Every day on the way back from work I walk next to this wall. The other day there were plenty of cannibal Harlequin ladybird larvae eating prepupae. Today there were many more pupae and a few prepupae. I have no idea how I noticed this tiny fly on the head of a prepupa. The ladybird pupa shook its body back and forth to no avail. Later I identified the fly as a scuttle fly, genus Phalacrotophora. Some species of this genus are endoparasites of ladybird pupae. The fly mounts guard on a prepupa and when it pupates it lays some eggs underneath. The fly larvae on hatching parasitise the ladybird and when fully developed they emerge and pupate on the ground. A common hypothesis on the rapid spread of invasive species is the "enemy release" hypothesis. This states that the invaders in the new range lack specific enemies - pathogens, parasites or predators and that this allowes uncheckered population growth. The success of the harlequin ladybird has been hypothesized to depend at least in part on escape from natural enemies. Recent studies indicate that generalist ladybird parasites might be starting to attack this ladybird in the invaded range and this includes pathogenic fungi, and endoparasitic nematode worms, wasps (Dinocampus coccinellae and Oomyzus scaposus) and flies. Prevalence can be quite high, with up to 33% of specimens in Danish samples infected with nematodes, but, on the other hand, lower fitness from the parasitoid Dinocampus reared from Harlequins, suggest than some of these enemies have yet to adapt to this invasive ladybird. If you live in the U.K. there is a survey you can take part into, by collecting ladybird pupae and rearing them, and then reporting what comes out of them (see the Ladybird Parasite Survey website).
Koyama, S., & Majerus, M. (2007). Interactions between the parasitoid wasp Dinocampus coccinellae and two species of coccinellid from Japan and Britain BioControl, 53 (1), 253-264 DOI: 10.1007/s10526-007-9138-5
Kenis, M., Roy, H., Zindel, R., & Majerus, M. (2007). Current and potential management strategies against Harmonia axyridis BioControl, 53 (1), 235-252 DOI: 10.1007/s10526-007-9136-7
Durska, E., Ceryngier, P., & Disney, H.L. (2003). Phalacrotophora beuki (Diptera: Phoridae), a parasitoid of ladybird pupae (Coleooptera: Coccinellidae) European Journal of Entomology, 100, 627-630 Other: 1210-5759