How temperature and relative humidity combine to affect mosquito traits

Temperature and water availability are two of the most important factors influencing ectothermic arthropod vectors. Humidity can have multiplicative effects on temperature, by significantly increasing or reducing the amount of desiccation stress organisms can experience.

To test the interactive effects of temperature and humidity on blood-feeding vectors, we focused on Anopheles stephensi, a malaria-transmitting mosquito from southern Asia. We devised two experiments to capture a variety of trait responses: one on larvae and one on adults. We combined 8 different temperatures with 5 different humidities to create 40 abiotic combinations. For larvae, 3 trays of 100 were placed in incubators with 1 litre of RO water and 4 pellets of fish food. We measured larval development rates and the proportion of adult emergence. For adults, we took 20 blood-fed females per treatment and individually aspirated them into cups with mesh lids. We measured their mortality, daily blood feeding rates, and daily egg production.

In both experiments we found that temperature and relative humidity had a significant impact on the traits measured, particularly at the extreme ends of thermal performance. Within a temperature treatment, relative humidity altered the larval development thermal maximum by as much as 5C and had a significant effect on the proportion of adults emerged. Similarly, adult mortality was significantly affected by humidity, both within and across temperature treatments. Our results collectively indicate that humidity and temperature must be considered together as interactive variables to better understand arthropod vector ecology.