Caterpillar parasitoids in cocoons photographed in England
by Stephen Hewitt | Published 4 February 2022
In September 2021 I put a normal-looking caterpillar into a glass jar, intending to photograph it. What happened afterwards was unexpected. The photographs here show what was in the jar later: clusters of yellow cocoons and a dead caterpillar.
There was a photograph of similar yellow cocoons with a similar caterpillar on a website selling stock photographs in February 2022. It was labelled: “A small white butterfly, Pieris rapae, caterpillar parasitised by a parasitoid wasp, Cotesia glomerata. with the wasp cocoons formed”, at https://www.alamy.com/parasitoid-wasp-cotesia-glomerata-cocoons-from-a-parasitized-small-image60417489.html.
At the same time a page on the Natural History Museum website stated: “Some wasps can use hosts that are a lot larger than they are, laying an entire batch of eggs inside the body of one caterpillar. A species of wasp called Cotesia glomerata does this inside the bodies of large white caterpillars.” 
A book of butterflies published in England in 1973 stated in its entry for the large white: “Among the caterpillars preparing for pupation there are always a number which are killed by the parasitic larvae of the Braconid wasp (Apanteles glomeratus L.). Instead of turning into a pupa, the caterpillar dies, surrounded by the yellow cocoons of the parasites which have been feeding inside its body.” 
In fact, the large white (Pieris brassicae) is not the only caterpillar used by Cotesia glomerata, according to more than one research paper on the web in February 2022. The small white (Pieris rapae) is also a host for it, at least.
One of those papers stated: “The small white butterfly Pieris rapae (L.) is a major pest of vegetable Brassica spp. in New Zealand. It first appeared in New Zealand in 1929-1930 and was without larval parasitoids until Cotesia glomerata (L.) (Hymenoptera: Braconidae) was introduced successfully”... 
- A web page with the heading ‘Body snatchers: eaten alive by parasitic wasps’ at https://www.nhm.ac.uk/discover/body-snatchers-eaten-alive.html in February 2022.
First sentence in Field Evaluation of Cotesia rubecula (Hymenoptera: Braconidae), an Introduced Parasitoid of Pieris rapae (Lepidoptera: Pieridae) in New Zealand P. J. Cameron, G. P. Walker Environmental Entomology, Volume 31, Issue 2, 1 April 2002.
The quotation above is from the version on the web in February 2022 at https://academic.oup.com/ee/article/31/2/367/347896 This page included a link to the paper as a PDF (SHA256 fdf040c505f78c204a09457ee6d9cba7b60f41aca0ec41649f532a2a89b122f1)
- Page 50, A colour guide to familiar Butterflies Caterpillars and Chrysalides, Josef Moucha, illustrated by Bohumil Vančura, Octopus Books Limited, London, 1983, ISBN 0 7064 1975 8
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