The viral disease dengue fever is often epidemic in Yangon. It is characterized by prostration, headache, severe joint pain, rash, and can result in death. The mosquito Aedes aegypti (Linnaeus) is a primary vector of dengue and yellow fevers, and malaria, in Asia and the Americas.
Members of the Myanmar Ministry of Health conducted a pilot study to determine the effect of releasing dragonfly larvae into A. aegypti breeding areas. C. servilia is a suitable predator for A. aegypti. It selects the same breeding site as the mosquito, reproduces year-round, and can subsist on a diet of mosquito larvae.
The treated area in Yangon was 0.02 square kilometers, with 2,200 inhabitants. An untreated community about 2 km away served as the control. The trial was conducted during the monsoon season, when rainfall is highest and mosquitos are most prolific.
Village awareness and cooperation were pivotal to the success of the study. For 20 years village water containers had been treated with the foul-smelling insecticide temphos (Abate). Villagers preferred introduction of larvae to treatment with insecticides. Some villagers were aware that their water vessels already harbored the dragonfly species B. geminata.
A. aegypti larvae, pupae, and adults were counted in each water source. C. servilia larvae were raised in pans and enamel bowls on a diet of pond weed and A. aegypti larvae. Four 3-week-old larvae were released in each of 800 water containers that served as A. aegypti breeding sites. Bimonthly evaluation of A. aegypti began mid-May and continued through mid-September 1979.
Within a month the mosquito larvae population had been reduced by up to 96 percent in the treated community. The population continued to decline for the duration of the study. In the control community the mosquito larvae population showed no appreciable change.
Periodic release of C. servilia larvae greatly reduced populations of A. aegypti. It is a safe, inexpensive biocontrol, requiring no complicated materials or technologies. However, dragonfly larvae must be continually replenished to maintain eating habits selective enough to satisfactorily control the mosquito larvae.
The success of this biocontrol is largely dependent on community understanding and participation. In this trial, cooperation was exceptional; villagers even asked that the larvae continue to be added after the study was over.
Young, S. Dragonflies help to defeat dengue fever. New Scientist, 27 April, 1991.