Laboratory tests of oviposition by the African malaria mosquito, Anopheles gambiae, on dark soil as influenced by presence or absence of vegetation

To test whether presence and height of grasses influenced oviposition, wild-caught gravid An. gambiae s. l. were offered paired choices between wet, bare soil and wet soil populated with mixed grasses or grasses of differing height. No-choice tests were also conducted by giving females either grassy soil or bare soil.In choice tests, females laid four times more eggs on bare, wet soil than soil populated with grasses. However in no-choice tests, egg output was not significantly different whether grasses were present or not. Females laid significantly more eggs on soil populated with short grass than with medium, or tall grass.This work shows An. gambiae s. l. has the capacity to oviposit into grassy aquatic habitats when typical puddles over bare soil are unavailable. This knowledge will need to be considered in the design and implementation of programmes aimed at reducing malaria transmission by suppression of An. gambiae s. l. immatures.Physical and chemical cues influence acceptance of ovipositional sites by mosquitoes [1-3]. Darkness and wetness are critical positive cues for Anopheles gambiae oviposition, while visual contrast strongly influences finding of prospective ovipositional sites [4,5]. Odours are not required for copious oviposition, however, they may increase egg output in some cases [6] and decrease it in others [7].Physical objects like vegetation can influence oviposition on soil or water substrates [8-10]. Rice plants or glass rods inserted into seepage water reduced Anopheles culicifacies oviposition [8]. However, oviposition by Anopheles hermsi increased in accordance with the density of aquatic macrophytes (Myriophylluym aquaticum) up to 1,000 stems m-2 [10]. In the field, Anopheles funestus breeds mainly in marshes and swamps that contain tall grasses and other plants [11]. An. gambiae is generally thought to utilize puddles over bare soil as its prime larval habitat [12-15] and to avoid standing water populated with vegetation. However, Muir