibos 1979; Vongtangswad & Trpis 1980; Lamb & Smith 1980;
O'Flynn & Craig 1982; McIver & Siemicki 1982; McIver & Beech 1986;
Robert et al. 1983; Schuler &Beier 1983; Russo 1986; Linley 1990;
Linley & Duzak 1989
T. haemorrhoidalis Lounibos et al. 1987
T. kaimosa Goma 1964; Sempala 1983
T. moctezuma Chadee 1985; Sherratt & Tikasingh1989; Rawlins& Ragoonansingh 1990;
Chadee & Small 1991; Rawlins et al. 1991; Tikasingh 1992;
Tikasingh & Eustace 1992
T. rutilus Focks et al.1978, 1980; Trimble & Smith 1978, 1979; Lamb & Smith 1980;
Padgett & Focks1980, 1981; Russo 1983, 1986; Schuler & Beier 1983;
Bradshaw & Holzapfel 1984; Frank et al. 1984; Lounibos 1985;
Lounibos et al. 1993; Hubbard et al. 1988
T. splendens Newkirk 1947; Russo 1986; Vongtangswad et al. 1983;
Linley & Duzak 1989; Toma & Miyagi 1992; Amalraj & Das 1996
T. theobaldi Russo 1986
T. towadensis (Matsumura) Yasuda 1996
ReviewArticle 107N
(Corbet & Griffiths, 1963) and Toxorhynchites amboinensis
(Doleschall) (Taylor, 1989). Just before pupation, fourth-instar
larvae kill but do not consume prey larvae. The most widely
accepted theory to explain this behaviour is that any other
potential predators in the same aquatic environment are killed
before the Toxorhynchites spp. larvae become vulnerable pupae
(Corbet & Griffiths, 1963). Russo & Westbrook (1986)
hypothesized that this behaviour is analogous to changes in
feeding behaviour at the same stage in other insects and attempted
to show that it was similarly governed by increases in ecdysteroid
levels.
The larvae of many species ofToxorhynchites,inadditiontobeing
predatory, are cannibalistic in all instars. The degree of
cannibalism displayed depends on prey density and behaviour,
size of prey relative to Toxorhynchites spp. larvae and the number
of hiding places in the container (Focks, 1982). The cannibalistic
behaviour of a few Toxorhynchites species has been described in
detail, along with the effects it has on the potential of the species
as a biological control agent. These species included T.
amboinensis (Focks et al., 1981; Linley, 1988; Linley & Duzak,
1989; Annis et al., 1990; Horio et al., 1990), T. brevipalpis
(Muspratt, 1951; Sempala, 1983; Linley & Duzak, 1989),
Toxorhynchites kaimosa van Someren (Sempala, 1983),
Toxorhynchites longgianeolata Macquart (Farghal, 1983) and
Toxorhynchites moctezuma (Dyar&Knab)(Sherrattet al., 1999).
Descriptions of egg cannibalism have also been made for T.
moctezuma (Chadee & Small, 1991), T. amboinensis and
Toxorhynchites rutilus (Coquillett) (Linley & Darling, 1993).
The duration of larval development in Toxorhynchites spp.
mosquitoes varies from 1-91 days, depending on species,
temperature and prey density (Steffan & Evenhuis, 1981),
followed by a pupal period of 3-12 days, which is dependent
mainly on temperature (Steffan & Evenhuis, 1981). Particular
aspects of the larval development of the following Toxorhynchites
species have been studied: Toxorhynchites splendens
(Wiedemann) (Jones, 1993), T. rutilus (Trimble & Smith, 1978,
1979; Trimble & Lund, 1983; Bradshaw & Holzapfel, 1984),
Toxorhynchites theobaldi (Dyar&Knab)(Rubioet al., 1980), T.
amboinensis (Robert et al., 1983), T. brevipalpis (Sempala, 1970;
Trpis, 1972, 1979; Robert et al., 1983) and Toxorhynchites
haemorrhoidalis Ficalbi (Lounibos et al., 1987).
At least one species, Toxorhynchites rutilus septentrionalis (Dyar
& Knab), is capable of diapausing. This occurs only in fourth-
instar larvae and is photoperiodically controlled. It allows the
larvae to survive temperatures as low as 7ºC (Steffan & Evenhuis,
1981) and hence, to survive as far north as New Jersey, USA,
where T. rutilus septentrionalis larval development varies
between diapausing populations in the north and non-diapausing
populationsin the south (Trimble & Smith, 1978, 1979).
Adults
There have been very few observations of Toxorhynchites spp.
adults in the wild. This is particularly true for the males since most
studies, both in the laboratory and in the field, have involved
observations of female oviposition behaviour. Eclosion in
Toxorhynchites spp. is not synchronized, it occurs during daylight
and, depending on the species,can be protogynous or protandrous.
