Effects of field and laboratory larval inhabited water physical chemicals and background colour on oviposition site selection by gravid female anopheline mosquitoes in Lusaka province.
Knowledge of factors that regulate oviposition site selection by gravid anopheline mosquitoes is still limited and yet it is of great importance in the quest to develop alternative malaria vector control methods. In order to contribute to data and knowledge on the subject, this study characterised and determined the effects of water physical chemicals including pH, temperature, turbidity, conductivity, larval pre-inhabited water and five different background colours on oviposition site selection behaviour by gravid anopheline mosquitoes. The study was conducted in three districts of Lusaka province, between July 2018 and February 2023. Potential anopheline oviposition sites were sampled by scooping in water at the sites using a 350mls dipper. The anopheline oviposition sites were confirmed by the presence of anopheline larvae which were identified by their parallel water surface resting positions. Larval density was determined as mean larval counts in scoops using the dipper. Portable water testers were used to record temperature, pH, turbidity and conductivity in the field. Late instar mosquito larvae were kept in breeding containers until adults emerged from them later for mosquito identification using both morphological and molecular techniques. Field oviposition water in 1 litre capacity containers was placed in an insulated cooler box which was transported to the laboratory to determine its efficacy in influencing oviposition behaviour in gravid Anopheles gambiae s.s. KISUMU mosquitoes. Effects of larval pre-inhabited water on oviposition site selection was determined by presenting 30mls of the treatment water in 36mls capacity oviposition cups to groups of 30 gravid Anopheles gambiae s.s., KISUMU mosquitoes in 30 x 30 x 30 cm cages. The effect of water background colour on oviposition site selection, on the other hand was tested by presenting 30mls of water in five differently coloured oviposition cups to groups of 30 gravid mosquitoes in 30 x 30 x 30 cm mosquito cages. After 24 hours, the oviposition cups were inspected for eggs deposited by gravid females. The data generated were analysed statistically using IBM SPSS Statistics software, version 26 and Windows Microsoft Excel. Out of a total of 43 sites sampled in this study, only three were confirmed as anopheline oviposition sites in the study area, two were in Nyamphande location (Site-A and Site- B) and one was in Shiala location, respectively. A total of 1,795 anopheline larvae were counted from the three sites. Two anopheline mosquito species namely, Anopheles gambiae s.s. (97.5%), and Anopheles pretoriensis (2.5%), were found to occur in Shiala location while Anopheles coustani (95%) and Anopheles gambiae s.s. (5%), occurred in the Nyamphande area. Larval density was higher at Nyamphande site-A when compared to site-B, t-test P < 0.05(95%CI), [t-test p = 0.0000]. There were no significant differences P > 0.05(95%CI) in physical, and chemical properties of water between the two sites of Nyamphande. The Spearman linear regression coefficient (rs), showed no significant correlation between larval density and water pH, temperature, turbidity, and conductivity at the three study sites. Laboratory reared gravid female Anopheles gambiae s.s., KISUMU laid more eggs in field oviposition water from Nyamphande site-A than in water from site-B or in distilled water, the control, P < 0.05(95%CI), one-way ANOVA (F(2,66) =11.172, p = 0.000 but laid fewer eggs in field oviposition water from Shiala when compared to the distilled water control t-test P < 0.05(95%CI), t-test p = 0.029. Gravid Anopheles gambiae s.s., KISUMU mosquitoes laid more eggs in water that had been pre-inhabited by the egg immature stages of conspecifics than in water of other instars or by a cohort, P < 0.05(95%CI), one-way ANOVA (F(3,88) = 3.663, p = 0.015. Gravid Anopheles gambiae s.s., KISUMU mosquitoes laid more eggs in red coloured oviposition cups than other coloured cups P < 0.05 (95%CI), one-way ANOVA (F(4,205) = 4.02, p = 0.004, in the order red (43%) > black (23%) > blue (16%) > green (11%) > yellow (7%). The larval density and egg counts of anopheline mosquitoes were not related to water pH, temperature, turbidity or conductivity but to factors within the larval pre-inhabited water and to the background colour of the water. The discovery of dominance in Rufunsa district by Anopheles coustani mosquito, a secondary malaria vector in this study is significant for purposes of malaria control in the area and calls for an expanded study to establish whether the species plays an important role in malaria transmission in the area. This study has generated information that may be relevant for use in improving the efficacy of some existing vector control methods or for the development of alternative vector control methods. The study recommends that more research be undertaken to identify and understand factors or sources of factors that influence oviposition site selection by gravid female anopheline mosquitoes at oviposition sites of the study area.
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