Cerceris: an almost-social wasp

When it comes to the Hymenoptera, social species tend to steal the headlines. The sharing of complex information by honey bees. The division of labour and specialised castes of ants. The extended families, cooperative childcare, and nest defence of paper wasps.

Yet the vast majority of wasps and bees are solitary. Some form nesting aggregations, but rarely do they show the elaborate behaviours and adaptations associated with true sociality.

So when a solitary species shows hints of a social life, it captures the attention of scientists. Cerceris is a good example. Although most Cerceris are solitary nesters, a small number of species in this huge genus show subsocial behaviours – including multiple, related females sharing a single nest.

Cerceris: a background briefing

• Cerceris is a huge genus, with nearly 900 species worldwide (GBIF). There are currently 50 described species in Australia (AFD; Evans, 1981; Evans, 1988).

• Cerceris construct multi-celled nests in firmly packed soil. Most provision their nests with beetles (Bohart & Menke, 1976) – hence the common names of ‘digger wasps’ and ‘weevil wasps’.

• Cerceris are mass provisioners. Each cell is packed with paralysed prey, a single egg is deposited, and then the cell is closed.

• Larvae feed upon the deeply paralysed beetles stored in their cell. Adults take only nectar and honeydew.

• Most Cerceris species are strictly solitary, but there is convincing evidence of nest sharing occurring in at least ten species (Polidori et al., 2006). Notably, this includes seven Australian species (C. antipodes; C. australis; C. goddardi; C. minuscula; C. unispinosa; C. windorum; C. xanthura).

• Cerceris antipodes is one of the most widespread and common species of Cerceris in south-eastern Australia. And it is the only Cerceris species we have recorded here in the forest.

• Although some Cerceris specialise in particular beetle taxa (e.g. C. australis preys on chafer beetles, Melolonthinae), C. antipodes is more opportunistic. They favour leaf beetles (Chrysomelidae) when available, but will also take weevils (Curculionidae), chafers (Melolonthinae) and other leaf-dwelling beetles (Evans & Hook, 1986).

Cerceris antipodes (Crabronidae: Philanthinae)

Lately I’ve been fascinated by the nesting behaviour of Cerceris antipodes. From October to April, this captivating wasp is a regular sight here in the forest. Year after year, their nests appear in the same patch of firmly packed, sandy soil. Their burrows form a cluster in an area shared with a variety of other crabronid wasps and bees. I’ve watched as females guard burrow openings or return laden with paralysed beetles, and as males cruise the nesting area or perch on nearby plants.

Cerceris antipodes - female
20 November, 2022

Cerceris antipodes - female
18 March, 2023

Cerceris antipodes - male
26 March, 2023

Evans and Matthews (1970) were the first to flag Cerceris antipodes as a species of special interest. Alcock subsequently confirmed that nests comprise multiple females and that they work shifts, sharing the tasks of provisioning and guarding (Alcock, 1980).

Extract from Evans & Matthews (1970) on the nests and prey of four species Australian Cerceris (p. 156).

Now, thanks to the efforts of David B. McCorquodale in the mid 1980s, C. antipodes is among the best known species in the genus. McCorquodale studied nest sharing in two separate populations in NSW, over three breeding seasons (Jan 1985 to Jan 1987) (1988a). He addressed issues such as the relatedness of nestmates (1988b), longevity of individual females (1989b), the role of soil softness in nest initiation (1989c), and signs of reproductive inhibition in shared nests (1990). McCorquodale’s PhD studies resulted in a significant body of published works, and presents most of what is known about the breeding biology of this species.

---

So with these publications in hand, I reimagine our local Cerceris. I review last season’s photos, seeing who’s who and what they’re up to. I walk by the sandy patch on a cold winter’s morning, envisaging those asleep beneath my feet. And I await the coming of spring, when I expect to see them out and about once more.

