If you have never heard of a trigonalid before, then you are not alone. It took me until yesterday to discover these wasps actually exist, and their life histories are absolutely fascinating!
The story begins back in July. While surveying a former coal tip in the county borough of Neath Port Talbot, South Wales, I collected a large wasp that looked somewhat reminiscent of an aculeate wasp (e.g. sphecid wasp). Using the book Solitary Wasps (Naturalists’ Handbooks) by Peter F. Yeo and Sarah A. Corbet as struggling to find a match, I soon discovered that this wasn’t a solitary wasp but instead some kind of parasitic wasp. Somewhat stumped as to what it could be, with it looking nothing like any parasitic wasp I had seen before, I turned to the power of the Twittersphere for answers. Posting a picture of the wasp on Twitter, I soon received several responses confirming the wasp’s identity as Pseudogonalos hahni – an apparently rarely collected wasp. And then the interesting bit began! Through some research, I discovered the incredibly strange biology of the species and thought I’d just have to share this with you invertebrate lovers.
Trigonalidae is a small family of parasitic wasps comprising little over 100 species worldwide. They are largely found in the tropics and subtropics, and only one species (P. hahni) occurs in Europe. Almost all trigonalid species are hyperparasitoids (parasites of parasites), attacking ichneumon wasps (Ichneumonidae) and tachinid flies (Tachinidae) that parasitise leaf-feeding lepidopteran or symphytan (sawfly) larvae.
The lifecycle begins with the trigonalid female, which presumably after mating, lays thousands of eggs on the leaves of herbaceous and woody plants during their short lifespan (believed to be 8 days at most). The microscopic eggs in turn are by chance ingested by leaf-feeding lepidopteran or symphytan larvae. This method of host ingestion is rather unusual amongst parasitoids and only evident in some tachinid flies. Once inside the herbivorous host, the trigonalid larva waits for the host to become parasitised (if it hasn’t already been parasitised). The trigonalid larva can stay viable within the herbivorous host for several months waiting for the host to be parasitised.
Once the herbivorous host is parasitised, the trigonalid larva searches within the body of the host for the ichneumonid or tachinid parasitoid larva, after which it develops as a hyperparasitoid. So essentially, trigonalids such as P. hahni need an herbivorous host (lepidopteran or sawfly larvae) to be parasitised by a primary parasitoid (ichneumon wasp or tachinid fly) before it can complete its development. How crazy is that! The likelihood of this sequence of events occurring to result in an adult trigonalid seems slim. The high fecundity of the female trigonalid (laying thousands of eggs) and the wide host spectrum, however, ensure that at least some progeny will reach adulthood every year.
The lifecycle of P. hahni centres around July in Northen Europe, coinciding with my sighting of the wasp on 20th July 2018. So next July, keep an eye out for large black wasps and you too could find this rarely collected wasp.
For further information on Pseudogonalos hahni, check out this recent paper available for download (FREE) at https://journal.fi/entomolfennica/article/view/71220/33017.