Bees & WaspsKerri-Lee HarrisBembix, wasp, digger wasp, batch 2, Crabronidae, sand wasp season blog link

Sand wasp season

Bees & WaspsKerri-Lee HarrisBembix, wasp, digger wasp, batch 2, Crabronidae, sand wasp season blog link
Sand wasp season

November 23, 2017:
After travelling for a week or so, we returned home yesterday to discover patches of the forest floor abuzz with sand wasps.

If anything is going to distract me from stalking native bees it is likely to be the intriguing behaviours of sand wasps. They buzz loudly as they patrol their territories before winging off to hunt or drink from flowers. 

Updated January 2026: When I first wrote this blog back in 2017 I knew next to nothing about Bembix. I wasn’t even able to distinguish males and females. Now, looking back on these images, I realise that I captured a rather special moment in the breeding season. Males not only engaged in their ‘sun dance’, but actively digging and competing for females – a behaviour I’ve not witnessed since.

So in a departure from our usual practice, I’ve decided to update this blog, drawing upon my improved knowledge of the genus and species.

Link: a detailed look at Bembix identification

Sexual differences

Back in 2017 I was unsure if these different colour forms represented two different species. I now know they are all Bembix furcata. The sexes differ markedly in colour pattern. Males also have a strong ventral projection beneath the abdomen and modified antennae.

Built to dig

Bembix are the quintessential ‘sand wasps’. They typically nest in sandy dunes, areas of riverine sand, or similarly friable patches of gravelly soils. Whatever the species’ preference, all are built for digging.

Both sexes dig. Females construct nesting burrows, and usually overnight within. Males dig individual, dedicated sleeping burrows, as do females without active nests. And as I discovered, some males employ their excavation skills in search of a mate. More on that shortly.

Featured
The outer edges of the front tarsi are adorned with an array of stiff spines. These form a rake for sweeping sand.
female
female

The outer edges of the front tarsi are adorned with an array of stiff spines. These form a rake for sweeping sand.

female
Incidentally, these ‘pecten spines’ are an important diagnostic character. Species vary in the number, colour & length of the spines, as well as in the shape of the tarsal segments
female
female

Incidentally, these ‘pecten spines’ are an important diagnostic character. Species vary in the number, colour & length of the spines, as well as in the shape of the tarsal segments

female
Mandibles are used to prise out larger stones.
male
male

Mandibles are used to prise out larger stones.

male
Digging a new burrow or opening a concealed one involves a similar set of actions. It is the front pair of legs that do the scraping as the wasps scuttles backwards with each sweep.
female
female

Digging a new burrow or opening a concealed one involves a similar set of actions. It is the front pair of legs that do the scraping as the wasps scuttles backwards with each sweep.

female
The action is fast and furious. With sand from the last sweep still flying, she is already reaching forward for another load.
female
female

The action is fast and furious. With sand from the last sweep still flying, she is already reaching forward for another load.

female
A few sweeps later, now dragging each load further from the entrance before kicking it out behind.
female
female

A few sweeps later, now dragging each load further from the entrance before kicking it out behind.

female
Males dig too, whether to create overnight retreats or (as here) in search of a female.
male
male

Males dig too, whether to create overnight retreats or (as here) in search of a female.

male
Those mandibles in action again. No wonder they become worn with time.
male
male

Those mandibles in action again. No wonder they become worn with time.

male

Despite B. furcata being among the largest of the Bembix, the nests are surprisingly shallow. On average, burrows are 13cm long, dug at a shallow angle, with the single terminal cell lying about 6cm below the surface (Evans & Matthews 1973).

A hunter of flies

Once completed, the female stocks the burrow with a paralysed fly and onto this fly she lays a rather large egg – about 5mm in length. And she keeps delivering fresh flies to her growing larva as it develops. Every time she leaves the nest to hunt she closes the entrance with sand, so on each return she must locate the somewhat concealed burrow and then dig her way in.

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On returning from a successful hunt, females fly directly to the site of their nest. The cues she follows may be visual, chemical, or both. Whatever the mechanism, it is remarkably reliable.
female
female

On returning from a successful hunt, females fly directly to the site of their nest. The cues she follows may be visual, chemical, or both. Whatever the mechanism, it is remarkably reliable.

female
Bee flies are a favoured prey of Bembix furcata , according to a study undertaken in the ACT (Evans & Matthews 1973). The fly in this photo is indeed a Bombyliidae – perhaps Staurostichus , a genus that takes flight in large numbers here durin
female
female

Bee flies are a favoured prey of Bembix furcata, according to a study undertaken in the ACT (Evans & Matthews 1973). The fly in this photo is indeed a Bombyliidae – perhaps Staurostichus, a genus that takes flight in large numbers here during November.

female
The fly is held securely by the wasp’s middle legs. And she continues to hold it while she rakes open the burrow.
female
female

The fly is held securely by the wasp’s middle legs. And she continues to hold it while she rakes open the burrow.

female
She is quick to disappear from sight, and for good reason. On the surface she may lose her prize to thieving neighbours – one of the downsides of a crowded nesting area. Then there are the ever-present ants that will grab any prey left unattended.
female
female

She is quick to disappear from sight, and for good reason. On the surface she may lose her prize to thieving neighbours – one of the downsides of a crowded nesting area. Then there are the ever-present ants that will grab any prey left unattended.

female

Male competition

Males patrol nesting grounds with good reason. They are on the lookout for newly emerging females. Evidence suggests that females of solitary wasps such as Bembix are receptive at the time they first emerge as adults … and that they mate just once (Evans & O’Neill 2007). Previously mated females reject the advances of males thereafter. So for males, it is all about being at the right place, at the right time. First.