Long, productive lives

Most solitary wasps live relatively short lives, dying well before their offspring emerge as adults. Not so for Cerceris antipodes. Some females live nearly a year, long enough to know their grandchildren!

• Young females that eclose in April, mate, then overwinter inside their burrows. They emerge again in late September or October and begin to reproduce. By late January, these ageing females may still be active in the nests.

• Females eclosing in Spring and Summer (the 1st & 2nd generations) may live for three months.

• After 10 days of adult life, females are capable of producing mature eggs.

• The first males emerge in late November. By March, the number of males has built noticeably.

• Females probably mate just once, or at most infrequently. Males tend to ignore older females in their pursuit of those newly emerged (Elliott, 1984).

• After three months of egg-laying, sperm-depleted females can continue to produce unfertilised eggs. These will give rise to female larvae (a feature of hymenopteran development).

• During winter, the concealed burrows contain only overwintering adult females. No adult males. And no larvae or pupae, in contrast to the overwintering arrangements of most crabronids.

Mating is thought to take place outside burrows, but it is rarely documented. Of around 250 sightings of Australian Cerceris on iNaturalist, just four show male-female interactions. I have seen the occasional tussle on the ground near nests, but the action is over before I manage a photo. It is also likely that mating is quite rare. Females probably mate just once (McCorquodale, 1990), so it's no surprise that the most male activity I see is late in the season, as young virgin females first venture above ground.

House-sharing is the norm

I’ve occasionally seen evidence of nest sharing – one wasp departs the nest and then another face appears at the burrow entrance. To really be sure what’s going on, however, I would need to tag individual wasps. I haven’t done this, but both Alcock and McCorquodale did! And they found that most nests contain several, actively nesting females.

• Early in the season, many females live alone or with one other female of the same cohort.

• By late December, most nests are home to one overwintered (‘older’) female and at least one young (‘1st gen’) female.

• The record stands at eight females in a single nest, although three to four is typical.

• Most housemates are immediate family – mothers and daughters.

• It is reasonably common for females to move into other, established nests, and these are often the nearby nests of a relative. So add a few cousins to the mix.

• Strangers (i.e. unrelated) females sometimes house-share too. There is even a record of C. antipodes female sharing with a C. australis female (McCorquodale & Thomson, 1988)!

• Despite the possibility of relocating to another burrow, housemates remain in stable groups for anything from a week to a month or more.

• No males remain in the nest. They depart soon after eclosing, and spend their entire lives above ground, sheltering among vegetation at night or during inclement weather.

Nesting tasks are also shared

Housekeeping and nesting chores include burrowing, provisioning, egg laying, and guarding. Of course, burrowing and egg-laying activities are hidden from view. What can be studied more readily are provisioning and guarding. But who undertakes what, and when? Again, I draw upon the works of McCorquodale, Alcock, Evans and Hook.

• There is no caste system. An individual female is capable of digging, hunting, guarding, and laying eggs.

• In shared nests, not all wasps head out to hunt on any given day. The non-provisioners do, however, tend to leave the nest once during the day to feed at flowers, returning empty-handed.

• Provisioners will often make several hunting trips in a day, sometimes in rapid succession. The prey is simply deposited in the burrow, and she is free to depart and hunt again.

• Some females simply do not hunt, presumably taking on other housekeeping roles.

• The beetles cached in the burrow are moved into cells as required, and by whomever needs them. One wasp’s catch can become food for another wasp’s offspring.

• It is common to see a wasp on guard duty at an open nest, particularly later in the season when the nest contains several occupants. Mutillids, ants, and ‘foreign’ females are effectively kept at bay by the rather formidable mandibles of the guarding female. When a provisioning female returns, the guard quickly backs down the burrow out of the way.

• Nest entrances are closed overnight and on many days they remain so, even when the weather seems conducive to hunting. The breeding season is long, so I guess there’s no rush, and a closed nest is safer from predators and parasites.

