Every spring, around the second weekend of April, a particular sound starts up across the suburbs. It's the sound of leaf blowers. People are cleaning up their yards. They are removing what nature spent the previous fall placing carefully on the ground.
Here's what's in those leaves.
The cocoon of a luna moth — that pale-green insect, with its long curved tails, that you maybe saw once at a summer porch light and never forgot. Luna moths overwinter as pupae, wrapped in a single oak or hickory leaf, sitting in the leaf litter under the tree. Rake the leaves up, bag them, and put them on the curb, and you've thrown away every luna moth that was going to emerge from your yard this June.
The eggs of fireflies. Fireflies lay eggs in late summer in damp soil and leaf litter. The larvae spend the next year and a half as predators, eating slugs and snails in the duff. Then they pupate. Then, on a humid evening in June, they emerge as adults and fill the air with light. Rake out the leaf litter and you've removed the larval habitat. No leaf litter, no fireflies, eventually.
The overwintering chambers of native solitary bees. Mason bees, leafcutter bees, sweat bees — the ones that do most of the actual pollination work in your area — spend the winter in hollow stems and under leaf cover. Rake everything to bare ground and you've removed the structural complexity they need.
Caterpillars and chrysalises of swallowtail butterflies, mourning cloaks, fritillaries. A whole list of other moth species. Spider egg sacs. Snail eggs. Ground-beetle larvae. The leaf layer is, ecologically, a city. We don't see the city because it's small and brown and looks like dead leaves. But it's a city.
The actual case.
The case for raking is aesthetic. Leaves on a lawn look untidy. Tidy yards are what we have been taught to want, and tidy yards require removing the leaves. We can decline this teaching.
The case against raking is ecological. The cost of a tidy yard is the entire next generation of moths, butterflies, native bees, fireflies, and ground-dwelling insects that were trying to overwinter on your patch. They don't have anywhere else to go. Their species evolved alongside the seasonal leaf-fall, and they are currently in the leaves you're about to bag.
The compromise.
If you cannot tolerate leaves on your lawn — and we mean this with no judgment, lawns are public-facing in a way that other parts of yards are not — here is the compromise. Rake the lawn. Don't rake under the trees, in the borders, in the back third of the yard, or anywhere out of public view.
Even better: when you rake the lawn, take the leaves and put them under your trees and shrubs as a thick mulch layer. You've created the same overwintering habitat in a different place. The luna moths don't care if their leaves are under an oak in the back of the yard or under an oak in the front. They care that the leaves are there.
The lawn-care industry will tell you that leaves left on lawns "smother the grass." This is mostly false — a thin layer of fall leaves decomposes by spring and feeds the soil. Only a thick mat of wet, matted leaves on a lawn causes problems, and you can avoid that with a single light raking or a mulching mower pass. You don't have to remove every leaf. You don't have to bag any of them.
If you can manage it, the right thing to do in your yard in early spring is mostly nothing. Wait until the daytime temperatures are consistently above fifty degrees for a couple of weeks. The overwintering insects will have woken up, emerged, and moved on by then. Then you can clean up, and you'll have lost very little.
"Leave the leaves" is now an actual campaign by the Xerces Society and the National Wildlife Federation, and they have wonderful detailed guides on this. But the short version is: don't rake. The luna moths are sleeping in there.
— The editors