For example, Toxorhynchites brevipalpis conradti Grünberg is
protogynous (Corbet, 1963), whereas T. rutilus septentrionalis is
protandrous (Crans & Slaff, 1977). Adult female Toxorhynchites
spp. mosquitoes are non-haematophagous, feeding on only nectar
and other plant derivedsugar sources. There have, however, been
only a small number of observations to confirm this. These include
records of T. moctezuma (as T. trinidadensis) feeding on the
flowers of the Christmas bush (Chromolaena odorata;
Asteraceae) and black sage (Salvia mellifera; Labiatae) (Urich,
1913), Toxorhynchites haemorrhoidalis superbus (Dyar & Knab)
feeding on Borreria verticilata (Rubiaceae) flowers (Heinemann
et al., 1980) and T. rutilus septentrionalis feeding on the nectar of
Hydrangea (Hydrangea macrophylla; Hydrangeaceae) flowers
(Williams et al., 1961) and on the sap of the black oak tree
(Quercus velutina; Fagaceae) (Nasci, 1986).
Very little is known about the adult dispersal and population sizes
of Toxorhynchites spp. in the wild. A mark-release-recapture
study of T. brevipalpis in a tyre dump in Dar es Salaam, Tanzania
estimated the population to be 1292 females and 2268 males per
hectare (Trpis, 1973). A method devised for 32P labelling
Toxorhynchites rutilus rutilus (Coquillett) in the field had the
potential of being used to track the dispersal of released adults, in
addition to the dispersal of eggs from released females (since 32P
is also passed on to the eggs) (Smittle & Focks, 1986). To date,
however, it has not been trialed in the field.
The oviposition behaviour of female Toxorhynchites spp.
mosquitoes is relatively well documented. The fullest description
of this is a study by Linley (1987a) of the oviposition flight of T.
amboinensis. The females rarely land on the water surface to
oviposit;instead theyperform an ovipositionflight which consists
of six to 43 elliptical loops, with the egg being released on the final
loop. The diel rhythm of oviposition in Toxorhynchites spp.
mosquitoes varies. For example, in the wild, peak oviposition
frequency in T. moctezuma occurs between 1200 h and 1700 h
(Chadee et al., 1987; Jordan & Hubbard, 1991a) compared with
the major oviposition peak for T. amboinensis in the laboratory,
which occurred between 1200 h and 1500 h, with a smaller peak
between 0800 hand 1000 h (Linley, 1987b).The reasons forthese
patterns of diel periodicity are unclear but may be related to
humidity (Jordan & Hubbard, 1991a) and rainfall patterns.
Toxorhynchites spp. mosquitoes oviposit during the rainy season
but not during the dry season. They survive the dry season as
fourth-instar larvae, then complete their juvenile development and
eclose at the beginning of the rainy season as the prey density
increases. For example, T. rutilus ecloses approximately two
weeks after the beginning of the rainy season (Bradshaw &
Holzapfel, 1984).
Distribution and Habitat of
Toxorhynchites Mosquitoes
Toxorhynchites mosquitoes inhabit most of the tropical regions of
the world. Edwards (1932) described the group as ‘tropicopolitan’
and Horsfall (1972) gave the distribution of the group as extending
to 40ºN and south to the edge of the southern tropics. They also
inhabit some subtropical and temperate regions. For example,
Toxorhynchites christophi (Portschinsky) has been found in the
Amur Valley in Russia (54°N) (Shamrai & Gutsevich, 1974) and
T. rutilus septentrionalis isfoundupto45°NinCanada(Parker,
1977). Members of the Lynchiella subgenus have also been
reported as far north in the USA as New Jersey and as far west as
Kansas and Texas (Jenkins & Carpenter, 1946).Stone et al. (1959)
detailed the regions inhabited by the three subgenera of the
Toxorhynchitinae subfamily (Table 3).
Tropical and subtropical forests are the main vegetation types
within which Toxorhynchites spp. mosquitoes are found
(Muspratt, 1951), although they also inhabit coastal palm belts;
for example T. splendens is found in this habitat in India, Sri
Lanka, Malaysia, Thailand, Papua New Guinea and the
Philippines (MacDonald, 1958). It was suggested that this wide
distribution may have been facilitated by transportation of the
larvae to new sites in water storage containers on ships.
108N BiocontrolNews and Information 2000 Vol. 21 No. 4
Classification of Toxorhynchites Mosquitoes and
Keys to the Group
Partly due to their unimportance as pest species, the taxonomy of
Toxorhynchites spp. mosquitoes has been largely neglected.
Toxorhynchites were first recognised as a distinct group in 1827
when Robineau-Desvoidy proposed that they be known as
Megarhinus (Robineau-Desvoidy, 1827). Since this work there
have been only two taxonomic revisions of the whole group.
These are the works of Theobald (1901) and Edwards (1932) on
Culicidae, in which the Toxorhynchites (as Megarhinus)were
included.