• There is also some evidence for variation in egg-laying capacity among females of a shared nest (McCorquodale, 1990), a variation not simply due to age or body size. The data is indirect, however, and interpretation complicated by movement of females between nests.

Well-built, long-lasting burrows

Like most Cerceris, C. antipodes nests are usually grouped and in aggregation with other ground-nesting wasps and bees. Our home population is typical for the species.

• A concentration of nests in a small area. By mid summer, there are at least 20 active nests in one small patch of a few square metres.

• The location is predictable. From one year to the next, the nests ‘appear’ in this same patch of firmly-packed, sandy soil. To date these are the only Cerceris nests I’ve found here in the forest … and I have been looking!

• They share the patch with various crabronid species including a large number of Sphodrotes (Crabroninae) and Austrogorytes (Bembicinae). Early in the season, the patch was also home to nesting nomia bees (Lipotriches). I’ve written about this busy patch in an earlier blog post (‘Neighbourhood watch’).

• Burrows descend almost vertically for about 10cm, then level out (Evans & Matthews, 1970: Alcock, 1980). Cells are constructed on short side branches off the main tunnel.

• A nest may be used over consecutive seasons, simply by extending the burrow and adding new chambers. In studies of another Australian, nest-sharing species (Cerceris australis), nests used over several years had been extended time and again (Evans & Hook, 1982). Alcock (1980) found a similar pattern of old, empty cells in the upper portion of second-year nests of C. antipodes.

There is limited data on the total number of cells in nests of C. antipodes. Based on results for C. australis, it may well exceed a hundred in an long-established burrow.

Extract from Evans & Hook (1982) showing the arrangement of cells in a Cerceris australis nest, in use for two seasons (Fig. 1b p. 560)

The advantage of nest sharing

Nest sharing and the continued use of existing burrows can be explained, at least in part, by the difficulty of starting a new burrow. Cerceris are not strong diggers, yet they select firm ground in which to nest. It is the first few centimetres of soil that present the greatest challenge. Sandy soils such as ours form a hard crust when dry. Therefore, during periods of hot, dry weather, newly emerging females either stay in their natal nest or seek to move in with neighbours. McCorquodale showed that new burrows are only started after rain has softened the surface (McCorquodale, 1989c). So once a burrow is established, it makes sense to reuse it in subsequent seasons.

Calling Cerceris antipodes ‘almost-social’ is potentially misleading. In evolutionary terms, these wasps are well adapted as they are. The species is “neither eusocial nor evolving toward eusociality” (McCorquodale, 1988a p.119). The study of their nest sharing behaviour simply helps entomologists identify the conditions that favour nest sharing over a completely solitary existence.

A puzzling three-way tussle

Most of the time, all is quiet at the nesting ground. Late in summer, several males can be seen cruising back and forth, and occasionally a female will fly from a burrow or return with prey and quickly disappear underground. But one day I spotted something very odd – two females and a male, tussling at a nest entrance. It was a protracted interaction, lasting several minutes. This was a well-established, active nest. I had watched wasps provisioning and guarding the nest all summer.

A closer look at the photos, and I now have a theory. A young, unmated female is trying to escape the attentions of a male, but he is reluctant to give up. At the same time, a young mated female from a nearby nest is looking for a place to overwinter. The drama of the clogged entrance and subsequent events provide just the distraction she needs, and she slips inside.

This is the first time I’ve ever seen a male enter a burrow. He was clearly not willing to give up on the young female.

Good years, and bad

Some years are better than others for Cerceris. Populations show seasonal fluctuations in response to weather conditions, prey and even nectar availability. The 2022-23 season was a particularly successful for our local population of C. antipodes.

Conditions next summer are predicted to be much hotter and drier. It will be interesting to see what effect this has on both the number of wasps and the number of nest burrows. I will watching the patch closely from November, onwards.

