Sydney University Department of Medical Entomology Westmead Hospital
agwetland.gif (2650 bytes) Irrigated Agriculture
(Pastures, Ricefields, Row Crops)
agwetland.gif (2650 bytes)
Mosquito production & management

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Introduction

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Mixed irrigation
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Irrigation channel with little marginal vegetation will not encourage mosquitoes

The provision of irrigation water for agricultural production has the potential to create mosquito habitat, either within the water storage systems, along the water delivery or drainage systems, or on the land that is receiving the water.

The water source, if a reservoir, may provide opportunities for various mosquitoes: Aedes species at the edges if there is repeated rise and fall in water levels, Culex and Anopheles species if the edges are more stable and support some emergent and floating vegetation, Coquillettidia species if dense emergent vegetation eventuates, and Mansonia species if floating vegetation proliferates.

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Water seepage from an unmaintained bank, creating ideal mosquito habitat (on left)
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Vegetation in channel will reduce water flow and encourage mosquitoes

Water delivery channels 'above' the land to be irrigated, are usually well maintained and free of marginal vegetation in order to enhance water flow. They may be lined with concrete, or if earthen lined any vegetation that establishes is periodically removed. In this state they usually do not provide a suitable habitat to support mosquito larvae.

Water drainage channels 'below' the irrigated land, designed to dispose of unused water, are usually earthen lined and often not well maintained. They can become heavily vegetated, restricting water flow, and providing suitable habitat with harbourage and nutrient resources for various mosquito species.

The different methods of irrigation itself also vary in their potential for the creation of habitat for various mosquito species.

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Flood irrigation

Flood irrigation, where areas of land are inundated by relatively large amounts of water provide the most obvious potential mosquito habitat. However, providing the water infiltrates, evaporates, or drains off and is delivered elsewhere, and does not persist as stagnant pools for more than 5 days, there is usually no concern for mosquito production.

Drip irrigation, where relatively small amounts of water are delivered to the base of individual plants and quickly soak into the soil, is the system least likely to provide mosquito habitat.

Sprinkler irrigation and channel irrigation, are of intermediate concern and these usually provide for mosquito habitat only when the water is over applied, the area is poorly drained, and stagnant pools are able to persist for more than 5 days.

Persisting pools of water will be exploited quickly in inland areas by Culex species such as Culex annulirostris, which is an important pest and vector of viruses such as Ross River virus and Murray Valley encephalitis virus. If the pools become semi-permanent and emergent vegetation becomes established, a range of species can be supported, including various Culex species, some Anopheles and Coquillettidia species, and even some Aedes species if there is sufficient rise and fall of edge waters. The species involved will vary with geographic region and also with local environmental circumstances.


Pastures

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Water pooling in a channel can result in mosquito breeding

Irrigation of pastures is usually accomplished by flooding, or by using fixed or travelling sprinkler systems. Invariably, the result will be very shallow water on level pasture but there may be deeper pools in depressed areas within the pasture or in the perimeter drainage zones.

With flood irrigation (and sometimes excessive channel irrigation) at the beginning of a season, there often will a large generation of Aedes species (e.g. Aedes clelandi, Aedes sagax, Aedes theobaldi, Aedes vittiger - depending on region) produced quickly from eggs deposited on site following the last flooding (perhaps from the previous season), but if water persists on the vegetated land, some Culex and Anopheles species (e.g. Culex annulirostris, Culex australicus and Anopheles annulipes) will follow within a week or two.

In parts of southern Australia, the southern saltmarsh mosquito Aedes camptorhynchus, normally associated with saline habitats as its common name implies, will breed in freshwater in pastures in coastal regions of Victoria (the east, particularly east Gippsland), Tasmania (the northeast, including the Tamar valley), South Australia (the southeast, particularly near the lower Murray River) and Western Australia (particularly the Mandurah to Busselton region).


