Wild With Nature

The tiny wasp shimmered between my fingertips, its hard exoskeleton glittering green and blue under the strong June sun. Nearby, along the base of the shaly orange slope, was the patch of leafy spurge where I had captured it as it had gathered nectar from the showy yellow spurge blooms. I had started seeing cuckoo wasps several weeks ago near this western Montana grassland stream, crawling across the ground and visiting flowers among the busy profusion of early-summer vegetation.

The cuckoo wasps I was seeing, roughly the size of rice grains, varied somewhat in their appearance. But all of them were striking. Most ranged from emerald to a deep blue-green. Some shone coppery red in the light. And the more I learned about these tiny, often-overlooked wasps, the more interesting they became.

When most people think of wasps, they think of the yellowjackets and paper wasps – that small handful of species that tend to show up uninvited at picnics and sometimes sting us. But the rest of Montana’s wasps make up a vast and seldom-seen world. Most of them would never show up at our picnics. Very few people even notice them. And cuckoo wasps (family Chrysididae) are part of that massive world. Unless you know what to look for, you’ve probably walked right past hundreds of them.

How to recognize them

Cuckoo wasps are extremely docile. Unlike most female wasps and bees, they actually can’t sting, even when they’re threatened. While most female wasps have a stinger, cuckoo wasps just have a harmless egg-laying tube. To protect themselves, most of Montana’s species can curl up into a ball when they’re attacked. Like the shell of a turtle, the hardened armor of their exoskeleton keeps them safe.

Cuckoo wasps aren’t Montana’s only small, wasplike, iridescent green insects. Certain sweat bees (family Halictidae) are similarly green – and these creatures can sting when they feel threatened. There are also other tiny wasps, such as the perilampids (family Perilampidae), that sport similar, shiny greens and blues.

How can you tell if you’re looking at a cuckoo wasp, then? Watch for compact, iridescent green wasps that typically play dead when disturbed, rolling into a ball. Their bodies usually have a pitted appearance, and many of them have a series of small teeth projecting from the rear of the abdomen. The veins in their forewings form a few box-like cells. Sweat bees have many more of these cells (over six of them), while perilampids have none.

A diversity of cuckoo wasps

Like many insects, cuckoo wasps are very diverse in Montana. According to Mike Ivie, curator of the Montana Entomological Collection in Bozeman, we have at least 71 species of them in the state. In fact, we have almost as many cuckoo wasps as we do mammals, a group that includes 109 species in Montana. But whereas most people are at least vaguely familiar with mammals, from elk to muskrats and deer mice, few of us have noticed even a single cuckoo wasp.

Until recently, even entomologists didn’t know much about cuckoo wasps in Montana. For many species the published range maps seem to curve around the state, barely missing it. But since the 1990s, when Mike Ivie and his colleagues began a much more thorough effort to inventory Montana’s cuckoo wasps, they’ve found a rather astounding diversity of them.

“Our diversity is that mix of eastern and western,” Mike says.

We have species from the Pacific Northwest that enter northwestern Montana, others from eastern North America, and still others associated with desert habitats near the Pryor Mountains. With cuckoo wasps, as with so many other animals, Montana’s diversity of habitats fosters an impressive diversity of species.

Species ID and where to find them

Identifying cuckoo wasps to the species level is generally quite challenging. Some are relatively large; others are as small as a mosquito. Many are bright green, while others have reddish tints. Details of the wing venation, ridges and indentations on the face, and the structures at the rear of the abdomen can help distinguish the different species. But for definitive identifications, it’s usually essential to study specimens under a microscope. 

Relatively few people may have the patience to identify cuckoo wasps to the species level. But for all of us, it’s relatively simple to keep an eye open for these creatures whenever we’re outside. Species ID is hard, but recognizing them as a family is fairly straightforward. And by noticing cuckoo wasps, we can understand another important part of the vast natural world that surrounds us.

Where can you find them? Sometimes, cuckoo wasps visit flowers, such as the brilliant green individual I found on the leafy spurge. I’ve also found certain cuckoo wasps visiting the striking white flowers of cow-parsnip (Heracleum lanatum) and the deep yellow arrays of goldenrod (Solidago spp.). But it’s more typical to see them on the ground.

“The place I see them most is on dead wood,” Mike Ivie tells me.

In forested settings, it’s common to find some species crawling busily along downed branches, large or small. Meanwhile, others frequent areas of bare ground.

A bizarre life cycle

Why all of the crawling? This behavior is the visible sign of the other 80% of these wasps’ life cycles, which take place in hidden burrows. And from here, the story of the cuckoo wasps gets increasingly bizarre.

