Do Coffee Grounds Really Keep Mosquitoes Away From Your Yard?

A single persistent mosquito is all it takes to send people indoors, even on the loveliest summer day. The low buzzing, the insistent presence in your face, and the threat of being bitten can spoil what should be a relaxing picnic or snooze in the hammock. While mosquito repellent can keep the insects at bay, some of us may not want to be slathered in chemicals.

People have often turned to natural ways of deterring mosquitoes from their yard. Some use thyme as a mosquito repellent. Others use cedar mulch to discourage them. And many people believe that certain flowers can repel mosquitoes. However, another option that is gaining traction is the use of coffee grounds, which in addition to keeping many folks awake, have alternative purposes like reviving dying houseplants. But is this pantry staple effective for this purpose? The answer depends on the type of mosquito.

Some studies have shown that certain types of mosquitoes do not like our morning Joe, and the grounds may actually be lethal to them. A 2015 study from Parasites and Vectors presented female Aedes albopictus (Asian tiger) mosquitoes with cups containing various concentrations of strained, roasted coffee bean powder steeped in water, in addition to cups containing only water. The researchers found that the higher the concentration of coffee, the less enticing they were to the insects. An International Journal of Mosquito Research study from 2019 found that the compounds in arabica coffee grounds (like tannins, alkaloids, and flavonoids) are toxic to Aedes aegypti (or yellow fever) mosquito larva and impede their development. 

Based on those studies, coffee grounds do have a negative impact on certain mosquitoes in one form or another, so using it as a repellent isn't too far-fetched. It may not deter all types of mosquitoes, but it could help — along with making sure there's no standing water anywhere outside. Pest control experts have suggested boiling grounds to create a spray or sprinkling them around the yard as the easiest possible way to do this.

However, burning coffee grounds may be the most effective solution. The aforementioned compounds seem to be most pungent and unappealing to mosquitoes when burnt, and create an invisible barrier that keeps them away. The smoke also can hide our bodily smells that would otherwise draw the insects to us.  When burning used coffee grounds, first dry them by spreading a thin layer out on a baking sheet or foil tray. 

Once they're dry, put them into a heat-proof container that allows for sufficient air flow (a clay or ceramic bowl is best; a glass jar would get too hot) or pile them into a fire pit or other nonflammable outdoor surface. Be mindful of how the space will be used to ensure that the container is out of reach of children or pets and won't get accidentally jostled.Light the grounds, making sure that the smoke is blowing upward and around the areas where mosquitoes are congregating. Similar to how you would keep a campfire alive, "stoke" or add more grounds to the flame as needed to keep the smoke strong and the mosquitoes away.