With around 6,000 species parasitic wasps make up by far the greatest proportion of Hymenoptera in Britain and they remain sorely under-researched, not least because some are incredibly small, less than one millimetre in length, and collection and identification are tricky in the extreme. One of the smaller species, a striking ant mimic Gelis species, is shown just below. This one measures 2mm.
Some of these parasites are extremely useful to humans, notably the Brachonid wasp Spathius exarator which can remove up to 90% of a Woodworm (Anobium punctatum) infestation in any property. Others significantly reduce numbers of pest species in crops, including aphids, so clearly they deserve a vote of thanks.
Ironically though, because there was insufficient information, none of this large group were included in the Red Data Book published in 1987 aimed at detailing our vulnerable invertebrates. Clearly if any of the hosts are vulnerable or endangered, which in a sample this size they are bound to be, the parasites are likely to be in the same parlous position. More information would be of great benefit in devising management policies to maintain this part of our biodiversity. After all, tiny species merit equal consideration with large ones.
Even one family containing a good proportion of pretty obvious species in both colour and size, the Ichneumonidae or Ichneumon wasps, was not in the Red Data Book. Be that as it may, a selection of Ichneumons, not all of which are identified to genus, are shown on this page. My thanks to Dr Gavin Broad of the Natural History Museum for his encouragement in trying to get to grips with these handsome and fascinating insects.
Adult Ichneumons, which tend to have long antennae with up to 40 segments in extreme cases, are not carnivores, gaining their sustenance from such as nectar and honeydew. Their larvae certainly are carnivorous though and predictably, given their numbers and dramatic variation in size, they use as hosts a huge range of invertebrates from spiders to Wood Wasps (Urocerus gigas) via Lepidoptera.
Arguably the most striking Ichneumons visually and behaviourally - not least because with perseverance they can be watched in action - are females with an ovipositor as long as the rest of the insect or longer. These devices, with all the penetration of a long hypodermic needle, are used with astonishing precision to insert eggs in the larvae or pupae of the host. The latter are located accurately by use of scent and sensory perception in the legs and the Ichneumon then can take up to half an hour applying her ovipositor to penetrate the wood, find the larva and lay an egg on it. Once seen, this action involving an astonishing degree of precision is never forgotten.
This is seen in the image of Dolichomitus imperator (55mm including the ovipositor), which deals with weevil larvae, while the impressive Rhyssa persuasoria (up to 75mm) goes for Wood Wasps or longhorn beetles. Ephialtes manifestator (up to 60mm) targets solitary wasps such as Ancistrocerus and Trypoxylon species, and possibly Ectemnius, while the smaller Perithous scurra (40mm) parasitises various wood and stem-nesting solitary wasps including Pemphredon species.
A hefty number of Ichneumons, and all the Ichneumoninae sub-family, parasitise moth or butterfly species, laying their eggs in larvae or pupae; adult emergence is always from the pupa. None of these need the long ovipositors required by wood and stem-boring Ichneumons. The evident decline in moth numbers over the last 20 years is a cause for concern for the parasites as well as the hosts.
Many Ichneumoninae hibernate as adults, including Pimpla instigator (16mm), one of our commonest, which deals with a variety of species including Gipsy Moth (Lymantria dispar) and Large White butterfly (Pieris brassicae) larvae. A female can lay up to 150 eggs in a single caterpillar. Ichneumon stramentarius (12mm) and Ichneumon sarcitorius (14mm) are also seen fairly frequently seen, with the latter visiting flowering plants and parasitising moth caterpillars.
Ambylteles armatorius is even larger at up to 16mm, a similar size to Ichneumon xanthorius. The latter is very similar in appearance to Ichneumon sarcitorius, showing what a nightmare proper identification of this family can be even with the larger species. That includes Ichneumon suspiciosus (12mm), which seemingly has markings dashing enough to set it apart from all its kin. Ischnus inquisitorius (12mm) is an ectoparasitoid of tortricid pupae, but little is known about the prey of Stenichneumon culpator (15mm) or Polytribax perspicillator (15mm).
One handsome and decidedly elongate species is Heteropelma amictum (17mm), which seems to use Geometridae and Tortricidae moth caterpillars as hosts. It is quite common and tends to be seen in flight much more than settled.
A handful of species are active in the winter including Ophion obscuratus (13mm), which is nocturnal. Together with Ophion luteus they are attracted to light and in the winter of 2006-7 several turned up in my kitchen during the evening. They are from the only sub-family of Ichneumons that can sting so care needs to be taken. The prey are larvae of Noctuid moths, as is the case with Ctenichneumon panzeri (15mm), the male of which is really striking in appearance.
One species which is host specific, and can have a massive impact on populations of the prey, is Listrodromus nycthemerus (11mm). This targets Holly Blue butterfly caterpillars, laying its egg in first instar larvae. This results in the emergence of a single adult wasp from the normally formed Holly Blue pupa. Parasitism of larvae can reach 99%, understandably causing a massive collapse in host populations. Listrodromus nycthemerus can take six or seven years to reach peak numbers, which together with its effectiveness as a parasite helps explain why the Holly Blue is subject to noticeable 'booms and busts'.
The thousands of other parasitic wasps include the Chalcidoidea, which tend to be tiny and are often known as chalcids. There are more than 1,000 species in Europe including a good number of Pteromalid wasps (3mm). They are predominantly parasitoids, affecting a wide range of insect groups. A few are parasitoids of the larvae of gall wasps, so can emerge from galls, but they are not the causers. For that, see below.
The effects of some Hymenoptera are much more evident than the insects themselves, Gall Wasps (Cynipidae) being a case in point. Most of these species, including the Oak Apple Gall Wasp (Biorhiza pallida), Marble Gall Wasp (Andricus kollari), Cherry Gall Wasp (Cynips quercusfolii) and Spangle Gall Wasp (Neuroterus quercusbaccarum), are pretty small, up to 5mm, but the effects of their labours on Oaks (Quercus spp) in particular are far from tiny.
Females lay eggs and secrete a fluid, and the combination leads to the development of the gall. These can be seen virtually all year and may be formed on roots, bark, twigs, leaves or fruits (acorns).
Marble Galls (maximum diameter 20mm) form in clusters which turn from green to brown as they mature; the adult wasps emerge in the autumn. Cherry Galls (20mm) are found on the underside of Oak leaves from late summer onwards, while Oak Apples (35mm) are evident on twigs or branches in spring. The much smaller Spangle Gall (5mm) is seen in bulk on the underside of leaves.
Perhaps the most spectacular of all the galls is the so-called Robin's Pincushion caused by the wasp Diplolepis rosae (4mm). The gall, seen in summer and early autumn, is green to begin with then turns red. It is constructed on leaves or stems.
Images © Jeremy Early. All rights reserved.
In 2013 I published My Side of the Fence - the Natural History of a Surrey Garden. Details may be found, and orders placed, via this hyperlink My Side of the Fence. In November 2015 Surrey Wildlife Trust published the atlas Soldierflies, their allies and Conopidae of Surrey, jointly written by David Baldock and me. Details are on this web page: Atlas.