If you were asked to describe good pollinator habitat, you’d probably come up with something like a wildflower meadow or a pollinator garden: green, flowery, lush, and diverse. You wouldn’t be wrong; pollinators do need flowers. But you’d be displaying a human bias toward the fertile and productive.
What you probably would not have envisioned would be a swath of bare, compacted mineral soil with nothing at all growing on it. It doesn’t sound like habitat for anything, much less flower-affiliated insects! And yet, for many kinds of pollinators, bare soil is an essential resource.
The reason is that feeding on flowers, as important as it is to a pollinator, is only one part of the life history of these important arthropods. There’s also the matter of reproduction, without which the whole glorious machine of nature would grind to a halt. And for many insects, successful reproduction requires nest tunnels dug into the ground. Approximately three-quarters of all the Vineyard’s bee species nest in the ground. And among two of the most important wasp families that visit flowers, Sphecidae and Crabronidae, ground-nesting is an almost universal rule.
Ground-nesting bees and wasps differ in some critical ways, to be sure. Bees (one friend of mine calls them “hippie wasps”) are vegetarians, stocking their nests with pollen or, occasionally, other plant products.
Wasps, on the other hand, may visit flowers as adults, but they feed their offspring largely on meat. In the case of many ground-nesting wasps, nests are provisioned with other arthropods, stung and paralyzed to keep them fresh, hauled underground, and fed on by the wasp larvae as they mature. As a rule, the prey of any particular wasp species is a fairly specific group: members of a specific family, genus, or even species.
But ground-nesting bees and wasps share many similarities, including an exquisitely refined sense of soil qualities. Each species has a very clear notion of the right kind of soil, and patches of the right kind of substrate can be busy places indeed, with many individuals of multiple species digging in to build their nests.
The driveway in front of the BiodiversityWorks office, just off Lambert’s Cove Road, has offered an unbeatable example this year. The soil is a mix of fine sand and clay; it’s been compacted by decades of pedestrian and automotive traffic; and I surmise that the conditions make it perfect for tunnelling, stable but not too hard to excavate. An area perhaps five by 20 feet has been the focus of a lot of my attention this season.
First to turn up were some early season bees. At least three species of the genus Lasioglossum nested there, the mouths of their tunnels marked by little volcanoes of sand. And tiny “fairy bees” — in this case, Perdita bradleyi — also favored this site, somehow digging burrows that produced no spoil. I assume the bee simply shoves grains of sand and clay aside, working her way down to whatever she deems a sufficient depth for safe egg-laying.
Along with the nesting bees came several species of “cuckoo bees” — other bee species that watch for nesting activity by an appropriate host species, and then lay their eggs in the nests of the other bees. When the cuckoo bee eggs hatch, they eat the provisions the host species stored for her own young (and sometimes eat the host’s offspring, as well).
The bee nests are all sealed up now, with the larval bees inside industriously feeding and growing. But now, ground-nesting wasps of several species are using the same area. Most prominent has been Cerceris fumipennis, sometimes known by the wordy but apt common name smoky-winged beetle bandit, which creates impressive excavations almost half an inch in diameter and surrounded by a mound of spoil that may be an inch high. This slender wasp, black with sparse yellow markings and dark wings, is easily recognized by three large, diffuse, pale spots on its face.
Cerceris itself is not a large wasp, and wouldn’t, on its own, need such a wide tunnel. But it preys exclusively on metallic wood-boring beetles (the family Buprestidae), which can be quite large. The tunnel’s width is designed to accommodate even the largest of these beetles. I found one Cerceris nest with a 15mm beetle I identified as Buprestis maculipennis dragged partway into the burrow.
While formidable, Cerceris is not immune to all threats. Flesh flies in the genus Metopia lay their eggs in Cerceris burrows, using the same strategy as cuckoo bees. Female Cerceris are aware of the threat, and defend their nests vigorously, backed into their tunnels and biting with their long mandibles, against the encroachment of these flies. The tension between staying home to defend the nest and leaving to hunt beetles must be a central fact of Cerceris life.
So there you have it. Flowers matter to pollinators. But for many species, bare, compacted soil is where the most important parts of their lives play out.