Insecticide resistance exerts signifcant ftness costs in immature stages of Anopheles gambiae in western Kenya

Abstract

Background: Despite increasing documentation of insecticide resistance in malaria vectors against public health insecticides in sub-Saharan Africa, there is a paucity of information on the potential ftness costs of pyrethroid resist ance in malaria vectors, which is important in improving the current resistant management strategies. This study aimed to assess the ftness cost efects of insecticide resistance on the development and survival of immature Anoph eles gambiae from western Kenya. Methods: Two-hour old, frst instar larvae (L1) were introduced and raised in basins containing soil and rainwater in a semi-feld set-up. Each day the number of surviving individuals per larval stage was counted and their stage of devel opment were recorded until they emerged as adults. The larval life-history trait parameters measured include mean larval development time, daily survival and pupal emergence. Pyrethroid-resistant colony of An. gambiae sensu stricto and susceptible colony originating from the same site and with the same genetic background were used. Kisumu laboratory susceptible colony was used as a reference. Results: The resistant colony had a signifcantly longer larval development time through the developmental stages than the susceptible colony. The resistant colony took an average of 2 days longer to develop from frst instar (L1) to fourth instar (L4) (8.8±0.2 days) compared to the susceptible colony (6.6±0.2 days). The development time from frst instar to pupa formation was signifcantly longer by 3 days in the resistant colony (10.28±0.3 days) than in suscepti ble colony (7.5±0.2 days). The time from egg hatching to adult emergence was signifcantly longer for the resistant colony (12.1±0.3 days) than the susceptible colony (9.6±0.2 days). The pupation rate (80%; 95% (CI: 77.5–83.6) vs 83.5%; 95% (CI: 80.6–86.3)) and adult emergence rate (86.3% vs 92.8%) did not difer between the resistant and sus ceptible colonies, respectively. The sex ratio of the females to males for the resistant (1:1.2) and susceptible colonies (1:1.07) was signifcantly diferent. Conclusion: The study showed that pyrethroid resistance in An. gambiae had a ftness cost on their pre-imaginal development time and survival. Insecticide resistance delayed the development and reduced the survivorship of An. gambiae larvae. The study fndings are important in understanding the ftness cost of insecticide resistance vectors that could contribute to shaping resistant management strategies