---

A note on species identification

I have been observing and photographing our local nesting patch of Cerceris for several years. Early on I had read that some species shared nests, and I suspected that our local wasps did this. The more I watched them, the more intrigued I became about the population dynamics of the nesting ground. I was keen to delve deeper into the literature on the genus, but first I needed to work out what species we have here. So at the end of the season (April 2023) I collected a male and a female, then set about working through the 44 described for eastern Australia.

Our local Cerceris are an excellent match for Evans' description of Cerceris antipodes. And luckily for me, this common and widespread species is one of the most thoroughly studied Australian Cerceris.

For full details of the identification process, see my working notes page.

---

References

AFD (Australian Faunal Directory) accessed 25/3/23 (link)

Alcock, J. 1980. Communal nesting in an Australian solitary wasp, Cerceris antipodes Smith (Hymenoptera, Sphecidae). Journal of Australian Entomological Society, 19: 223-228

Bohart, R,M. & Menke, A.S. 1976. Sphecid Wasps of the World: A generic revision. University of Calfornia Press.

Elliott, N. B. 1984. Behavior of males of Cerceris watlingensis (Hymenoptera: Sphecidae. Philanthinae). The American Midland Naturalist 112(1): 85-90

Evans, H.E. 1981. The genus Cerceris in eastern Australia (Hymenoptera: Sphecidae). Transactions of the American Entomological Society, 107(4): 299-380

Evans, H.E. 1988. Further studies on the systematics and nesting behavior of eastern Australian Cerceris wasps (Hymenoptera: Sphecidae). Transactions of the American Entomological Society, 114(1): 1-13

Evans, H.E. & Hook, A.W. 1982. Communal nesting in the digger wasp Cerceris australis (Hymenoptera: Sphecidae). Australian Journal of Zoology, 30: 557-568

Evans, H.E. & Hook, A.W. 1986. Prey selection by Australian wasps of the genus Cerceris (Hymenoptera, Sphecidae). Journal of Natural History, 20: 1297-1307.

Evans, H.E. & Matthews, R.W. 1970. Notes on the nests and prey of Australian wasps of the genus Cerceris. Journal of Australian Entomological Society, 9: 153-156

GBIF (Global Biodiversity Information Facility) accessed 25/5/23 (link)

McCorquodale, D.B. 1988a. Nest Sharing in the Sphecid Wasp Cerceris antipodes Smith. PhD thesis, submitted May 1988. Australian National University.

McCorquodale, D.B. 1988b. Relatedness among nestmates in a primitively social wasp, Cerceris antipodes (Hymenoptera: Sphecidae). Behavioral Ecology & Sociobiology, 23: 401-406

McCorquodale, D.B. 1989a. Nest defense in single- and multifemale nests of Cerceris antipodes (Hymenoptera: Sphecidae). Journal of Insect Behaviour, 2(2): 267-276

McCorquodale, D.B. 1989b. Nest sharing, nest switching, longevity and overlap of generations in Cerceris antipodes (Hymenoptera: Sphecidae). Insectes Sociaux, Paris, 36(1) 42-50

McCorquodale, D.B. 1989c. Soil softness, nest initiation and nest sharing in the wasp, Cerceris antipodes (Hymenoptera: Sphecidae). Ecological Entomology, 14: 191-196

McCorquodale, D.B. 1990. Oocyte development in the primitively social wasp, Cerceris antipodes (Hymenoptera: Sphecidae). Ethology, Ecology & Evolution, 2: 345-361

McCorquodale, D.B. & Thomson, C.E. 1988. A nest shared by the solitary wasps, Cerceris antipodes Smith and C. australis Saussure (Hymenoptera: Sphecidae). Journal of Australian Entomological Society, 27: 9-10

Polidori, C. Federici, M., Papadia, C. & Andrietti, F. 2006. Nest sharing and provisioning activity of females of the digger wasp, Cerceris rubida (Hymenoptera, Crabronidae). Italian Journal of Zoology, 73(1): 55-65