#74 Giant Horntail & Sabre Wasp by Dan Lombard
Meet senior ecologist at Wold Ecology, Dan Lombard!
Dan works as a senior ecologist at Wold Ecology, where he primarily undertakes surveys for protected species and habitats. Dan has fostered an interest in natural history throughout his life from his beginnings as an inquisitive child and any free time he gets is now spent looking for, recording and watching a wide variety of Yorkshires wildlife. He has a wide range of interests including invertebrates, plants, mammals, birds and herptiles.
Dan currently operates as co-recorder for reptiles and amphibians for the YNU and has previously operated as Ringing Officer at Filey Bird Observatory, where he also helped oversee conservation projects including the restoration of Filey Dams. Dan’s passion for conservation has also seen him volunteer with Butterfly Conservation Yorkshire, The North York Moors, RSPCA, Flamborough Bird Observatory and Natural England through various stages of his career.
Giant Horntail
Dan’s chosen species are the Giant Horntail Urocerus gigas and its parasite the Sabre Wasp Rhyssa persuasoria. These two behemoths of the insect world predominantly occur in coniferous and mixed woodlands, with a scattered distribution across the region. The horntail, our largest species of sawfly can reach an impressive 4cm in length and with its ovipositor and black and yellow marking gives the impression of a formidable insect, it is however completely harmless. Equally as impressive is the Sabre Wasp which also achieves a length of around 4cm, with a further 4cm long hair like ovipositor. Whilst not fully restricted to horntails as its host, they are the preferred choice. More information can be found in the profiles of Giant Horntail here and for the Sabre Wasp here.
The Giant Horntail is a wood boring species which lays its eggs into the wood of dead and dying coniferous trees as well as recently felled timber. The larva then develop by feeding internally on the softwood, where they create tunnels filled with compact frass. Development in this situation usually takes between 2 and 3 years, but may last as long as 5 years, at which stage the adult insect emerges from quite a sizable tunnel of between 6 and 7mm diameter. This impressive insect may then be found in similar situations looking for suitable places to lay its eggs.
You could be forgiven for thinking this existence away from predators and the elements tucked away in the safety of their woody catacombs leads to a relatively secure existence for the developing larvae. This assumption would be very wrong, for there is another insect, the Sabre Wasp, which considers a large juicy grub, hidden away in the safety of its log the perfect place to rear its own progeny. Parasites and their hosts are some of the most fascinating aspects of natural history, with some truly amazing associations, but few are quite as heavy weight and as easy to observe as these two.
Sabre Wasp
Whilst sat in a woodland clearing, during the height the summer, a female Sabre Wasp lands on a nearby Scot Pine log and proceeds to massage its surface with her antennae, frantically searching across the logs surface like a dog, nose to the ground, searching for a scent. Eventually her searching excitedly intensifies and becomes more localised, constantly pattering the wood’s surface with her antennae. Unsuspectingly within the wood below the Sabre Wasp, a horntail larva is slowly chewing a chamber through the log. Wood is eaten, then passed by the larvae as frass which fills the chamber behind it as it slowly moves. Back on the surface the irresistible smell of fresh frass resonates through the wood, which is then picked up by the Sabre Wasp’s antennae. She uses them to smell her victim beneath the wood, focusing on more recently deposited fresh strong-smelling frass excreted from the horntail larvae, to locate its exact location deep in the log. Even if the horntail knew of its peril and stopped moving and gave no other indication to its presence, its fate would still be sealed by its smell.
Still caressing the wood’s surface with her antennae, the Sabre Wasp now knowing the location of her prey starts what is quite a long process of ovipositing. The reason for her long tail is now abundantly clear, to allow her to access larvae deep in the wood. The tail acts as a drill, which is amazingly strong for such a thin hair like structure. These wood boring drills in Ichneumon wasps are reinforced with Zinc and Magnesium metals to give them the strength and durability to penetrate wood often multiple times over the breeding season. Initially she drills several probe holes, of just a few millimetres in the wood whilst she decides on the best angle to approach her prey: finally she appears to be content and begins the process of drilling into the wood.
Depending on the depth of the larvae and compactness of the wood, drilling may take between 30 minutes and an hour before she reaches the tunnel in which the larvae is living. This is a particularly vulnerable time for the wasp, with many plucked from the logs surface by birds, as they are attached to the log. The abdomen is contorted during this process almost giving the impression it is going to split or snap. Upon reaching the tunnel the wasp lands a catastrophic sting on horntail larvae paralysing it, leaving it immobilised in its tunnel. The wasp then lays her egg on its now paralysed body through the hole it has just drilled. The horror for the larvae has now only just begun, as it is now slowly eaten alive by the wasp grub, starting with non-essential organs and fat reserves. Eventually the host is killed, the larvae pupate and then overwinters in its hosts lair before emerging during the summer to continue the cycle.
The fascinating life history of parasitic wasps have become immortalised in pop culture most notably through the inspiration behind Dan O'Bannon’s sci-fi works Alien. Later he co-produced the block buster Alien films with Ridley Scott. In these works, a wasp inspired extra-terrestrial species called a xenomorph uses humans as its host.
Parasites occur in a huge range of different shapes and forms, with complex and varied ecology and often have more of a significant impact on host populations than predation, making it a hugely important ecological process to understand and study. Aside from the gruesome fascination, this story also mirrors a much wider and significant factor which is the importance of deadwood habitats. Deadwood is just as important as living wood, yet sadly it is chopped out and cleared, considered unsafe, unsightly and used as firewood. Our landscapes are sadly significantly lacking in deadwood habitats, just as important for bats, cavity nesting birds and fungi as they are for insects. All areas of woodland need deadwood increasing and retaining, ash dieback should be managed to reflect this and the consideration of creating diverse deadwood habitats should always be considered during any site management.
Dan’s first experience with great horntails came as a child playing in his garden, where he witnessed a horntail emerging from some new garden furniture (given how long larvae live inside felled timber this is a common situation in which this species is found). A child who unfortunately, through ignorance and fear, killed it, but who would quickly become inspired and fascinated by this same insect’s natural history to pursue a career and lifelong passion for wildlife and conservation. Only a few weeks before writing this post, successful communication and advice to a woodland manager result in the retention of a large log pile destined for firewood. This log pile was teeming with both species, along with a wide range of other insects and will now become a permanent feature with an interpretation board, designed to help inspire and educate future naturalists.
Recording and monitoring
The recording of our wildlife is fundamental to conservation and the understanding of natural processes. If you observe either of these insects, or indeed any other species then please enter your records via iRecord. These are verified by experts, and can usually be accessed by your local LERC. Photos really help with verification, so should be included where possible.
Further information and acknowledgements
NEYEDC would like to thank Dan for his time and expertise in helping to create this blog.