Effect of temperature manipulation on pupation and emergence rates
Logistic regression analysis indicated that the reduction in temperature, to 19oC, during early larval development had no overall impact on pupation rates of An. coluzzii or An. gambiae (Likelihood ratio Chi-square = 3.63, df =1 P = 0.057). There was a significant difference in pupation rates between the two species (LR = 144.23, df =1, P < 0.001). Higher pupation rates were observed for An. coluzzii at both 19oC (98%) and 27oC (97.5%), however, An. gambiae also achieved high pupation rates at both temperatures; 85% (19oC) and 89% (27oC) (Figure 1a).
High overall emergence rates (> 85%) were observed for both species, however the effect of temperature depended on species (Table 1). Indeed, higher emergence rates were observed for An. coluzzii at 19oC, whereas for An. gambiae emergence was higher at 27oC (Figure 1b).
Table 1 Logistic regression (Likelihood ratios) of the effect of temperature and species on emergence rates
Source
|
DF
|
LR ChiSquare
|
Prob>ChiSq
|
Species
|
1
|
30.61
|
< 0.001
|
Temperature
|
1
|
5.79
|
0.016
|
Temperature*Species
|
1
|
24.15
|
< 0.001
|
DF Degrees of freedom, LR Likelihood Ratio
Effect of temperature manipulation on sex ratio
None of the An. coluzzii and An. gambiae treatment groups differed from a male:female ratio of 1:1. For An. coluzzii at 19oC the proportion of males was 0.52 (95% CI 0.48-0.56) and at 27oC the proportion of males was 0.54 (CI 0.50-0.58). For An. gambiae the male:female ratio for both temperatures was 0.51:0.49 (CIs +/- 0.04). Logistic regression analysis indicated that sex-ratios did not significantly differ between species (LR Chi-square= 2.14, df =1, P =0.144) nor by temperature condition (LR Chi-square =0.002, df =1, P =0.965).
Effect of temperature manipulation on emergence times
Proportional hazards analysis revealed that the emergence time of both An. coluzzii and An. gambiae was significantly affected by the 5-day cooling period (Table 2). An. coluzzii took on average 2.4 and An. gambiae 3.5 days longer to emerge compared with those maintained at 27oC (Figure 2). There were also significant differences in emergence times between species and sex. The interactions between species, sex and temperature were also found to be significant (Table 2).
Table 2 Logistic regression (Likelihood Ratios) for the effect of species, temperature and sex on emergence time
Source
|
DF
|
LR ChiSquare
|
Prob>ChiSq
|
Species
|
1
|
259.71
|
< 0.001
|
Temperature
|
1
|
2025.21
|
< 0.001
|
Sex
|
1
|
19.51
|
< 0.001
|
Temperature*Species
|
1
|
46.01
|
< 0.001
|
Temperature*Sex
|
1
|
5.60
|
0.018
|
Effect of temperature manipulation on adult fitness parameters
Multivariate analysis showed that mosquito wing length was significantly affected by species, temperature and sex (Table 3). An. coluzzii individuals were significantly smaller than An. gambiae and male mosquitoes significantly smaller than females. Those exposed to the 19oC 5-day cooling period were significantly smaller than their counterparts maintained at 27oC (Table 3, Figure 3a).
Table 3 General linear model effect data for the effect of species, temperature and sex on mosquito wing lengths
Source
|
DF
|
Sum of Squares
|
F Ratio
|
Prob > F
|
Species
|
1
|
1.20
|
70.86
|
< 0.001
|
Temperature
|
1
|
0.51
|
30.29
|
< 0.001
|
Sex
|
1
|
0.89
|
52.09
|
< 0.001
|
Insemination rates were similar at both temperatures for both species (Figure 3b). When female size was also considered, differences were apparent between species, between temperatures and at different wing lengths (Table 4). Overall, inseminated females were larger in all conditions (Figure 3c).
Table 4 Logistic regression (Likelihood Ratios) for the effect of species, temperature and wing length on mosquito inseminations rates
Source
|
DF
|
L-R ChiSquare
|
Prob>ChiSq
|
Species
|
1
|
6.63
|
0.010
|
Temperature
|
1
|
4.16
|
0.041
|
Wing length (mm)
|
1
|
94.00
|
< 0.001
|