Feeding Mosquitoes to Beetles: A Biological Control Study

In some parts of the world mosquitoes carry and transmit infectious agents, including malaria, Dengue fever, West Nile virus, and Yellow fever. Controlling the population of such medically important mosquitoes is essential. Insecticides have typically been the method of choice for mosquito control; however, their potentially harmful effects on humans and animals must be considered. Overuse of insecticides can also lead to the cultivation of more resilient mosquito strains. Therefore, some scientists are seeking alternative, more sustainable methods for controlling mosquito populations. Biological control methods employ the use of natural mosquito predators that will feed on immature, larval mosquitoes. Some of these predators include certain types of fish, crustaceans, and other insects. The larvae of dytiscid beetles, also known as predaceous water beetles, are natural predators that feed on mosquito larvae. In India dytiscid beetles in their larval form play the role of dominant predator in rice fields and temporary pools of water, prime habitats for breeding mosquitoes. Researchers at the University of Burdwan, West Bengal, India, studied the Acilius sulcatus species of dytiscid beetle as a possible biological control agent, and published their findings in BMC Infectious Diseases.

The study was performed in both laboratory and field settings. Laboratory experiments were performed at Burdwan University's Department of Zoology Mosquito Research Unit. Beetle larvae were collected from ponds and rice fields, and mosquito larvae from the city drainage system. Pond water for mosquito breeding was filtered to exclude larvae from other competing predator species. Combinations of predator, prey, and pond water were varied, and the experiments were repeated three times, in order to authenticate results. The number of prey eaten by each predator was recorded during a twelve-hour lights-on period, from 6 am to 6 pm, and a lights-off period, from 6 pm to 6 am. The number of prey consumed by each predator in a 24-hour period was used to calculate the clearance rate, a determination of the efficiency of dytiscid beetle larvae as a biological control agent.

For the field experiments, ten cement tanks were used as breeding containers for the larvae of several different mosquito species. Twenty beetle larvae were introduced into each of the first five tanks, while the remaining five tanks served as controls, in order to rule out any influences by other predators that might be present in the tanks. Thirty days after beetle larvae were introduced, the population of mosquito larvae was measured. The beetle larvae were then removed from the tanks, in order to assess the affect of their absence on the population of mosquito larvae.

In the laboratory it was observed that the predators captured their prey at the head with the help of powerful jaws. This action triggered the release of certain enzymes into the tissues of their prey. Predators sucked out the body fluid, and discarded the body parts. Each predator killed an average of 34 mosquito larvae per day, depending on the various combinations of prey, predator, and water volume. In the field a "dip" of water from each tank was taken for measurement. The authors found that the number of prey decreased from 23.03 to 13.28 mosquito larvae per dip, after 30 days. Once predators were removed from the tanks, the population of prey rose, from 13.28 to 20.97 per dip. No significant changes in the mosquito larvae population were observed in the control tanks, an indication that the dytiscid beetles were responsible for decreases in the mosquito larvae population.

The authors concluded that the larvae of the dytiscid beetle have potential as biological control agents against medically important mosquitoes. However, they stress the need for further study. Introduction of any species as a biological control agent can affect the ecological food chain in unpredictable ways. The authors point out that the natural habitats of dytiscid beetles and their larvae need careful evaluation, in order to determine the potential impact of their use as a biological control agent, and to determine the best method for maintaining and releasing beetle larvae for wide-ranging applications. Mosquito control is a critical issue in areas where mosquitoes present a serious health threat. Results of this study make an important contribution to a growing effort to find alternatives to mass pesticide application. This well-designed study demonstrates a careful effort to address a serious health problem, without altering the balance of nature.

Reference

Chandra, G., Mandal, S., Ghosh, A., Das, D., Banerjee, S., Chakraborty, S.

Biocontrol of larval mosquitoes by Acilius sulcatus (coleoptera: Dytiscidae)

BMC Infectious Diseases

http://www.biomedcentral.com/bmcinfectdis/

http://www.biomedcentral.com/1471-2334/8/138