Constant temperature and time period effects on Anopheles gambiae egg hatching

Abstract

Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) egg development and its relation to environmental parameters is an understudied aspect of vector biology. Although several studies have illustrated the dramatic effects of temperature on egg development, egg hatching dynamics remain unclear. The objective of this study was to expose An. gambiae eggs to various temperatures for different lengths of time and determine the impact on egg development and hatching count. Batches of mosquito eggs (n = 30 eggs/replicate) were incubated under moist conditions at temperatures of 12, 22, 27, 33, and 42 degrees C for intervals of 1, 3, 7, and 10 days. After that, they were flooded with distilled water at 27 degrees C, and hatching counts were observed for up to 7 days. Mosquito eggs held at 22 and 27 degrees C had the highest overall mean hatching count. During early incubation periods, eggs held at 33 degrees C had hatching counts comparable to 22 and 27 degrees C, but counts decreased drastically during later incubation periods. Temperatures of 12 and 42 degrees C reduced mosquito egg viability, because few eggs hatched in these temperature regimes. Other experiments revealed that during early embryonic development, temperature had a major effect on the developing embryo, while later in embryonic development it had no dramatic effect. Microscopic observation of the An. gambiae embryo showed that extreme low and high temperatures affected the normal development of the embryo. A regression model was developed to describe the effect of incubation temperature and incubation period on egg hatching counts, which demonstrated that the optimum temperature for egg hatching ranges from 24 to 30 degrees C, irrespective of incubation period. The interaction between temperature and time period may have implications for dry-season survival and climate-based models of malaria risk.

Fig. 1

Egg hatching count of An. gambiae monitored for 7 days after being incubated at constant temperatures of 12, 22, 27, 33, and 42°C for incubation periods of 1, 3, 7, and 10 days, then being flooded and kept in 27°C double-distilled H₂O.

Fig. 2

The predicted surface response of An. gambiae egg hatch counts to incubation days and incubation temperature.

Fig. 3

Eggs of An. gambiae held at 3 constant temperatures of 12, 27, and 42°C and flooded with room temperature double-distilled H₂O to test for viability and to show presence or absence of major developmental hallmarks (V, ventral end; D, dorsal end; P, posterior pole; A, anterior pole).