Semi-field experiments and sibship analyses reveal larval habitat size preferences and skip oviposition behaviour in female Anopheles arabiensis.

BACKGROUND: Since the turn of the century, long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS) have played a major role in malaria control. However, the effectiveness of these tools is declining due to the development of insecticide resistance and other factors, creating an urgent need for complementary strategies. Larval source management (LSM), through the application of biological larvicides or the autodissemination of larvicides by ovipositing female mosquitoes, offers alternative avenues to target malaria vectors. Predicting the effectiveness of such approaches requires a detailed understanding of oviposition site selection behaviour in female Anopheles gambiae sensu lato. This study investigated the oviposition strategy of female Anopheles arabiensis in relation to aquatic habitat size under semi-field conditions in south-central Tanzania.

METHODS: An array of twelve alternating small (30 cm diameter, 20 L capacity) and large (60 cm diameter, 40 L capacity) artificial larval habitats was established in two compartments of a semi-field system. In quadruplicated experiments, forty wild gravid An. arabiensis females were released into each compartment and allowed to oviposit in their preferred habitats. The resulting third-instar larvae were collected, counted and preserved in ethanol. Larvae from one replicate were subjected to DNA extraction and microsatellite genotyping. Sibship analysis using Bayesian-likelihood methods (COLONY software) was conducted to reconstruct individual female oviposition behaviour.

RESULTS: Rather than distributing eggs in proportion to habitat size, An. arabiensis females significantly preferred smaller habitats. Sibship analysis showed that 49.2% of females skip-oviposited across two to four habitats, while others deposited eggs in only one. Skip-ovipositing females laid more eggs overall, but fewer eggs per habitat, and were more likely to alternate between small and large habitats than to use habitats of the same size, consistent with a potential bet-hedging strategy.

CONCLUSIONS: Gravid An. arabiensis females exhibited a strong preference for smaller aquatic habitats and frequently engaged in skip-oviposition. These behavioural traits have potential implications for optimising larval-stage vector control tools, particularly those based on autodissemination and attract-and-kill strategies.