‘Parasite’ is the term used for a creature that lives on or in another ‘host’, but doesn’t actually kill it. A ‘parasitoid’ is something whose presence does kill, or at least prevents the reproduction of its host. Parasitoid insects are a very varied group that reproduce by laying their eggs inside another animal and as their young develop, they eat away at the insides of their host and kill it before emerging as an adult wasp or fly. There are thousands of different species, many of which are seeking out very specific hosts. In our samples we regularly find these parasitoids and continue to be astonished at their diversity, beauty and size.
In particular, the Chalcid wasps (Hymenoptera; Chalcidoidea) are incredible. You have probably never knowingly seen one, but there are over 1500 species in Britain and probably well over 100,000 species worldwide. They include some of the most beautiful insects alive. Many have shining metallic bodies and bright red eyes, or they may be yellow with very long, sting-like ovipositors, while others may have strange antler like antennae. But for all their beauty and variety they are so small that they are easily overlooked. We use different sized sieves to sort through the specimens that we collect and the final sieve has a mesh of only 1mm, but most of the Chalcids go through those holes. They include the tiniest of all insects, the fairy flies, which may only be 0.25mm long!
Like many hymenoptera (wasps, ants and bees), adult Chalcids feed on nectar, or sometimes on the fluids from animal wounds. But the larvae of most species are parasitoids of other insects. There is a parasitoid Chalcid wasp for almost every kind of insect, as well as various ticks and mites, which means that these tiny wasps feed on many species that are considered as pests. It is actually possible to buy tubes of Chalcid wasps to release into greenhouses, to protect them from specific caterpillars that might eat the crops.
As well as finding a range of adult Chalcids, we have recently also been seeing evidence of their gruesome life histories. One particular fly grub had an unusual appearance under the microscope. By shining a light from underneath we could see that, instead of a fly ready to emerge from within, the larval skin contained a fully formed adult wasp. The parent wasp would have injected an egg into the fly grub when it was small and as it grew, the wasp’s own grub would have hatched and begun to eat away at the inside of its host. Once it had eaten all but the outer skin, the wasp grub would change into the adult, ready to break out and fly off.
Another fly, which this time had pupated and was ready to turn into the adult, was brought to my attention. On closer inspection, inside the fly, inside the pupal case were no fewer than 36 tiny wasps that had all been developing as the maggot grew and turned into a pupa.
In case it sounds like the flies are all victimised by parasitoids, there are several groups of flies that feed in exactly the same way. The ‘big headed flies’ (Diptera; Pipunculidae) lay their eggs inside leaf and plant hoppers (Hemiptera; Auchenorrhyncha). They are very distinctive when found in our samples due to their enormous eyes, which completely cover their heads.
On another occasion I was asked to identify an unusual aphid, with a large curved body. It turned out to be the skin of an aphid, within which was a large wasp grub. There are a whole series of parasitoid wasps that feed on aphids. They lay their eggs inside them, turning the dying carcases into “mummies”. In our samples these are usually seen as a round, dead aphid with a single, neat, circular hole where the wasp has emerged.
Some larger parasitoids can be observed during the summer months if you look closely. When undertaking the visual surveys of the meadow plots in July 2015, I counted a huge number of pollen beetles (Coleoptera; Nitidulidae) on the flowers. On closer inspection, small wasps could also occasionally be seen tapping around and inserting their long ovipositors into the flowers in search of beetle larvae. Pollen beetles can be a serious pest of Oilseed rape, so a parasitoid wasp is a useful ally for farmers protecting their crops.
One spectacular parasitoid was seen in the meadows at Goldington Green in Bedford. It was flying among one of the tall plots feeding on the flowers. It had long antennae, a large head and long, tripod-like rear legs, balanced by an equally long, upturned abdomen, at the end of which was a straight black ovipositor with a white tip. The ovipositor was longer than the entire length of the rest of the body. It was a parasitoid of solitary bees and wasps called Gasteruption.
Research has shown that Parasitoids struggle with the isolation caused by fragmenting the greenspace within an urban area (Denys & Schmidt 1998, Bennett & Gratton 2012). As the gradient from green, vegetated areas at the edge of a town changes towards sealed, man-made surfaces in the centre there are fewer flowers, with fewer insect species feeding on them. Some species can cope reasonably well with this increased isolation, but natural enemies of these insects may decline, the more they are separated from their main population. Alternatively, an increase in flowers, due to urban gardening can help to increase the numbers of parasitoids (Bennett & Gratton 2012).
These influences highlight one of the issues faced by reduced or fragmented urban green spaces. Some of the species from that habitat may re-establish and cope very well with living in an urban island, but these may be the pest species, like aphids. The delicate parasitoids that would keep them in check, may not be so mobile, or able to span ever increasing hard urban landscapes. A healthy ecosystem needs the two parallel life stories running side by side. An increase in floral sites, such as road verges or urban meadows might be a solution to encouraging these insect relationships that maintain the ecological balance. The parasitoid way of life may be the thing of nightmares, but they provide a significant ecological service. These beautiful murderers may be too tiny to see, but their impact can have consequences for all of us.
Bennett, A.B. & Gratton, C. (2012) Local and landscape scale variables impact parasitoid assemblages across an urbanization gradient. Landscape and Urban Planning 104, pp 26– 33
Denys, C. and Schmidt, H. (1998) Insect communities on experimental Mugwort (Artemisia vulgaris L.) plots along an urban gradient. Oecologia Vol. 113, No. 2 , pp. 269-277
Noyes, J.S. 2003. Universal Chalcidoidea Database. World Wide Web electronic publication. http://www.nhm.ac.uk/chalcidoids
J.P.Richards Aug 2015