Insect repellent was a substance applied to skin, clothing, or other surfaces to discourage insects from interacting with it.
In 2373, Neelix told Captain Kathryn Janeway that he grew up near the Rinax marshlands where they had the hottest summers in the sector and vicious lavaflies that grew to be six centimeters long. As a result, insect repellent was a booming business. (VOY: "Macrocosm")
In the late 31st century, subspace damage caused by the the Burn shifted the orbit of Kwejian's moon, which in turn changed the tides on the surface. As a result, vast numbers of sea locusts emerged from the ocean and devoured the Kwejian harvest, leading to millions starving. The Emerald Chain offered them repellent to drive the locusts away, in exchange for the planet's trance worms. This deal ended when in 3189, Osyraa withheld the sea locust repellent from Kwejian, threatening them again with famine, as leverage for her demand that Steward of the Sanctuary Kyheem deliver Booker to her. In response to Kyheem's message, the USS Discovery arrived and forced her to withdraw after a brief standoff. Discovery crew then devised a means to solve the problem without the need of the repellent. (DIS: "The Sanctuary")