Geosmin and the tale of mosquitoes

At FIELDWORKERS Co., Ltd., we raise many mosquitoes to study their ecology and test products we have developed. As we take care of them every day, we are often surprised by their mysterious ecology. This time, we would like to talk about the ecology surrounding mosquito egg-laying. By the way, mosquito eggs are white when they are freshly laid, but the photo shows how they gradually turn black!

One of the essential aspects of raising mosquitoes for research is to have them lay eggs. Mother mosquitoes don't just think that water is fine, but choose a place where their young ( mosquito larvae ) can grow properly to lay their eggs. So what are the conditions for a water source that meets the mother mosquito's standards, that is, a place suitable for the growth of mosquito larvae?

 

Trivia 1: Various mosquito breeding sites

Here we are talking about egg-laying of Aedes mosquitoes, which are commonly found in Japan, but the egg-laying locations of mosquitoes vary depending on the species, and some mosquitoes choose unique egg-laying locations. For example, the Yanbaru silver mosquito of Okinawa lays eggs by throwing them into holes made by long-horned beetles in bamboo joints, and the larvae live in the water that accumulates in the bamboo joints.1 Also, the American mosquito Wyeomyia smithii lays eggs in the digestive fluid of carnivorous plants, and the larvae grow by eating insects that fall into the digestive fluid.2 There are many interesting mosquitoes in the world.

 

Of the many conditions, let's consider here the importance of an abundance of food for mosquito larvae. When a mother mosquito finds a water source, she tastes the water before laying eggs to determine whether it contains organic matter and bacteria that the mosquito larvae will feed on.3 One of the factors she uses to make this judgment is a substance called geosmin. Geosmin is a substance produced by bacteria such as actinomycetes that are abundant in nature, especially in the soil, and while it smells moldy to us humans, it seems to have a pleasant taste to mosquitoes, and is an indicator that the water contains organic matter and bacteria that the mosquito larvae will feed on.

 

Trivia 2: Substances that inhibit egg laying

In contrast to geosmin, there are substances that inhibit mosquito egg-laying. Mosquitoes avoid laying eggs in water that contains kairomones released by fish and other predators of mosquito larvae4,5,6 . It's amazing that mother mosquitoes can tell if there are fish in the water just by drinking it.

 

Taking this into consideration, we mosquito researchers wondered if we could create a mosquito trap using geosmin. An overseas study conducted an experiment targeting a species of Aedes mosquito in Brazil that transmits dengue fever7 . As a result, traps containing beet juice containing geosmin were observed to lay more than twice as many eggs on average compared to traps containing only water.

It is interesting that geosmin, an unpleasant odor for humans, is an indicator of good breeding sites for mosquitoes. If we could create a mosquito trap that utilizes this, we might be able to efficiently reduce the number of disease-carrying mosquitoes.

 

Responsible for the text: Naoki Matoba@London

 

References

  1. Miyagi, Ichiro (2002). The Mysteries of Mosquitoes: Biological Diversity . Tokai University Press .
  2. Arellano, AA & Coon, KL Bacterial communities in carnivorous pitcher plants colonize and persist in inquiline mosquitoes. Anim Microbiome 4, 13 (2022).
  3. Day, JF Mosquito Oviposition Behavior and Vector Control. Insects 7, (2016).
  4. Hurst, TP, Kay, BH, Brown, MD & Ryan, PA Melanotaenia duboulayi influence oviposition site selection by Culex annulirostris (Diptera: Culicidae) and Aedes notoscriptus (Diptera: Culicidae) but not Culex quinquefasciatus (Diptera: Culicidae). Environ. Entomol. 39, 545–551 (2010).
  5. Vonesh, J.R. & Blaustein, L. Predator-Induced Shifts in Mosquito Oviposition Site Selection: A Meta-Analysis and Implications for Vector Control. Isr. J. Ecol. Evol. 56, 263–279 (2010).
  6. Silberbush, A. Fish-Released Kairomones Affect Mosquito Oviposition and Larval Life History. J. Med. Entomol. 59, 78–82 (2022).
  7. Melo, N. et al. Geosmin Attracts Aedes aegypti Mosquitoes to Oviposition Sites. Curr. Biol. 30, 127-134.e5 (2020).

 

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