Edwards (1932) divided the 52 species known at the time into
three groups based on the differences in the maxillary palps of the
adult females. Group A, Megarhinus, was composed of 20
Neotropical species; Group B, Ankylorhynchus, contained two
Brazilian species and Group C, Toxorhynchites, was composed of
30 species from the Old World. There have,however, been many
regional studies of the taxonomy of Toxorhynchites mosquitoes
(Table 4), although the taxonomy of the Toxorhynchites species of
the New World is still thought to need clarification (Steffan,
1975). More recently, descriptions of type specimens in museums
have been produced, including those held at the United States
National Museum of Natural History (Steffan, 1980) and those
held at the British Museum (Natural History) and Oxford
University(Steffan & White,1981). A key to and descriptions of
the Afrotropical Toxorhynchitinae have also been produced
recently (Service, 1990).
Descriptions of new Toxorhynchites species or species new to a
particular area have been added to the literature as they have been
identified. Recent examples include descriptions of
Toxorhynchites yaeyamae Bohart from Japan (Sâto & Arita,
1968), Toxorhynchites bengalensis Rosenberg & Evenhuis from
Bangladesh (Rosenberg & Evenhuis, 1985), Toxorhynchites
auranticauda Lane from Indonesia (Lane, 1992), T. rutilus
septentrionalis from Rhode Island (Lawson et al., 1994) and
Toxorhynchites macaensis Ribeiro from Macau (Ribeiro, 1997).
Due to the fragmented nature of the taxonomy of the
Toxorhynchites group, the same species has often been described
more than once but has been named differently on each occasion.
Also, the interspecific relationships of Toxorhynchites spp.
mosquitoes are unclear and most of the species are very similar to
each other. This has resulted in serious implications for biological
control attempts using Toxorhynchites species. For example, the
species introduced into Hawaii as T. splendens was later found to
be T. amboinensis and this error was repeated when T.
amboinensis was introduced into other Pacific Islands as a
biological control agent (Steffan, 1975).
Stone et al. (1959) and Stone (1961, 1963, 1967, 1970) in their
Synoptic Catalog of the Mosquitoes of the World and its
supplements listed 66 Toxorhynchites species, divided into three
Table 3. Regions inhabited by the three subgenera of Toxorhynchitinae (Stoneet al., 1959).
Subgenus Region/country
Ankylorhynchus (Lutz) Argentina, Brazil, Bolivia
Lynchiella (Lahille) South America: Argentina, Paraguay, Brazil, Peru, Ecuador,
Surinam, Guyana, French Guiana, Venezuela, Colombia
Central America: Panama, Costa Rica, Nicaragua, Honduras,
El Salvador, Guatemala, Mexico
Caribbean: Greater Antilles (Puerto Rico, Haiti, Cuba),
Lesser Antilles, Trinidad
Southeast USA (Florida, Georgia, South Carolina).
Toxorhynchites Theobald Australasian Region: Australia, Papua New Guinea
(including Bismarck Archipelago), Tuvalu, Hawaii
Oriental Region: eastern Siberia, western Himalayas, Indochina,
including Japan (including Ryukyu Islands), Taiwan, Philippines,
Malaysia (Malaya) Singapore, Indonesia (Sumatra, Java, Maluku,
Sulawesi, Borneo), Thailand, India, Sri Lanka
Ethiopian Region: Gambia, Sierra Leone, Liberia, Ghana, Nigeria,
Cameroon, Gabon, Uganda, Kenya, Tanzania (including Zanzibar),
Democratic Republic of Congo, Malawi, Mozambique,
South Africa (Transvaal, Natal, Cape Province), Madagascar
Table 4. Regional reviews of the taxonomy of Toxorhynchites species.
Region Review
New World Dyar 1928; Vargas 1953a, b; Lima et al.1962
Central and North America Howard et al.1917
Neotropical Region Lane 1953
Caribbean Region Belkin et al. 1970
Afrotropical Region Hopkins 1952; Service 1990
India Barraud 1934
South Pacific Belkin 1962
ReviewArticle 109N
subgenera: Ankylorynchus, Lynchiella and Toxorhynchites.
However, by 1975 only 53% of these species were known in all
stages and the eggs of only five species had been described
(Steffan, 1975). There have been some more recent descriptions to
add to this, particularly of the eggs, including those of T.
splendens (Mattingly, 1969; Linley & Seabury, 1990), T.
brevipalpis (Lamb & Smith, 1980), T. rutilus (Lamb & Smith,
1980; Linley 1989), T. amboinensis (Steffan et al., 1980; Russo &
Westbrook, 1986; Linley, 1989; Linley & Seabury, 1990) and T.
moctezuma (Chadee et al., 1987). In addition, the pupa of T.
rutilus septentrionalis has been described (Steffan & Evenhuis,
1980)
The most recent classification of Toxorhynchites spp. dealt with
the Toxorhynchites group regionally; for example, the first section
covered the Australasian, east Palaearctic, and Oriental species-
groups (Steffan & Evenhuis, 1985). The authors tried to clarify the
complex interspecific relationships in the group by taking into
account both intuitive taxonomic methods and numerical
phylogenetic studies. They also included keys to the
Toxorhynchites species found in each region.