Ricefields

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A flooded ricefield

Irrigation of ricefields is usually accomplished by flooding, and the water persists at depths of approximately 20-30 cm over some months during the summer growing season. This persistence of water, and the emergent rice plants provide good habitat for a number of mosquito species, particularly Anopheles species (e.g. Anopheles annulipes in the southeast, Anopheles bancroftii and Anopheles meraukensis in the north) and Culex species (e.g. Culex annulirostris and Culex australicus in the southeast, Culex annulirostris in the north).

Ricefields can be relatively productive of mosquitoes, although this persistence of habitat also allows for the buildup of invertebrate and vertebrate predators of mosquito larvae, and often the end result is a lower level of production than might be expected. Density and height of the plants (both increasing through the growing season) will variously influence mosquito (and predator) populations depending on species; at times, the perimeter ditches may provide better habitat and produce more mosquitoes than the rice field itself.


Rowcrops (fruit and vegetables)

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Irrigated vineyards can produce large mosquito numbers

The irrigation of orchards, vineyards or vegetables is usually accomplished by either channel irrigation, drip irrigation, or overhead sprinklers. It is the pooling and persistence of water in the rows that provides habitat for mosquitoes; quick infiltration or rapid drainage will preclude formation of mosquito habitat.

Various mosquitoes can be produced - Aedes, Anopheles and Culex species - depending on the region and local circumstances, and the particular species are usually related to the amount and persistence of the water and whether there is grass or other vegetation present. At the beginning of the season and where the habitat is short-lived and sporadic, some Aedes species may be produced, but Culex species usually predominate and Anopheles may also be found where there is persisting or more regular habitat created.


The concerns

Most of the species mentioned above can be significant pests and are often produced in substantial numbers. The Aedes species are often more immediately noticeable after irrigation events because of their rapid appearance (eggs are already present) and because many will bite avidly during the daytime. The Culex and Anopheles species may take a few weeks to become apparent and are principally evening and night time biters.

Culex annulirostris is the species of greatest concern associated with irrigation waters. It is the major inland vector of disease-causing organisms, such as the arboviruses Ross River virus and Murray Valley encephalitis virus, although a number of the above-mentioned Aedes species have also been found with viruses.

Initial seasonal virus activity can be through Aedes mosquitoes, and this can reflect their early seasonal abundance prior to the buildup of the Culex populations. However, there is evidence that some of the arboviruses can be transmitted from one mosquito generation to the next through the eggs, and it may be that the Aedes species are responsible for the persistence of these viruses in some areas. Although the initiation of virus activity may occur with Aedes species in the early part of the season, it appears that Culex annulirostris then becomes responsible for high-season activity and is the principal inland vector of arboviruses to humans during epidemics.

 

Mosquito control strategies

Environmental options

Water management is the essence of good irrigation practice, and water management should be the essence of mosquito management in irrigation areas. Overuse of water and inadequate drainage are the principal cause of mosquito production associated with irrigation areas.

Ensuring water does not persist on pastures and in row crop channels for more than 5 days will usually prevent mosquito production. Effective slope drainage, and infiltration and evaporation rates associated with the soil, must be considered with respect to the water application rates and frequencies. Ensuring free flow of water in the delivery and (particularly) the drainage channels, by clearing emergent and floating vegetation, will help to prevent buildup of mosquito populations.

With respect to ricefields, it is the persistent presence of water on the crop that provides the habitat. In parts of eastern Asia, where mosquito-borne disease (Japanese encephalitis) is associated with ricefield habitats, cultivation practices (wherein water is drained repeatedly from the fields during the mosquito season) have been developed to control the peak buildup of mosquito populations. This strategy may be successful in parts of Australia if appropriate varieties of the crop that can sustain this periodic drainage can be planted.


Biological options

Although, other aquatic insects such as dragonfly nymphs, and various beetles and bugs, can reduce mosquito larval populations to some extent, they are rarely available in the temporary pools often associated with irrigation water habitats. Even in the more permanent sites such as ricefields and drainage channels with restricted flow because of dense vegetation, these predators are difficult to manage and only rarely can be relied upon for adequate control.

