[Taken from the Bulletin of the Amateur Entomologists' Society  59 (October 2000) (p182-184) and reproduced here with their permission.]

An unexpected attack by the parasites!

by Reg Fry (2333)

Most of us who enjoy breeding insects are sometimes disappointed, but not too surprised, when wild caught larvae, pupae and even ova yield one or more parasites when they should be changing to the next stage. All stages are at risk, as illustrated by the following specific examples from the butterflies, which I have copied here to set the scene for those who have recently taken up breeding insects (an extract from Dickson, 1992). "The White Admiral butterfly, Ladoga camilla, has a parasitic wasp of minute proportions which lays its eggs in the butterfly's egg deposited on the upper surface of a honeysuckle leaf. The parasite larva devours the contents of the egg and instead of a caterpillar emerging from the egg there issues a wasp. The same butterfly has another enemy, a wasp of the genus Meteorus (Braconidae) which lays its egg in the small larva soon after it hatches. This parasite spends the winter inside the host and in the spring, soon after the caterpillar starts to feed again, the parasite bores its way out of the dying caterpillar, spins a thread and from this it makes its cocoon. However the most devastating parasites for the breeder of butterflies are those chalcid wasps which lay their eggs on or in the soft pupa immediately after it has emerged from the last larval skin. The commonest of these is called Pteromalus puparum (L.). It is gregarious and often the female wasp can be seen sitting on the back of the fully grown caterpillar, waiting for it to change into a chrysalis"

When breeding insects in captivity we expect to be able to avoid any such problems after we have reared the first generation to the adult stage, particularly from the larger parasites. There is always a risk that the tiny chalcid wasps (some less than 2mm long) can get through small cracks in cages and coarse netting but usually we manage to keep both Hymenopterous and Dipterous parasites at bay.

This is not always the case however and I was reminded of other, less obvious, risks when I was captive breeding the "yellow" form of the Garden Tiger, Arctia caja a few years ago. On several occasions when checking that the larvae had successfully pupated I was surprised to find the brown puparia of one of the Tachinid flies alongside dead caja larvae. This has happened on a few occasions before with other livestock but whilst I made a mental note of it at the time, it was several months later that I decided to read up on the life history of the Tachinids to find out how these parasites managed to attack my livestock.

There is not a great deal of published information which is readily available on the parasitic Tachinids, although the AES publication A Dipterist's Handbook gives a broad description of the ways they attack Lepidoptera and other Orders (however I hope that the AES will be able to produce a more detailed leaflet on this topic in the next few months). Most parasitic Diptera do not possess the sort of ovipositor that allows them to pierce the skin of other insects and most Tachinids lay their eggs on or close to the larva they hope to parasitise; for example, Carcelia lucorum is known to lay eggs on the hairs of the Garden Tiger and the parasite larvae burrow through its skin when they hatch. However this can usually be ruled out for captive bred stock because the flies are relatively large and should not be able to gain entrance to well maintained cages.

How then were my caja larvae attacked? Well I was continuously breeding the larvae, feeding them on dock for most of the season and so-called "spring greens" from late autumn to spring. The parasite attacks occurred during the time that the larvae had been feeding on dock and the most likely source is from a species such as Pales pavida (Meigen) which lays thousands of eggs at random on foliage in the hope that a few of them will be eaten by larvae. These species (e.g. from the subfamily Goniinae) lay from 2000 to 6000 eggs which are viable for up to six weeks and are very small (less than 0.2mm); the eggs are ingested by the larvae as they consume the foodplant.

It is perhaps surprising that these occurrences are fairly rare even when breeding large numbers of larvae in captivity (unless you have found it to be otherwise!). In the case of the caja larvae, I was picking dock leaves from a small paddock almost as fast as they were growing to maturity so that there must have been a good chance that any eggs laid on the dock would be picked up with the foodplant. In addition when breeding such large numbers of larvae there is a much greater chance that every leaf will be consumed in it's entirety and hence any parasite eggs deposited would have a very better chance of being ingested than in the wild.

There is still a need to record parasites emerging from Lepidoptera and and even more so from other Orders (Shaw, 1997) and I regret now that I did not record how many flies (if more than one) had parasitised each larva and that I did not send the flies off for a positive identification. The AES has two advisors who welcome specimens for identification, providing accurate details are given of the host species - date of capture, location and foodplant. It is essential that you are accurate in establishing which insect host the parasite emerged from and that, for example, it did not turn up in your breeding cage from cocoons already on leaves or deep within a potted foodplant (send the remains of the host also if possible). Particular care should also be taken in packing the specimens to ensure they are not crushed or shaken to bits in the post and please remember to enclose a stamped addressed envelope for a reply. The current AES advisors are:

For Hymenoptera: Dr. Mark Shaw, Royal Scottish Museum, Department of Natural History, Chambers St., Edinburgh EH1 1JF

For Diptera: Tom Ford, 39 Ashbury Drive, Sheffield, Yorks S8 8LE.

References

Dickson, R. (Second Edition 1992). A Lepidopterist's Handbook. The Amateur Entomologist Volume 13.

Shaw, M. R. (1997), Rearing Parasitic Hymenoptera. The Amateur Entomologist Volume 25

Stubbs, A.E. & Chandler, P. (Second impression 1996). A Dipterist's Handbook. The Amateur Entomologist Volume 15. (All published by The Amateur Entomologist's Society).