# Exposure to copper metal enhances the tolerance of An. gambiae s.s. over multiple generations while reducing both fertility and fecundity in this primary malaria vector

## TL;DR

Exposure to copper reduces mosquito survival and reproduction over generations, but may help control malaria by lowering their fitness.

## Contribution

This study shows copper exposure increases An. gambiae tolerance but reduces fertility and fecundity over multiple generations.

## Key findings

- Copper exposure increases larval mortality initially but decreases over generations.
- Copper exposure prolongs pupation time and reduces fecundity and fertility in adult mosquitoes.
- Copper exposure lengthens the gonotrophic cycle and lowers egg hatching rates.

## Abstract

Anopheles s.l.
displays the potential to develop tolerance to heavy metals, particularly copper, this may occur at a significant biological cost, which can adversely affect its ecological fitness. This study investigated the larval metal exposure on larval development and reproduction of
An. gambiae s.s., a laboratory susceptible strain,
kisumu.

Stage 2 larvae of
Anopheles gambiae,
Kisumu were exposed to C
1 = 484 μg L
-1, C
2 = 300 μg L
-1 and 0 μg L
-1 (control) of copper chloride. Larval mortality, pupation time, pupation rate, gonotrophic cycle length, fecundity and fertility of larvae/adults were assessed over six generations.

Results revealed that larval mortality rate was significantly higher in the C
1 groups of each group (p = 0.000), but this mortality rate decreased over generations. Pupation time was extended to 13 and 14 days respectively for C
2 and C
1 groups (p = 0.000) compared to the control group. Similar results were observed for the gonotrophic cycle, which increased from 4 days at G0 to more than 6 days at generation 5 in adults of C
1. The pupation rate in generation 4 (C
1) and generation 5 of the same group (p = 0.000) as well as the emergence rate in generation 4 (C
2, p = 0.000) and generation 5 (C
1 and C
2, p = 0.000) decreased significantly compared to the control group. The average number of eggs laid was lower in the test groups from generation 4 to generation 5 (C
1 and C
2, p = 0.00) and egg fertility was also negatively affected by exposure of the larval stage of
An. gambiae s.s. to copper.

Although studies have already shown that copper induces resistance in
An. gambiae s.l. to insecticides, this study has shown that this adaptation requires a non-negligible biological cost in the life of the insect.

The efficacy of insecticides used in vector control depends on the control of factors that contribute to the resistance of malaria vectors to these insecticides. Previous studies have reported the adaptation of
An. gambiae s.l. larvae to breeding sites polluted by chemical contaminants. In this study, we pre-exposed 2
nd instar larvae of
Anopheles gambiae, kisumu for 6 consecutive generations to 484 μg L
-1 (C
1), 300 μg L
-1 (C
2) and 0 μg L
-1 (control) until pupation. We found that after 3 days and 7 days, the mortality rate of larvae was a function of the concentration and was higher in the first generations before reaching zero at the end of the experiment. This pre-exposure negatively affected the pupation time, the pupation rate and the emergence rate. In adults, pre-exposure of larvae to copper lengthened the duration of oviposition after the blood meal and decreased the average number of eggs laid by females as well as the hatching rate compared to the control group. On the one hand, our study revealed the advantage of copper in vector control considering the larval mortality, the prolongation of the gonotrophic cycle and the pupation time that it induces and its negative action on fecundity and fertility on the other hand. However, the decrease in the mortality rate over the generations requires further work to elucidate the possibility of copper to select for resistance to insecticides.