Although dragonflies are often associated with ponds, streams, and swamps, they have rarely been studied as a potential biocontrol agent against malaria and other insectborne diseases. Dragonflies belong to the order Odonata, which includes dragonflies (suborder Anisoptera) and damselflies (suborder Zygoptera). Worldwide, nearly 5,000 species of dragonfly are known (Borror & White, 1970). The principal methods of distinguishing the various dragonfly families are based on wing venation, eye structure, color, and size. B. pratense belongs to the Darner family (Aeshnidae) of dragonflies and is commonly known as the "hairy hawker dragonfly" because of the numerous long hairs on its thorax.
B. pratense feeds voraciously on insects during its larval stages as well as in its adult stage. Its larger size allows for greater consumption. The natural feeding behavior of the nymph is also innately effective. The nymphs of various dragonfly species generally feed using one of three techniques: passively feeding from burrows in the bottom sediment (burrowers), hunting from the bottom sediment surface (sprawlers), or actively stalking their prey among the vegetation. The nymphs of B. pratense have good vision and are vegetation stalkers that actively hunt prey.
The life cycle of dragonflies is relatively long, and the nymph stage, consisting of 10 to 15 nymphal instars between egg and imago, may last for one year or more (Tillyard, 1917). The presence of dragonflies is not seasonal, but the adults are found in greater numbers during summer and monsoon seasons (March through September). Dragonflies have some preference with respect to habitat (preferring mesotrophic habitats), but the factors responsible for this preference are not clearly understood. The presence or absence of wild vegetations in and around ponds and lakes may be one factor affecting the level of nutrients in the water.
Anopheles subpictus is a house-frequenting mosquito among the anophelines reported from various parts of the world. It has also been reported as a malaria vector in the Austral-Asian zone (Russel, West, Manwell, & Macdonald, 1963) and as a secondary vector of malaria in Sri Lanka. It has also been reported as a malarial vector in various parts of India (Kulkarni, 1983; Panicker, Geetha Bai, Bheema Rao, Viswam, & Suryanarayanamurthy, 1981; Roy, 1943). In the Tarakeswar area of West Bengal, India, this mosquito species is very prevalent and acts as a primary vector of malaria (Chatterjee & Chandra, 2000). It is resistant to chemical insecticides in Pakistan, Nepal, Sri Lanka, Indonesia, and Bangladesh, as well as in various parts of India such as Delhi state, Punjab, and Pondicherry (Das, Chandrahas, & Panicker, 1980; Rathor, Taqir, & Reisen, 1980; Sharma, 1996). Anopheles subpictus breeds mainly in the small ponds and submerged fields of the study area. Although the larvae are present throughout the year, more than 70 percent of them occur during the rainy season (July to October) (Chatterjee and Chandra, 2000).
Mosquitoes serve as a vector not only for malaria, but also for yellow fever, dengue, filariasis, encephalitis, and other diseases. Owing to growing resistance to commonly used organochlorine and organophosphate chemicals in the field, control of mosquitoes through eco-friendly biological means may be advantageous. Other benefits of biocontrol agents include an ability to kill the targeted species while being safe for nontargeted ones; ease of field application; inexpensive production; and the lack of infectivity and pathogenicity to mammals, including humans (Rathor et al., 1980; World Health Organization [WHO], 1975a; WHO, 1984). The alternative malaria control strategy of bio-environmental improvement techniques gives primary importance to antilarval operations.
Although the larvae of Toxorhynchites sp. (mosquito) and Diplonychus indicus (water bug) …