Montana’s cuckoo wasps are parasitoids. When we see them crawling busily across wood or soil, we’re actually watching them hunting. A few of Montana’s species, in the genus Cleptes, attack sawfly cocoons in forested habitats across the state. The rest of our cuckoo wasps look for certain groups of wasps and bees. In this case, the female will sneak into the nest of her host species, laying an egg there. When her young larva hatches, it will devour its host and whatever food is in the nest cell. The next summer, the new cuckoo wasp will emerge as an adult, having usurped its host’s would-be nest.

What can these shiny green wasps teach us? It turns out that noticing the cuckoo wasps themselves is just the tip of the iceberg. They’re a window in on a whole community of bees and wasps that nest in the ground, or in downed branches where wood-boring beetles have excavated. Each of these species, which the cuckoo wasps target, has its own story. Many are highly specialized, collecting pollen from particular flowers or hunting certain insects to stock their nests. It’s an intricate, miniature world that most of us don’t even think of. And the cuckoo wasps can help us notice.

A miniature world of bees and wasps

“They’ll be crawling around on pieces of dead wood, branches…. Those old beetle borings are full of bees,” Mike says.

A closer look at a few species helps to illustrate the complexity of this miniature world. On a sunny early July afternoon, I found a pair of deep green cuckoo wasps mating within a streamside leafy spurge patch. Later, studying one of them under the microscope, I identified it as Holopyga ventralis. Like all cuckoo wasps, Holopyga has a unique life story. This species attacks Bicyrtes, a group of pale-striped sand wasps that visit flowers and dig their nests in sandy soil. 

Like many wasps, Bicyrtes are rather-specialized predators. In order to feed their larvae, the females hunt stink bugs and other true bugs. Stinging these bugs to paralyze them and then stocking their underground nests with them, they’re ensuring that their larvae will have all the food they need to develop into an adult. If everything goes according to plan, that is. 

Enter the cuckoo wasp, Holopyga. If a female Holopyga manages to sneak into a Bicyrtes nest and lay her egg, then her larva will devour the developing sand wasp, along with the paralyzed stink bugs that were supposed to feed it. This is game over for the Bicyrtes larva. It’s also why the female Holopyga can curl into a ball, protecting herself with her super-tough exoskeleton. If the adult Bicyrtes wasp finds her sneaking into the nest, the armored-ball defense will allow her to survive. The angry Bicyrtes may throw her out of the nest, but the Holopyga will live to nest-search another day.

Different species, different story

It’s a similar story with Hedychrum parvum, a cuckoo wasp that I found visiting flowers in a dry, disturbed area around the same time of year. But instead of searching for Bicyrtes nests, this one has a different host. Hedychrum sneaks into the underground nests of Cerceris or Eucerceris, slender predatory wasps that hunt weevils. 

Then there’s the bewildering diversity of cuckoo wasps in the genus Chrysis. For many of these species, entomologists don’t yet know which wasps or bees they attack. Others are better-studied – such as the widespread cuckoo wasp Chrysis pellucida. This one relies on wasps in the genus Trypoxylon, which nest in wood and hunt spiders.

Aren’t cuckoo wasps bad news for the wasp, bee, or sawfly larvae that they attack? For these particular larvae, yes. But on the population level, one Finnish study suggests that they don’t seem to threaten the long-term survival of their hosts. Indeed, because they rely on specific host species, cuckoo wasps tend to be less common than their hosts, more vulnerable to extinction. For a species like Holopyga ventralis to thrive, it depends on an entire system in miniature: the Bicyrtes wasps, their true bug prey, and the plants those bugs feed on. 

So as June approaches again – and with it, the beginning of cuckoo wasp season – keep your eyes open. Watch for jewel-like, iridescent wasps crawling across downed branches or patches of bare soil. How many can you see? Can you spot the nests they’re searching for, or some of their host wasps and bees? Welcome to the miniature world of the cuckoo wasps.

Further reading

Bohart, R.M. & Kimsey, L.S. (1982). A synopsis of the Chrysididae in America north of Mexico. Memoirs of the American Entomological Institute no. 33. Ann Arbor, MI: The American Entomological Institute.

Evans, H.E. (1970). Ecological-behavioral studies of the wasps of Jackson Hole, Wyoming. Bulletin of the Museum of Comparative Zoology 140(7):451-511.

Paukkunen, J., Pöyry, J. & Kuussaari, M. (2017). Species traits explain long-term population trends of Finnish cuckoo wasps (Hymenoptera: Chrysididae). Insect Conservation and Diversity 11(1):58-71. Retrieved from https://resjournals.onlinelibrary.wiley.com/doi/full/10.1111/icad.12241