These males were so impatient that they were digging into the ground in their search. Males are known to dig sleeping burrows for themselves, but based on their behaviour on this day it was all about mate seeking. And competition was quite fierce at times!

Featured
Males spend much of their time flying back and forth over patches of the nesting area. This is a behaviour I typically associate with our local Bembix . But on this day they were keen to dig. Very keen!
male
male

Males spend much of their time flying back and forth over patches of the nesting area. This is a behaviour I typically associate with our local Bembix. But on this day they were keen to dig. Very keen!

male
Back in 2017 I wasn’t at all sure what this was about. Indeed, I assumed these were nesting females. Now I know better.
male
male

Back in 2017 I wasn’t at all sure what this was about. Indeed, I assumed these were nesting females. Now I know better.

male
Males can detect the presence of females as they prepare to emerge for the first time. Such females are virgin and receptive. They will mate just once, then quickly get under way with nest digging, egg laying, and provisioning.
male
male

Males can detect the presence of females as they prepare to emerge for the first time. Such females are virgin and receptive. They will mate just once, then quickly get under way with nest digging, egg laying, and provisioning.

male
Males can sense the presence of females well before they reach the surface. Studies have shown the both chemical and vibrational cues are involved, and that males recognise the sex of the hidden wasp. They can even distinguish newly-emerging virgins
male
male

Males can sense the presence of females well before they reach the surface. Studies have shown the both chemical and vibrational cues are involved, and that males recognise the sex of the hidden wasp. They can even distinguish newly-emerging virgins from previously mated, nesting females!

male
The digging male soon attracted the attention of a second, cruising male.
males
males

The digging male soon attracted the attention of a second, cruising male.

males
Initially I didn’t see any overt physical attack, but the interloper was clearly seeking to drive the other male away.
males
males

Initially I didn’t see any overt physical attack, but the interloper was clearly seeking to drive the other male away.

males
And then a third one sought to crash the party!
males
males

And then a third one sought to crash the party!

males
The newcomers were persistent, but the first male did his best to guard the excavation site.
males
males

The newcomers were persistent, but the first male did his best to guard the excavation site.

males
Well, perhaps not all just intimidation. There was the occasional physical stoush!
males
males

Well, perhaps not all just intimidation. There was the occasional physical stoush!

males
Perhaps his larger size was an advantage. Bembix furcata males range from 14mm to 21mm in length.
males
males

Perhaps his larger size was an advantage. Bembix furcata males range from 14mm to 21mm in length.

males

The males’ digging is unlikely to be random. Firstly, the area would be one where this species nested last season. It is probably also the site from which these males recently emerged. Second, there is evidence that Bembix can detect females several centimetres below ground (Evans & O’Neill 2007). Chemical and vibrational clues even enable them to distinguish virgins from older females and males!

Despite my patience, and that of the males, I failed to sight an emerging female or the ensuing coupling. It is likely that the females can also detect the presence of males above, and bide their time.

Elsewhere on the nesting ground a female was returning with prey. One of the cruising males sought to grab her, but she was having none of it! She held tight to her catch and soon proceeded to dig open her nest, apparently unfazed. In fact, this is the same female as shown in the earlier panel, on the same provisioning flight.

Featured
Returning with a paralysed fly, this female prepared to dig open her burrow.
female
female

Returning with a paralysed fly, this female prepared to dig open her burrow.

female
In a lightning attack, a cruising male made a grab for her. She was clearly not receptive! Unfortunately, it all happened a little too fast for me to capture on camera.
female, male
female, male

In a lightning attack, a cruising male made a grab for her. She was clearly not receptive! Unfortunately, it all happened a little too fast for me to capture on camera.

female, male
Within seconds she was again aloft, lining up her approach. The earlier photo series under ‘A hunter of flies’ show her subsequent success. The male did not make a second attempt.
female
female

Within seconds she was again aloft, lining up her approach. The earlier photo series under ‘A hunter of flies’ show her subsequent success. The male did not make a second attempt.

female

The action I witnessed on 22nd November was a rather short-lived event. This is not unexpected. Males are the first to emerge during breeding season. After the initial few weeks, their numbers rapidly decline. I was lucky to be in the right place at the right time. In the years I have spent prowling our forest in spring, I’ve never again stumbled upon such a frenzied male behaviour.

References

Evans, H.E. & Matthews, R.W. 1973. Systematics and Nesting Behavior of Australian Bembix Sand Wasps. Memoirs of the American Entomological Institute, Number 20.

Evans, H.E. & O’Neill, K.M. 2007. The Sand Wasps: Natural History and Behaviour. Harvard University Press