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How to battle the mosquitos

July 15, 2010

Although we have no malaria in Thailand, mosquitos may still annoy you, and some mosquitos (Aedes spp) may transmit diseases like dengue fever. What can a home gardener in northern Thailand do to limit the mosquitos? Below we share our experience from Dokmai Garden.

1. As the larvae develop in standing water, not streaming water, you need to make sure there are no unnecessary water traps anywhere. The tiny amount caught on a flower pot tray might be enough, so use your imagination to make sure there is nowhere they can reproduce. Put anything in your nursery that can hold water upside down.

2. If you collect water in barrels, the best way to prevent larval development is to have a lid on your barrel. A mechanical barrier is far better than using chemicals. If that is not possible, put fish there. The fish will feed on the mosquito larvae. Red swordtails (Xiphophorus spp) are good as you will be able to see where the fish are when you fill your watering can. You can buy fish at numerous places in Chiang Mai. The largest aquarium shop is ‘Shogun’ at the Khamtieng flower market near Tesco Lotus north of town.

3. If you have an ornamental pond, you need to have fish which catch the larvae. Some gardeners dislike fish as they eat up all ornamental plants, but such behaviour is largely depending on the fish species. Select small species that are uncapable of uprooting a water lily: Siamese fighting fish (Betta splendens) and archer fish (Toxotes microlepis) are small and native species. Guppies (Poecilia reticulata) are exotic popular fish.

4. You may have small ornamental water pots with ornamental plants. Such pots are oases for thirsty birds and butterflies, and amphibians may take a refreshing bath or even lay eggs in it. Such pots must have fish too. At Dokmai Garden we use the native tiger barb (Puntius partipentazona) successfully. A large pond keeps a cool temperature, but a small ornamental water pot should be kept in the shade, or the temperature will be so high that  predatory fish and tadpoles die. We have an example where we pruned back a large Clerodendrum, thereby allowing the sun to shine straight at a water bowl.  The water became so hot that it burnt our hands. All fish and all tadpoles died, but not the mosquito larvae! As we did not want to use chemicals, we decided to drain the pot. We allowed the sun to shine a few hours extra to really dry it out. When we poured water back again, dozens of mosquito larvae came out from hidden pockets of water under the stones! In such a case, empty it permanently or move the pot to the shade and put back some fish!

5. If you create a garden with many predators (spiders, bats, geckos) they will help you with the adult mosquitos that may fly in from your careless neighbour. Such a garden contains meadows with tall wildflowers and hollow trees for the bats. In addition, you may want to plant Cymbopogon nardus, citronella grass, near your favourite evening corner. This grass is an ingredient of many commercial repellants. Its mere presence seems to limit the abundance of mosquitos, and by gently rubbing your skin with crushed leaves (be careful, they are very sharp) you protect yourself. The classical insect repellant from Ketsanee’s hometown is to put moist rice straw on fire and then smoke out the adult mosquitos. That is a good cure before dinner, but not during dinner. During dinner, you can turn on a fan.

6. We can not really see the reasons for applying chemicals to reduce mosquitos in a home garden, as fish do the same job and are more fun to watch, but if you want to try a chemical you go to your subdistrict’s health centre (satani anamai). They hand out the larvicide Vactor 1G (manufactured by Zagro) for free. It contains 1% temephos which has been attached to sand for slow release. You add 5 g (one tea spoon) for 50 litres of water. One treatment lasts one month. The temephos inhibits the enzyme cholinesterase, which normally breaks down the transmitter substance acetylcholine, allowing a nerve to return to its resting state. Without a functioning cholinesterase the nerve does not shut down, resulting in cramps and the larvae die within a day. Although low doses of temephos are not dangerous to humans, temephos affects every organism with a nervous system, why tadpoles and fish may suffer too, and it is not recommended to let pets drink such water or let children play with it. Temephos was mentioned as one factor explaining the current worldwide decline in amphibians. Also, the wild bees at Dokmai Garden frequently sip from our water pots, and since temephos is highly toxic to these humble fellow gardeners, we wish to avoid this chemical. Resistant strains of mosquitos were discovered in Brazil in 1999.

7. In Sweden a Bacillus related to B. thuringiensis has been successfully used to specifically wipe out mosquito larvae in marshlands. Apparently resistance develops over time. Does anybody know about its use in Thailand?

Ketsanee Seehamongkol and Eric Danell

Temephos is distributed for free by the Thai government in an effort to control mosquito borne diseases. Like Sweden, Thailand has made an effort to treat all malaria cases. This policy has wiped out the disease from the two countries, as the malaria parasite is restricted to the human reservoir. The last case in Sweden was in the 1930’s, and Thailand is virtually free, although refugees from neighbouring countries suffering from communism may carry the disease.

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