In the summer, an alternative method to get rid of mosquitoes is popular on social media, which involves burning coffee residues, because compounds with a repellent effect on mosquitoes would be formed. Let’s clarify right away that There are no studies to support this thesis and coffee does not contain insecticides. It must be said that many insects are not attracted by the smoke. So, if you notice a decrease of mosquitoes after burning coffee grounds, it is likely the smoke to keep them away. Therefore, no matter what you burn, you may get this effect. In fact, coffee grounds may be useful in other ways. Research has shown that water solutions containing coffee in various concentrations influence the deposition and development of larvae, effectively reducing the number of surrounding mosquitoes. Let’s see in detail the effects of coffee on mosquito oviposition.
Coffee alters mosquitoes’ oviposition habits
Mosquitoes lay their eggs near stagnant water, from rivers and swamps to our pets’ water bowls and plant saucers. If they have multiple places to lay their eggs (oviposition), usually prefer dark, colored waters to clear water, but not in the case of coffee, indicating that they are able to perceive substances harmful to the development of the larvae.
In fact, analyzing the oviposition preferences of 60 female mosquitoes, a group of researchers discovered that they preferred to lay their eggs in water or glucose solutions, while avoiding those containing coffee. Of 3306 eggs, only 11.77% was deposited in the solutions containing coffee extractsboth roasted powder and residues.
Indeed, by eliminating the deposition sites containing water and leaving only those containing coffee, the mosquitoes preferred to retain (and therefore do not lay down) a large percentage of mature eggs! So: coffee-containing solutions reduce mosquito egg-laying, and that already seems like good news. But what happens to the eggs laid in coffee?
The percentage of eggs that hatch is significantly lower than those laid in water and depending on the concentration of coffee it varies from 28% (low concentrations of coffee) 85% (high coffee concentrations) fewer eggs hatched for Aedes albopictus and from 49% to 85% less for Aedes aegypti.
The few that survive, live less and if they are females they will tend to lay their eggs in coffee solutions, further reducing the population.
Substances present in coffee alter the development of larvae
That coffee negatively affected the development of insects and mammals is well documented, but it is not clear how.
However, we know that in the larvae that mature in coffee extracts there are a increase in enzymessuch as caspases and peroxidases, that trigger cell death and alterations in the motor and cellular development of the larvae, in particular the morphology of the midgut. The higher the concentration of coffee, the lower the larval development and maturation.
One of the main problems is the lack of characterization of the molecules that make up the extracts used in the studies. Depending on where and how it is produced, each extract has different concentrations of the molecules that compose it. And in addition, processes such as roasting itself generate up to 950 new compounds.
On the other hand, many molecules present in coffee, such aschlorogenic acid and alkaloids, are synthesized by the plant specifically for the purpose of keeping insects and other predators away, but it is not clear which molecule or molecules induce the changes observed in the larvae.
What is clear is that both green coffee (i.e. unroasted beans), roasted powder and even residues had similar effects. And no, caffeine would be only partly responsible, because Even decaffeinated coffee achieved the same results as regular coffee.
Does it make sense to burn coffee grounds?
The simple answer is: no. Perhaps you think that by burning the dust or residues you get a sort of “natural” mosquito coil, but There is no scientific evidence to confirm the creation, or presence, of repellent molecules in coffee smoke. In fact, many insects do not like smoke, so it is more likely that if you notice fewer mosquitoes after setting fire to the coffee grounds, it is simply the smoke that is driving them away.
As we have seen, the studies present in the literature all speak of aqueous solutions, containing coffee residues, roasted coffee powder and green coffee and which would reduce the mosquito population at the source influencing their reproduction and development.
These are studies of great interest especially for reducing the arbovirosisthat is, those diseases transmitted by insect bites, such as dengue and the La Crosse virus, against which solutions with very high concentrations of caffeine (much higher than those found in the residues of our coffee makers) also seem to reduce the infection of the virus in the mosquitoes themselves. But, although promising, there is still a long way to go in research, so for now let’s rely on the currently approved repellents.
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