Have you ever noticed a group of moths flying toward a street light at night? Or a large population of mosquitoes hurtling toward a lit pool in the summertime nights? On any given camping trip through the woods, one may expect to be bitten by many avian insects, particularly the not-so-forgiving mosquito. A careful eye may observe a pattern: Mosquitoes tend to attack and feed off of certain individuals more than others. A can of “OFF” may serve well at a camping trip, but for some, the problem is perpetual and annoying.
An introductory biology course investigates the process known as phototaxis, or the innate movement toward light. Phototaxis is widely used by many organisms in nature, from fireflies to junebugs, and is widely regarded as a fundamental process for insect survivability. Although phototaxis plays a great role in mosquito movement, why do many mosquitoes possess this innate host discrimination? In a study by Zermoglio et al., it is investigated that thermal (heat) energy plays a great role in mosquito orientation behavior.
The study was conducted to investigate three fundamental hypotheses: (1) Whether female mosquitoes are able to discriminate between different temperatures of different heat sources, (2) Whether female mosquitoes exhibit a preference among heat sources as a function of size and location (distance from organism), and (3) Whether or not female mosquitoes use IR radiation as an orientation cue. The study utilized an acrylic Y-Tube as the study chamber and periodically altered the size, location, and temperature of the heat sources at the end of the chambers. The mosquitoes were introduced and allowed to host discriminate. It is important to note that male mosquitoes were not subjects in the experiment due to the exclusivity of host feeding by females mosquitoes.
So what were the results? Zermoglio and his colleagues found that Ae. aegypti females are capable of orientating toward a heat source which temperature resembled that of a host (roughly 34°C). Researchers also found that mosquitoes distinguish among sources that are presented at different distances. Zermoglio and his colleagues also found that heat source size did not evoke any preference among the mosquitoes. Lastly, the researchers found that female mosquitoes were not able to use IR Radiation as a means of orientation behavior.
The data concluded that, when given the option, female mosquitoes will actively choose the thermal source of host temperature, regardless of source distance and size. When given sources of non-host temperature, the female mosquitoes will often choose the source of closest proximity. No evidence could be concluded on the importance of host size for host discrimination (Fig 1).
What significance does this experiment offer us? The experiment was able to provide us concrete evidence that thermal (heat) energy and location of the source play a vital role in mosquito host discrimination; the size of the heat source plays absolutely no role in the host preference process, and IR Radiation does not play a large role in host discrimination and preferences. The study gave an accurate characterization of mosquito behavior to thermal (heat) sources in the absence of other stimuli. The study also sets up the framework for additional research in mosquito host discrimination as a function of location and temperature, which in turn, will give us a better understanding of their complex sensory systems. Additional areas of research such as avian insect host discrimination, sensory systems, and trade-off mechanisms are now being actively explored.
Zermoglio, Paula F, et al. “What Does Heat Tell a Mosquito? Characterization of the Orientation Behaviour of Aedes Aegypti towards Heat Sources.” Journal of Insect Physiology, vol. 100, no. 9-14, July 2017, pp. 9–14.