Biological Control of Pest and Vector Mosquitoes
The increasing problem of how to control pest and vector
mosquito populations effectively was recognised in 1982 when
the World Health Organization (WHO) Expert Committee on
Vector Biology and Control metin Geneva. The then Director of
the Division of Vector Biology and Control, Dr N. G. Gratz, stated
that although the use of residual pesticides had provided a cheap
and simple way to control vectors of disease, the increase in
pesticide resistance and the environmental damage caused by
these pesticides had rendered their long-term use unfeasible
(WHO, 1982). The 23rd World Health Assembly in 1970 had
previously recommended the developmentof alternative methods
of vector control (World Health Assembly, 1970).
Alternative methods of mosquito control had been investigated
and evaluated even before this time. In 1950 H. H. James reported
to the Quebec Society for the Protection of Plants on a number of
possible biocontrol agents of mosquito larvae (James, 1950). The
natural predators that they identified as possible biocontrol agents
were dragonfly nymphs, damselfly nymphs, dytiscid beetles,
phantom midge larvae, corixid water bugs and two species of
stickleback fish. Ducks have also been identified as natural
predators of mosquitoes, for example the Australian whistling tree
duck (Dendrocygna arcauta) and the black duck (Anas
superciliosa) feed on at least two species of Culex mosquitolarvae
(Marks & Lavery, 1959). Ramoska & Sweet (1981) have also
observed a spider (Agelenopsis naevia (Walckenaer) (Araneae,
Agelenidae)) which is a natural predator of adult mosquitoes
ovipositing into discarded tyres. Mosquito fish (Gambusia affinis)
have been introduced to storm drains in California to control
Culex quinquefasciatus Say (Mulligan et al., 1983). Culex larvae
have also been controlled using the planarian Dugesia
dorotocephala (Ali & Mulla, 1983) and in New Zealand, Aedes
australis (Erichson) mosquitoes have been successfully controlled
in experimental pools by the fungus Coelomomyces opifexi Pillai,
which has a copepod as its intermediate host (Pillai, 1985).
The bacterium Bacillus thuringiensis subsp. israelensis (Bti)has
been used widely and successfully as a biological control agent
against mosquito larvae. For example, it was lethal to eight
mosquito species, including Aedes aegypti (L.) and Culex
quinquefasciatus in the laboratory and was effective in the field
against Culex tarsalis Coquillett (Mulla et al., 1982). In addition,
it successfully controlled Culex sitiens Wiedemann, a vector of
Ross River Virus in Fiji (Pillai, 1985). The related Bacillus
sphaericus hasalsobeenusedagainstmosquitolarvae.For
example, in a field trial in Recife, Brazil, it successfully controlled
C. quinquefasciatus (Regis et al., 1995). There is, however,
evidence that C. quinquefasciatus is capable of developing
resistance to B. sphaericus (Rodcharoen & Mulla, 1994) and at
least one other species of mosquito, Aedes albopictus (Skuse), is
susceptible to Bti but not to B. sphaericus (Ali et al., 1995). In
addition to resistance, another major problem of the use of
Bacillus biolarvicides against mosquitoes is that their toxic
proteins are not sufficiently residual in the environment, although
there have been attempts to overcome this problem, including
genetically engineering the micro-organisms on which mosquito
larvae feed to express Bti and B. sphaericus toxic proteins (Orduz
et al., 1995).
There is one main disadvantage in using any of the above
biological control agents: theyhave to be manually introduced to
the majority of pest and vector larval habitats to yield adequate
control, which requires regular maintenance. Toxorhynchites spp.
mosquitoes offer an alternative form of biological control. Their
predatory larvae often feed upon the larvae of vector species of
mosquito and hence, Toxorhynchites spp. adult females seek out
these same aquatic habitats to lay their eggs. An additional tool in
this respect might be the exploitation of oviposition cues for
Toxorhynchites species. They could, for example, be used to
encourage released adult female Toxorhynchites spp. mosquitoes
to remain in the target area, in addition to increasing the efficiency
of any release programme since both predator and prey may be
attracted to the same oviposition sites. Although oviposition
aggregation cues or pheromones have yet to be used on a regular
basis with Toxorhynchites spp., potential oviposition cues for T.
moctezuma and T. amboinensis have been identified in
electroantennogram studies and oviposition bioassays.
Electroantennograms were recorded for seven compounds found
commonly in water containing decaying leaves and known to be
attractive to other mosquito species: 4-methylcyclohexanol,
phenol, indole, 3-methylindole, m-cresol, o-cresol and p-cresol
(Collins & Blackwell, 1998). In laboratory oviposition bioassays
4-methylcyclohexanol, 3-methylindole, m-cresol, o-cresol and p-
cresol were found to act as oviposition attractantsfor both ofthese
species (Collins & Blackwell, in press). For other species,
oviposition aggregation cues have been used successfully to
monitor pest mosquito populations of A. aegypti (Reiter et al.,
1991) and there has been a successful field trial in Kenya using
oviposition pheromone combined with an insect-growth regulator
to control C. quinquefasciatus (Otieno et al., 1988).