Larvivorous fish are the only biological agents available for practical use, and these can be a valuable component of an integrated control program, particularly in persistent habitats such as ricefields. There is a view in Australia that because of undesirable impacts on native fauna by the introduced 'mosquitofish' Gambusia holbrooki, indigenous fish species should be given priority in mosquito control programmes. There is a range of local fish that may be useful in some areas but none has yet been shown (in controlled studies in field situations) to be highly effective in controlling mosquitoes in Australia. The relevance and effectiveness of native fish will vary from place to place, and local species should be used where possible, but the exotic Gambusia may be difficult to exclude from wetlands if it is present in local watersystems. Additionally, in shallow waters vegetation is critical in determining the effectiveness of fish as mosquito predators, because it can inhibit the fish gaining access to the mosquito larvae.


Chemical options

The principal advantage of chemical control methods is that pesticides can be quickly applied with rapid results at relatively low cost. However, chemical usage should not be viewed as a long term strategy, and should be resorted to only when there are occasional episodes of heavy uncontrolled breeding.

Relatively few chemicals can be recommended currently because of environmental concerns. The agents that are available and can be useful in some irrigation situations are:

(i) the organophosphate temephos is a contact pesticide relatively target-specific for mosquitoes and generally suitable for environmentally sensitive freshwater habitats, although above recommended dosage it is also highly toxic for many invertebrates. It has low mammalian toxicity, but is moderately to highly toxic for fish and birds and recommended application rates must be followed. Weekly use may be required during summer months, and its persistence can be reduced to a few days or less in polluted or colloidal waters.

(ii) commercial products of the bacteria Bacillus thuringiensis israelensis (Bti) are toxic to larvae on ingestion; they are environmentally acceptable because of their relative specificity for mosquitoes amongst invertebrates and negligible toxicity for vertebrates. These should not be thought of as biological agents; they are not 'live' organisms but toxic products of culture of the bacteria, and they don't 'recycle' in the environment. Bti has little persistence and mosquito populations can rebound in 1-2 weeks, even in nonpolluted sites.

(iii) the insect growth regulator methoprene is a juvenile hormone mimic that does not kill larvae but prevents them becoming adults. It is environmentally benign because of its relative specificity for mosquitoes. Because larvae are not killed, and it is adult emergence from the pupal stage that is prevented, the effectiveness of the product is note readily apparent in the habitat - adult populations and/or sentinel larvae must be monitored to evaluate its effectiveness. Slow-release formulations can provide control over some months, but at 1/99 only the liquid formulation was registered for use throughout Australia.

Because of the extensive area of habitat that occurs in many irrigation areas (e.g. with pastures or ricefields), often the control agents of choice will be most efficiently applied from aircraft.

It must be remembered that prolonged use of pesticides will lead to development of resistance in mosquito populations, thereby limiting overall management options.


Summary

Mosquito control in irrigated agriculture can be problematical if water usage is not controlled and unused water is allowed to pool and thus provide habitat for pest and vector species. Notwithstanding that rice cultivation is a separate issue with its requirement for impounded water persisting over some months, the general principles for mosquito control in irrigation areas can be listed as:

1. Avoiding overuse of irrigation water will limit habitat creation.

2. Ensuring efficient drainage to avoid pooling at the lower end of the area will reduce availability of habitat.

3. Maintaining weed free margins in drainage channels will reduce opportunity for mosquito colonisation.

4. Chemical control can be effective, but in many instances can be avoided with improved water usage strategies. 

5. Biological control using predatory fish can be appropriate for ricefields, and local native fish should be used although further information on useful species is required for most regions of Australia.


In conclusion, the nature of the crop and method of water delivery and removal will determine the mosquito risk and indicate the most appropriate methods of prevention / management. Expert advice should be sought as to the risks associated with particular situations and regional activities.

See 'Contacts' for further information.

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