Fireflies spend the first 2 years of their lives underground. Lawn chemicals kill them.
During those early years in the soil, they're working double time dealing with your pests. They eat slugs, snails, earthworms and other soft-bodied insects.
The three biggest threats to fireflies are habitat loss, light pollution, and pesticides, pretty much in that order according to experts surveyed across 350 firefly researchers worldwide.
What actually helps: stop all lawn chemicals, leave the leaf litter in the fall, mow less frequently and higher, and turn off outdoor lights at night.
How are the fireflies doing in your yard?