Mosquito Larvae Eradication

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QUESTION

What measures can be taken to eradicate mosquito larvae?

ANSWER

The main methods to control mosquito larval populations are by reducing areas available for adults to breed and also directly killing larvae. So, for example, adult mosquitoes need standing, stagnant water in order to lay their eggs. Getting rid of these sources of standing water, for example by filling in unwanted ditches and removing empty containers that can become filled with rainwater, therefore reduces the available breeding habitat for mosquitoes. Similarly, dripping a small amount of oil on top of standing water creates a thin film of oil over the surface of the water, preventing adults from laying eggs. In addition, the oil prevents the larvae from gaining access to air, suffocating them and killing them. There are other methods for killing larvae too. Directly spraying insecticides onto water is one method, but often not very specific, and can have a negative effect on other forms of aquatic life. One popular method for biocontrol is to introduce other animals which eat mosquito larvae, thus keeping numbers down. Examples include certain species of fish, as well as small crustaceans called copepods.

Mosquito Larvae Eradication

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QUESTION

What measures can be taken to eradicate the mosquito larvae?

ANSWER

The usual, traditional method of eradicating mosquito larvae is through the application of insecticides. However many of these are known to have severe negative effects on water quality, particularly through being non-selectively toxic and therefore killing lots of other aquatic life as well as the mosquitoes. Moreover, some are known for their effects on animals further up the foodchain; the most famous example of this being DDT, which was used to kill adult mosquitoes. It was discovered that this pesticide resulted in birds laying eggs that had very thin shells, preventing the chicks from hatching successfully. As such, it was banned in most developed countries. Modern insecticides used for mosquito larva reduction have been developed to target mosquitoes specifically; a popular one in the USA is methoprene, which interferes with the mosquitoes’ growth hormones, preventing development into adults. Microbial compounds, which are not dangerous to other organisms, are also sometimes used.

Another approach is through the use of natural enemies of the mosquito larvae, notably certain species of fish and dragonflies. These will eat mosquito larvae and pupae, thus naturally reducing numbers, and with little undue effect on water quality (although in some contexts, such as reservoir water, there may be concerns with stocking the water source with large numbers of fish).

Use of Fish for Malaria Eradication

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QUESTION

Why is very little research being done on the possibility of mosquito fish being used as a means of controlling the hatching of new eggs?

ANSWER

Actually there is a lot of scientific literature on the use of fish as a biological control of mosquitoes, usually through consumption of the mosquito larvae or pupae while they are in freshwater. The most common species used for this purpose are the guppy (Poecilia reticulata) and the appropriately named mosquito fish (Gambusia affinis). Other fish groups, like cyprinodontids (i.e. Aphanius species), tilapia and minnows will also eat mosquito larvae.

In addition, other freshwater predators, such as copepods, have been shown to be highly effective in  killing mosquito in laboratory and field experiments, though results are sometimes inconsistent (see Lardeaux, 2008 “Biological control of Culicidae with the copepod Mesocyclops aspericornis and larvivorous fish (Poeciliidae) in a village of French Polynesia,” in Medical and Veterinary Entomology, vol 6, issue 1, pages 9-15, for a comparison of the anti-mosquito effect of these different groups).

The Lardeaux paper also describes the failure of the program: despite introduction of larvivorous animals, biting rates of mosquitoes did not significantly reduce, indicating some of the complications that can be associated with biological control programs.

However, in Vietnam, field studies have shown significant reductions of local Aedes mosquitoes associated with the presence of copepods in standing water sources, which shows the potential positive effects of incorporating natural biological control agents within part of an integrated vector control strategy (Nam et al., 2000 “National progress in dengue vector control in Vietnam: Survey for Mesocyclops (Copepoda), Micronexa (Corixidae) and fish as biological control agents,” in the American Journal of Tropical Medicine and Hygiene, vol 62, issue 1, pages 5-10).

Another example of successful introduction of fish as biological control agent comes from Ethiopia (Fletcher et al., 1992 “Control of mosquito larvae in the port city of Assab by an indigenous larvivorous fish, Aphanius dispar,” in Acta Tropica, vol 52, issue 2-3, pages 155-166).

Malaria Month

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QUESTION

What is the national month for malaria?

ANSWER

I’m not exactly sure I understand your question. If you are asking if there is a specific month when malaria is the worst, that depends on where you are. Malaria transmission is highly seasonal, because the mosquitoes that transmit the disease require pools of standing water in order to reproduce. Therefore in many places, transmission is highest during the rainy or wet season.

However, if you mean if there is a specific month dedicated to raising awareness about malaria and control, then the answer is yes, at least for certain countries. India, for example, has declared each June between 2005 and 2012 to be “Anti-malaria Month,” when a big push is made nation-wide to educate people about preventing malaria, implement early diagnosis and treatment and organise vector control efforts. June was selected as it is the month before the start of the monsoon season, when malaria transmission increases due to the heavy rains.