Malaria is caused by Plasmodium, a protozoan parasite and transmitted by the bite of infected mosquitoes of genus Anopheles. Annually 250 million people are infected and close to one million people, mostly children in sub-Saharan Africa were killed by malaria [1]. Insecticide-based control such as massive use of insecticides and insecticide-impregnated bed nets has been one of the most successful methods to control malaria [2]. However, emergence and spread of insecticide-resistant Anopheles observed throughout the world including Africa pose a threat to the gains made in malaria control [3, 4]. Novel approaches to the control of vectors are therefore urgently needed to combat malaria.
Advances in insect molecular biology, in particular the development of germ-line transformation, have opened a new avenue for the control of malaria [5]. One of the major goals of gene-modification studies is to generate malaria parasite-resistant (refractory) An. gambiae s.s., the most important vector of human malaria in Africa, and to replace the wild-type An. gambiae s.s. population that is susceptible to malaria parasites with the genetically modified mosquito population.
The first key step for germ-line transformation is to obtain a requisite number of mosquito eggs for injection. Since mosquito eggshell becomes hard within two hours after being laid [6], it becomes difficult to inject genes into mosquito eggs using a glass needle after that period. Therefore clarification of the conditions in which mosquito eggs laid within two hours can be efficiently obtained is a crucial prerequisite for gene-modification studies. Compared to Anopheles stephensi, which is the predominant Anopheles vector in the Indian subcontinent, An. gambiae s.s. has been used less frequently in embryo transgenic studies and relatively little information is available regarding its ovipositional behavior [7]. In the present study, we analyzed ovipositional behavior of An. gambiae s.s. with reference to the following conditions: age of mosquitoes, time post blood meal to access ovipostion substrate, and light conditions at oviposition.