I’ve not noticed this before, but it seems to have been a pretty good year for Large White caterpillars and therefore presumably for their predators. I was confused as to what was going on at first, what the little yellow “eggs” were and why the caterpillar hadn’t pupated.
After a bit of research I discovered that the “eggs” are not eggs, but pupae cases themselves, for Cotesia glomerata, a small wasp that parasitises the caterpillar. The parent wasp injects its eggs into the caterpillar and when the larvae hatch they “take over the mind” of the caterpillar. Having had it take them to somewhere “safe” to pupate, they eat their way out of the caterpillar and it even defends them from predators until it finally expires.
Quite horrifying in a way, but I often feel that Large Whites particularly don’t deserve a good end 😀 Even more astonishing perhaps is the fact that the wasp in question is also parasitised by two additional wasps.