Biological Control of Pest and Vector Mosquitoes
Using Toxorhynchites Species
Toxorhynchites spp. mosquitoes are potentially ideal biological
control agents, as highlighted by Trimble (1983): the adults do not
blood feed and therefore cannot themselves act as vectors of
disease; the larvae are predatory on other mosquito larvae and
show ‘prepupal killing’ behaviour (before pupation they kill but
do not consume large numbers of potential prey) and in addition,
female Toxorhynchites spp. oviposit into pools of water which are
not accessible to chemical control methods. However, biological
control using Toxorhynchites spp.mosquitoes has not alwaysbeen
successful, often because introduced Toxorhynchites spp.
populations have not always become established and, even if they
have, established Toxorhynchitesspp. populations have frequently
failed to give an adequate level of control of pest mosquitoes. One
reason for this failure is that there is a delay between the pest
mosquito and Toxorhynchites spp. population increases, since
Toxorhynchites spp. mosquitoes have a generation time which is
approximately three times longer than that of their prey; resulting
110N BiocontrolNews and Information 2000 Vol. 21 No. 4
in a high peakin the pest mosquito population before it is able to
be challenged by the Toxorhynchites spp. (Trpis, 1973; Lounibos,
1979; Service, 1983). Additional failures in control can also occur
if the established Toxorhynchites spp. populations inhabit and
oviposit into different aquatic habitats from the target species
(Focks, 1982; Service, 1983).
The Pacific islands have been popular test sites for introductions
of Toxorhynchites spp. as biocontrol agents, with the first attempts
carried out on Hawaii in 1929 (Swezey, 1930; Pemberton, 1931),
followed by releases of T. brevipalpis from Africa in 1950
(Bonnet & Hu, 1951), T. theobaldi from Panama in 1953, and T.
amboinensis (as T. splendens) from Manila in 1953 (Steffan,
1975). Integration of Toxorhynchites spp. into filariasis-control
programmes on the Hawaiian Islands was the aim of the
introduction of T. amboinensis (as T. splendens) in 1955, although
with relatively little success (Hu, 1955). For example, by 1963 it
was clear that although T. amboinensis had become established in
Hawaii it did not adequately control its target organism, A.
albopictus (Nakagawa, 1963) and on the island of Oahu, onto
which both T. brevipalpis and T. amboinensis had been
introduced, T. amboinensis has displaced T. brevipalpis (Steffan,
1970). Greater success was, however, recorded in other areas. For
example, following the introduction of T. amboinensis (as T.
splendens)andT. brevipalpis onto American Samoa in 1955 to
control the filariasis vector Aedes polynesiensis Marks (Peterson,
1956); it was reported in 1978 that T. amboinensis had become
established and was effectively controlling the target mosquito
(Engber et al., 1978).
Problems relating to the mis-match of oviposition sites between
the target and control species were highlighted when T. splendens
was introduced onto Fiji in 1934 to control A. polynesiensis
(Paine, 1934), followed shortly by Toxorhynchites inornatus
(Walker) (Lever, 1938). Toxorhynchites splendens became
established but did not provide adequate control of A.
polynesiensis, since their oviposition sites did not coincide
(Toohey et al., 1985). An attempt was made to overcome this
problem in 1979 with the introduction of T. amboinensis, and by
1985 it was clear thatthis had beensuccessfulin controlling the A.
polynesiensis population (Toohey et al., 1985). The introduction
of T. amboinensis larvae was less successful in 1987 when
introduced into domestic water storage containers on Java in an
attempt to control A. aegypti and A. albopictus, vectors of the
dengue virus. Little control was achieved due to the large number
of untreated natural containers in which the target mosquitoes
were breeding (Annis et al., 1990).
Attempts to control pest and vector mosquitoes using
Toxorhynchites spp. mosquitoes have been made in many regions
of the world. For example, T. splendens has been used
successfully in Pudapet, a coastal village near Pondicherry on the
Coromandel coast of India, where there were significant
reductions in the numbers of A. aegypti, Armigeres subalbatus
(Coquillett) and C. quinquefasciatus breeding in domestic water
containers six months after treatment began (Panicker & Geetha
Bai, 1983). Second-instar T. splendens larvae were also used
successfully to suppress Aedes aegypti and A. albopictus in
domestic water containers in Malaysia (Chuah & Yap, 1984).
However, when first-instar T. splendens were used similarly
against the same target mosquitoes in Jakarta, Indonesia in 1987,
the A. aegypti population was not affected, probably because the
first-instar larvae were unable to withstand starvation (Annis et
al., 1989). Fourth-instar T. splendens, however, were used
successfully to reduce the A. aegypti population on Sa-Med
Island, Thailand in 1979 (Vongtangswad et al., 1983).
There have been a number of studies of the suitability of
Toxorhynchites spp. mosquitoes as biological control agents in
Africa. For example, Muspratt (1951) on T. brevipalpis, Corbet
(1963, 1964) on T. brevipalpis conradti in Uganda, and Sempala
(1983) on T. brevipalpis conradti and T. kaimosa, also in Uganda.
Toxorhynchites spp. have also been considered as possible
biological control agents in the USA. For example, Focks et al.
(1979) released adult T. rutilus rutilus into an area of Gainsville,
Florida to control A. aegypti and although the adults moved out of
the urban area into the forest and deposited some of their eggs into
non-target oviposition sites, there were enough ovipositions into
target sites to give adequate levels of A. aegypti control. In an
urban area of New Orleans, Louisiana, the introduction of first-
instar T. rutilus rutilus larvae resulted in a 74% reduction in the
levels of A. aegypti and C. quinquefasciatus (Focks et al., 1982).
Another study using T. rutilus rutilus in Louisiana (Focks et al.,
1983) was less successful, since oviposition occurred more often
into tree holes than into the artificial containers in which the A.
aegypti were breeding. Bailey et al. (1983), however, reduced A.
aegypti populations by 50% in tyre dumps in Jacksonville,Florida
by introducing T. rutilus rutilus larvae to the tyres. In a later study
in New Orleans, T. amboinensis releases were combined with
ultra-low volume (ULV) malathion spraying, resulting in a 96%
reduction in the A. aegypti density, whereas treatment with ULV
malathion alone reduced the A. aegypti by only 29% (Focks et al.,
1986).
Over the last twenty years, there have also been attempts to use
Toxorhynchites spp. mosquitoes as biological control agents in the
Caribbean islands. For example, on the island of St. Maarten, T.
brevipalpis larvae released into domestic water containers
reduced the house index of A. aegypti to zero (Gerberg & Visser,
1978) and in 1985 it was reported that a native species in Trinidad
and Tobago, T. moctezuma, was potentially a good biological
control agent for A. aegypti since it tends to oviposit in the same
places as the target mosquito (Chadee, 1985). A study of the
effectiveness of a single introduction of T. moctezuma larvae on
Union Island resulted in a reduction in the number of adult A.
aegypti, although there was some doubt as to whether this
reduction was actually due to the introduction of the T. moctezuma
larvae (Rawlins et al., 1991). In a laboratory study it was found
that the introduction of between five and ten T. moctezuma larvae
to each water container reduced the A. aegypti population to 0-
0.6% for 16 weeks (Tikasingh, 1992), and sequential releases of
T. moctezuma larvaeinavillageonUnionIslandresultedina
significant reduction in the A. aegypti population (Tikasingh &
Eustace, 1992).
It can be concluded from the examples above that Toxorhynchites
spp. mosquitoes do not all oviposit into A. aegypti infested
containers in urban environments. Also, that higher levels of control
are produced if Toxorhynchites spp. larvae are introduced rather
than adults. This led Gerberg (1985) to recommend sequential
introduction of Toxorhynchites spp. eggs and larvae to infested
containers rather than releasing adults. In addition to this, Jones
(1993) recommended the introduction of fourth-instar
Toxorhynchites spp. larvae to containers before the target mosquito
population increases. The overall belief of workers is that, although
Toxorhynchites spp. mosquitoes are unable to control target species
instantly, they could provide long term control once the introduced
population has become established (Focks, 1982).
Integrated Mosquito Control Using
Toxorhynchites Mosquitoes, Bacillus Toxins and
Traditional Chemical Pesticides
Recently, there have been investigations into the suitability of
Toxorhynchites spp. mosquitoes for use in integrated pest
management (IPM) programmes, both with Bacillus toxins and
ReviewArticle 111N
with traditional chemical insecticides. Problems have arisen in
combining Bti and B. sphaericus with some Toxorhynchites spp.,
in that effective Bti doses may be lethal to the Toxorhynchites
larvae. For example, in laboratory studies, Bti effective doses for
the control of A. aegypti larvae were lethal to first and second-
instar T. rutilus rutilus, although not to older instars of the same
species (Lacey & Dame, 1982). Bacillus sphaericus toxin,
although only moderately effective against most Aedes species,
does control Culex and Anopheles species. Furthermore, although
B. sphaericus toxin was lethal to T. rutilus rutilus, it was not to T.
theobaldi, T. brevipalpis and T. amboinensis (Lacey, 1983).
In a laboratory study of the susceptibility of T. amboinensis to a
variety of insecticides used for control of A. aegypti, the vector
mosquito larvae were significantly more susceptible than T.
amboinensis to naled and chlorpyrifos. Most insecticides,
however, were lethal to both predator and prey mosquitoes. Only
a few insecticides appear promising for combination with
Toxorhynchites spp. larvae. For example, the organophosphate
temephos has a sufficiently low toxicity to T. moctezuma larvae to
be useful in IPM programmes (Rawlins & Ragoonansingh, 1990)
and naled was the least toxic organophosphate compound to T.
splendens larvae (Tietze et al., 1993). Malathion also appears
promising; releases of T. splendens along with ULV applications
of malathion have been used in four cities in Florida. This
combination produced better levels of control of A. aegypti and
other pest mosquitoes than the use of ULV malathion applications
alone (Schreiber & Jones, 1994). An alternative suggestion has
been the sequential alternation of insecticides with
Toxorhynchites spp. releases, with the aim of reducing insecticide
selection pressure on vector mosquito populations (Djam &
Focks, 1983).
Conclusions
It is evident from attempts to use Toxorhynchites spp. mosquitoes
as biological control agents that it is essential that the chosen
species can become established in the area in which control is
required, and that it will oviposit into the same aquatic
environments as the target mosquito species. Toxorhynchites spp.
have been used successfully to control vector mosquito species,
although the fact that these cases have been successful is due, at
least in part, to chance, since very little was known of their biology
and taxonomy at the time.
The modern techniques of molecular biology should be used to
clarify the complex relationships between species and to define
species complexes which cannot be defined using traditional
taxonomy. Although much is now known of the general biology
of some Toxorhynchites species, there are still large areas in which
a greater understanding of their biology would be advantageous in
the successful choice of particular Toxorhynchites species as
biocontrol agents. For example, it is necessary to understand fully
both the population dynamics of Toxorhynchites spp. in their
natural habitat, and the predator-prey relationships between
Toxorhynchites spp. larvae and their prey. Most importantly, there
is a great need for studies of the oviposition site choices and
oviposition cues of a variety of Toxorhynchites species in their
natural habitat. These oviposition cues could then be matched to
the oviposition cues of target species and increase the efficiency
of adult release programmes. They could also be exploited to
attract both Toxorhynchites spp. and target species to oviposition
traps.
Acknowledgements
This work was funded by a Natural Environment Research
Council Postgraduate Studentship.
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- Our study system includes three co-occurring, container-dwelling culicids Culex mollis, Limatus durhamii and Aedes albopictus, and the dominant generalist predator Toxorhynchites theobaldi, which preys on all three culicid species in natural environments in Brazil (Lopes, 1997). Toxorhynchites species are widely distributed in the tropics and subtropics (Collins & Blackwell, 2000) and prey voraciously upon mosquito larvae (Steffan & Evenhuis, 1981). In Viçosa, Minas Gerais (MG), Brazil, T. theobaldi naturally co-occur with C. mollis, L. durhamii and A. albopictus in container habitats (M.
- At present, biological control of vectors and continuous monitoring of the three systems studied are recommended . The options for controlling mosquitoes in these aquatic systems include maintaining ecological integrity, so that natural predators control mosquito populations, the introduction of larvivorous fish [26, 27] or other natural predators [28][29][30][31] and testing of target-specific bio- pesticides [32, 33]. At the same time, the general public should be encouraged to use mosquito nets and medical practitioners should be provided with facilities for screening immigrants coming from destinations where malaria is suspected to exist.
[Show abstract] [Hide abstract] ABSTRACT: Background In 2015 alone there were an estimated 214 million new cases of malaria across the globe and 438,000 deaths were reported. Although indigenous malaria has not been reported in Sri Lanka since 2012, to date 247 imported cases of malaria have been identified. Knowledge of the locations, behaviour and vectorial capacity of potential malarial vectors is therefore needed to prevent future outbreaks. Attention is now being focused on some previously ignored habitats. Methods Active and abandoned granite and clay quarry pits, located in wet and intermediate zones, and agro wells located in the dry zone of Sri Lanka were mapped and sampled for 1 year, as potential mosquito breeding sites. Species composition and spatio-temporal variation in both malarial and other mosquito larvae were recorded. Results A total of 18 species of mosquito larvae were identified. Other than Anopheles culicifacies, the primary malaria vector, five species of potential malaria vectors (Anopheles vagus, Anopheles varuna, Anopheles nigerrimus, Anopheles peditaeniatus and Anopheles barbirostris) were found in all three aquatic systems. Additionally, Anopheles annularis was found in granite quarries and Anopheles subpictus and Anopheles pallidus in both types of quarry, but only during the initial sampling. Apart from potential malaria vectors, mosquito larvae such as Anopheles jamesii, Culex tritaeniorhynchus, Culex infula and Culex malayi were found in all three habitats at least once during the sampling period. Apart from potential malaria vectors and other mosquito larvae common to all three aquatic systems, Culex gelidus, Culex mimulus and Culex pseudo vishnui were detected in agro wells. Culex gelidus was also detected in granite quarry pits. Culex mimulus, Culex lutzia and Culex fuscocephala were detected in clay quarry pits. Accordingly, a total of 14, 13 and 15 mosquito species were identified in agro wells, granite and clay quarry pits, respectively. Conclusions Although zero occurrence of indigenous malaria has been achieved in Sri Lanka, the current study emphasizes the potential for future epidemics. The presence of native flora and fauna in abandoned granite and clay quarry pits and the need to extract drinking water from agro wells demand bio-sensitive control methods in these three aquatic systems.- albopictus was found to be resistant towards DDT and permethrin (Wesson 1990). Since chemical insecticidal approach causes much worry over increasing resistant traits in mosquitoes, not to mention the impact they posed on the environment, biological control has received much attention and interest as an alternative management action on mosquito borne diseases' vector (Collins & Blackwell 2000; Focks 2007; Wijesinghea et al. 2009; Nyamah et al. 2011). Fish predator especially Gambusia species has been one of the interest in biological of mosquitoes and identified as significant predator with wide range of habitat (Griffin 2014).
[Show abstract] [Hide abstract] ABSTRACT: To understand the effects of fish predator’s kairomones on Aedes mosquitoes’ oviposition, we established an experiment using gravid Aedes females. Kairomones concentrations were established using Hampala macrolepidota. One individual fish was placed inside containers with varying water levels (1 L, 5 L, and 10 L of water). The fish were kept in the containers for 24 hours and were removed immediately at the start of each trial in order to have the kairomones remnants. Twenty gravid adult females of Aedes aegypti and Aedes albopictus were allowed to lay eggs on oviposition site with various treatments: (1) control without any kairomones; (2) kairomone remnant in 1 L of water; (3) kairomone remnant in 5 L of water; and (4) kairomone remnant in 10 L of water. There are significant differences between the numbers of eggs laid by both Aedes species for each different treatment (F = 9.131, df = 16, p<0.001). However, fewer eggs were laid by Ae. albopictus compared to Ae. aegypti in the presence of kairomone remnants. This suggested that Ae. albopictus are significantly affected by the kairomones itself and have ability to detect the residual kairomone presence from H. macrolepidota.- Our study system includes three co-occurring, container-dwelling culicids Culex mollis, Limatus durhamii and Aedes albopictus, and the dominant generalist predator Toxorhynchites theobaldi, which preys on all three culicid species in natural environments in Brazil (Lopes, 1997). Toxorhynchites species are widely distributed in the tropics and subtropics (Collins & Blackwell, 2000) and prey voraciously upon mosquito larvae (Steffan & Evenhuis, 1981). In Viçosa, Minas Gerais (MG), Brazil, T. theobaldi naturally co-occur with C. mollis, L. durhamii and A. albopictus in container habitats (M.
[Show abstract] [Hide abstract] ABSTRACT: Prey organisms can perceive cues to predation hazard and adopt low-risk behaviours to increase survival. Animals with complex life cycles, such as insects, can exhibit such anti-predatory behaviours in multiple life stages. 2. Cues to predation risk may induce ovipositing females to choose habitats with low predation risk. Cues to predation risk may also induce larvae to adopt facultative behaviours that reduce risk of predation. 3. One hypothesis postulates that anti-predation behaviours across adult and larval stages may be negatively associated because selection for effective anti-predator behaviour in one stage leads to reduced selection for avoidance of predators in other stages. An alternative hypothesis suggests that selection by predation favours multi-component defences, with both avoidance of oviposition and facultative adoption of low-risk behaviours by larvae. 4. Laboratory and field experiments were used to determine whether defensive responses of adult and larval mosquitoes are positively or negatively associated. The study tested effects of waterborne cues from predatory Toxorhynchites theobaldi on oviposition choices and larval behaviours of three of its common prey: Culex mollis, Limatus durhamii and Aedes albopictus. 5. Culex mollis shows strong anti-predator responses in both life stages, consistent with the hypothesis of a multi-component behavioural defence. The other two species showed no detectable responses to waterborne predator cues in either adult or larval stages. Larvae of these unresponsive species were significantly more vulnerable to this predator than was C. mollis. 6. For these mosquitoes, species appear either to have been selected for multi-component defences against predation or to act in ways that could be called predator-naïve.- Our study system includes three co-occurring, container-dwelling culicids Culex mollis, Limatus durhamii and Aedes albopictus, and the dominant generalist predator Toxorhynchites theobaldi, which preys on all three culicid species in natural environments in Brazil (Lopes, 1997). Toxorhynchites species are widely distributed in the tropics and subtropics (Collins & Blackwell, 2000) and prey voraciously upon mosquito larvae (Steffan & Evenhuis, 1981). In Viçosa, Minas Gerais (MG), Brazil, T. theobaldi naturally co-occur with C. mollis, L. durhamii and A. albopictus in container habitats (M.
- Although the application of Toxorhynchites sp. as biological control to the dengue vector was not capable to many occasion, nevertheless, the predatory mosquitoes was believed to provide a long-term control when large enough population had been established (Focks 1982). In addition, Toxorhynchites sp. also not capable to integrate with other conventional control method such as inseciticidal treatment as the insecticides will kill both predator and prey of mosquitoes (Collins & Blackwell 2000).
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