The impact of Anopheles gambiae egg storage for mass rearing and production success

Mass rearing requires a large colony from which male individuals can be harvested for sterilization and release. Attention is needed when monitoring life parameters of the reared population, knowing that any variations within the target population would lead to mismatching between two populations. The aim of this study was to assess the impact of Anopheles gambiae sensu stricto (s.s.) egg storage on hatchability and life history traits. For each parameter, comparison was made between freshly laid and stored eggs in three densities (40, 80, 120 eggs).

Anopheles gambiae s.s. freshly laid eggs were collected from the Tropical Pesticide Research Institute (TPRI) insectary. Eggs to be stored were kept at - 20 °C for 10 min and then transferred to refrigerators at 4 °C for intervals of 5, 10, 15, 20, and 25 days. After respective storage days, the eggs were transferred from refrigerators to ambient temperature of (25 ± 2) °C for 24 h and then placed in incubators for 24 h. Thereafter eggs were hatched. The egg hatchability, emerged larvae development, larvae survival and emerged adult sex ratios were monitored.

This study found that hatching rates decreased with increase in storage time. The difference was significant in eggs stored for 10 and 15 days (P < 0.05). There were no significant differences in hatching rates between An. gambiae eggs stored for 5 days and freshly hatched eggs (P > 0.05). Anopheles larvae development (L1 to pupae) was not significantly affected by storage time across all hatching densities. The study also found that larvae survival decreased with increase in egg storage time. However, there was no significant difference between larvae from freshly hatched eggs and those from eggs at 5 and 10 storage days (P > 0.05) but not for eggs stored for 15 days. Furthermore, there was a decrease in emerged adult males and increase in females relative to increased time of egg storage. The difference was significant (P < 0.05) at 15 storage days but not for eggs stored for 5 and 10 days (in triplicate densities).

From this study it was concluded that storing An. gambiae eggs at 4 °C and 48 ± 2% relative humidity (RH) for 5 days is the optimal condition and time that did not affect egg hatching rates, larval development and survivorship and emerged adult mosquito sex ratio.