Can the Smell of Soil and Beetroot Guide Mosquitoes to Egg-Laying Sites?

The smell of soil after rain and the earthy aroma released when you cut a beetroot share a compound: geosmin. Known to people as an earthy smell, this molecule also provides yellow fever mosquitoes with a cue for finding places to lay eggs.

A 2020 study combined laboratory experiments, brain imaging, and field oviposition traps to investigate how mosquitoes detect a microbial odor and use it to guide their behavior. It reveals how the same molecule can mean different things to organisms with different ways of life.

An earthy smell signals microbes in water

Geosmin is a volatile compound produced by various microorganisms in soil and water. Earlier research in fruit flies had shown that it signals the presence of harmful microbes and triggers avoidance.

Mosquito larvae, by contrast, live in water and feed on microorganisms and other material. When adults choose egg-laying sites, microbial odors could provide clues to suitable habitats for their offspring.

Nadia Melo and colleagues focused on the yellow fever mosquito (Aedes aegypti). They examined whether geosmin influences egg-laying choices and larval behavior, how the olfactory system detects it, and whether it could serve as an oviposition lure in the field.

Females chose geosmin-containing water, and larvae approached it too

In the laboratory oviposition experiment, groups of 20 mated, blood-fed females were placed in cages offering a choice between plain water and water containing geosmin. The main comparison involved six replicates. Females laid more eggs in the geosmin-containing water.

The team also tested mutants lacking Orco, a co-receptor needed for olfactory function. These mosquitoes showed no preference for geosmin-containing water. This indicates that an olfactory pathway contributes to the choice, rather than visual or other cues alone.

In two-choice experiments that tracked individual larvae, larvae also tended to spend time on the geosmin side. This preference disappeared in larvae whose antennae had been removed and in Orco-mutant larvae.

Importantly, geosmin did not influence every behavior in the same direction. Its effects on drinking sugar solution and approaching a human hand to attempt blood feeding were limited, with weak repellency at some high concentrations. Attraction to egg-laying sites and behavior toward a human host were tested separately.

Comparing microbes to examine the odor’s role

The researchers next examined actual cyanobacteria. Chemical analysis confirmed that one strain produced geosmin, and they compared water containing that strain with water containing culture medium alone. Females preferred the microbial water for egg laying, and larvae also preferred that side.

Microbes produce many substances besides geosmin, however. The team therefore also used a different strain that did not produce it. Water containing this strain did not elicit an oviposition preference over the control. In a direct comparison, mosquitoes preferred the geosmin-producing strain to the non-producing strain.

When geosmin was added to the non-producing strain, the egg-laying preference between the two strains disappeared. Because these were different strains, not every difference can be attributed to a single compound. Nevertheless, this additional experiment supports geosmin’s importance as a cue for selecting microbe-containing habitats.

This does not mean that all geosmin-producing microbes are safe for larvae. The authors note that some strains of the cyanobacteria used can produce toxins. How mosquitoes select feeding habitats while distinguishing harmful microbes remains an open question.

A region of the mosquito brain was highly sensitive to geosmin

Electrical recordings from the antennae confirmed responses to geosmin. The team then examined olfactory regions of the mosquito brain using optical measurements of calcium changes associated with neural activity.

The small compartments where odor information converges are called glomeruli. Most did not respond strongly to geosmin, but one showed a strong response even to very dilute stimulation. Based on its location, the researchers tentatively identified it as PD3.

This compartment responded more strongly to geosmin than to the other odorants tested. Its response was not exclusively limited to geosmin, however: it also responded to other compounds associated with mosquito oviposition and host seeking. The corresponding mosquito receptor itself was not identified in this paper.

Fruit flies and mosquitoes are both sensitive to the same microbial molecule, yet their behavior differs in direction. What an organism detects, and whether it approaches or avoids the source afterward, must be considered in the context of its way of life.

Field tests were still exploring the conditions that work

Around Miami, the researchers tested oviposition traps at 21 sites over a total of 30 weeks. They compared geosmin-containing lures with solvent-only controls and counted the eggs laid.

Of three dilution conditions, only the intermediate condition produced significantly more eggs than the control. Neither the more concentrated nor the more dilute condition showed a significant preference.

The authors suggest that there may be an effective concentration range, but also note that the concentrations were tested at different times, so differences in weather and other conditions may have mattered. The results do not establish that stronger concentrations attract more mosquitoes or that one concentration is optimal everywhere.

Testing beetroot peel as an accessible source of geosmin

Pure geosmin is expensive and can be difficult to obtain. The researchers therefore turned to beetroot. Chemical analysis confirmed that the peel of earthy-smelling beetroot contained more geosmin than the flesh.

Peel extracts strongly stimulated the brain compartment that responded to geosmin. In oviposition experiments, females also preferred containers with peel to those with flesh. Chemical analysis, neural responses, and behavior thus pointed to the same candidate.

A small six-week field trial on a university campus in Brazil also found that traps using beetroot peel collected more eggs than water-only controls. This suggests the potential to use an accessible material as an oviposition lure.

What was measured here was egg collection. Whether such an approach reduces local mosquito populations, bites, or disease incidence requires further testing. The study does not recommend simply leaving water and peel out at home. The challenge is to incorporate the lure into control or surveillance systems that appropriately manage the attracted mosquitoes and eggs.

Summary

The earthy-smelling compound geosmin influenced egg-laying site selection and larval attraction in yellow fever mosquitoes, and these responses required olfactory pathways. An adult brain compartment responded with high sensitivity to geosmin, detecting a cue associated with microbial presence.

Some field conditions also yielded more eggs, and beetroot peel showed promise as a geosmin source. Yet effects varied with concentration and environment, and practical mosquito-population reduction remains a separate question. An earthy smell to people can be a signpost for the next generation to mosquitoes: this study explored a molecule’s meaning through animal behavior.

The research received support from the Swedish Research Council, US public agencies and research grants, foundations, and other sources. The authors declared no competing interests.

References

• Melo, N., et al. (2020). Geosmin Attracts Aedes aegypti Mosquitoes to Oviposition Sites. Current Biology, 30(1), 127–134.e5. DOI and original article. Public author manuscript.