All is not well with the large white caterpillars (Pieris brassicae). A large number of the caterpillars did not pupate and seemed lethargic. The reason for this lethargy is now clear.
Many of the caterpillars have been parasitised by the Braconid Wasp (Cotesia glomerata L.). The pupae of these wasps are now breaking out of the bodies of the caterpillars – all of which are still alive.
There are roughly 20-30 parasites within each host and I’d estimate that more than 50% of the population of these caterpillars have been parasitised:
Reading Michael Chinery’s book “The Natural History of the Garden” (1977) he suggests that the density of parasitism on Pieris brassicae by Cotesia glomerata would be enough to make the large white extinct in the UK if it weren’t for the rate of migration of this species from Europe each year. I can’t help thinking that the populations would regulate each other before extinction occurs but, based on the evidence from my own garden, the rate of parasitism is indeed very high.
However, some seem to make it: