In Passing Unnoticed - Spider Hunting wasps

In passing unnoticed – observations on the smaller denizens of Dove Stone Reservoir.

 This is my first blog post of a series in which I will introduce some of the overlooked invertebrate inhabitants of our local reserve, Dove Stone Reservoir.

The start of the ‘invertebrate season’ has passed us by for 2020 but there are definitely many miniature miracles of nature taking place at Dove Stone as Summer continues. I want to share some that I’ve seen and hope to encourage you to search out others for yourselves. These events are easy to find; just by slowing down or standing still, and with a bit of practice it’s easy and rewarding to refocus awareness into a different scale of life that is going on, unnoticed, beneath our feet.

In future posts I’ll look at some easily observed species; especially our amazing solitary bees, but for this post I’d like to introduce one of the most exciting (for me) observations so far this year; the Spider Hunting Wasp (Pompilid) Anoplius nigerrimus transporting it’s prey to it’s nest.

Pompilids are, as their common name suggests, spider hunters by profession and they are very good at it. Anoplius nigerrimus is a super-cool, all black, ninja like hunter able to take on and subdue a spider four times it’s size, even though the spider has more legs and is a venomous insect hunter itself. As with most aculeates (the group of insects to which pomilids belong) spider hunting wasps nest solitarily and it is the female that does all the work.

Like us and most other aculeates these wasps love hot, dry and sunny conditions. If you find an area of sparsely vegetated, grassy ground and stand still for a minute or two you may see one on the hunt, moving swiftly and erratically over the ground, dodging nimbly amongst vegetation, her antenna frantically waving. 

You will be ignored as she is completely focused on hunting and if you are very lucky you may see one find her arachnid (spider) prey. Many species hunt for particular types of spider and once they find a suitable prey a brief struggle ensues with the pompilid usually being victorious as she darts in and swiftly stings the spider. 

The sting paralyses the spider and the female wasp then begins her herculean task of dragging the spider across uneven and vegetated ground to the nest she has previously located. Imagine having to carry three months worth of shopping single handed, through a jungle, backwards! She can only travel backwards as she has to grasp the spider with her mandibles (jaws). She will often stop and scout out the terrain ahead (or perhaps just for a breather) before returning to drag her prize a little further.

Eventually she reaches her nest and drags the spider inside. At last safe inside the nest she will lays a single egg on the spider.

Anoplius nigerrimus females can dig their own nests in soft ground but are usually ‘renters’ using hollow stems, cavities in the ground, snail shells or the abandoned burrows of ground-nesting invertebrates. In this case most likely the abandoned nest of an Andrena mining bee.

The egg will hatch in a few days as a ‘wasp grub’, which will devour the paralysed but still alive spider, then slowly transform from a grub into a pupa. The pupa will rest in the nest over winter and will finally metamorphose into a new pompilid.

The female wasp seals the nest with particles of sand, soil and vegetation to ensure her offspring remains safe until ready to emerge next year when the weather is warmer and spiders are once again abundant.

        Anoplius nigerrimus is one of the UK’s most common pompilids and especially likes dry, grassy scrubland so is likely to be found in the wildflower meadow areas and on sloping ground with exposed soil near to footpaths.

Dove Stone Reservoir is sure to be home to a number of different species of Pompilid, but so far this year I have only seen Anoplius nigerrimus and Evagetes crassicornis (which is black with red markings on it’s abdomen.) Evagetes crassicornis is not a true spider hunter but is actually a 'cuckoo wasp'. This means that, much like the avian cuckoo, it doesn’t forage to feed it's young but rather lays it’s eggs in the nest of another wasp, in this case Anoplius nigerrimus. This way of life is also known as cleptoparasitic and another aspect of the fascinating diversity of our native invertebrates.

There will definitely be more species of spider hunting wasp out and about at Dove Stone at the moment so definitely worth keeping an eye out when you next visit.

If you have any questions about this post or other invertebrates you discover at Dove Stone please feel free to email me: richard.dawson@rspb.org.uk

References:

Online references freely available:

BWARS account

https://www.bwars.com/wasp/pompilidae/pompilinae/anoplius-nigerrimus

The Biology of British Pompilidae (Hymenoptera) Trans of Soc Brit Ent (Vol6 30^th August ) by Richards O W & Hamm A H 1939 part 4)

https://www.biodiversitylibrary.org/item/254326#page/26/mode/1up

Published literature:

Spider-hunting wasps (Hymenoptera: Pompilidae) of Poland

Bogdan Wisnioswski

PL-ISBN 83-60337-15-4