This is only the second oil beetle I’ve ever found. Meloe proscarabaeus, the black oil beetle is a giant among British insects, up to 45 mm long and a great tub of a beast. Partly distinguished by her swollen abdomen, she’s full of eggs, up to a thousand of them according to the literature. She was half way into digging her egg burrow at the edge of the path down to Woody Bay, at St Lawrence, Isle of Wight last week. I nearly squeaked.
It’s usually a given, in insects, that it is the adult stage which does the flying off to start new colonies after the larva does all the eating and growing, but she can’t fly; not only is she weighed down with all her eggs, she doesn’t have any wings anyway. In oil beetles, this maxim is thrown out of the window, and it is the larvae which to the dispersing. The adult beetle is little more than a barely mobile egg storage facility. Here’s a link to the Buglife oil beetle page which goes into the life cycle of these curious insects. In brief, the tiny but very active larvae (triungulins) hatch from the batch of eggs, scurry up plant stems and congregate on flowers waiting for a soil-nesting solitary bee to visit. They then grip tight to legs, feet, antennae, hair, whatever they can, to hitch a lift back to the bee’s nest where they invade the brood chambers, eating bee grubs and whatever nectar/pollen food stores the bee has laid up in there.
This is, as you can imagine, a rather precarious hit-and-miss strategy. The success rate of the the thousand or so triungulins is probably around the 0.1 per cent mark. So a ‘thriving’ oil beetle colony can only exist where there are bounteous bees nesting. And like all parasitoids it does not take much of a dip in host numbers to render them impossible to find for the budding brood thieves waiting atop flowers. The adult beetles are very short-lived, and only about for a few weeks in early spring; this is a good time to find the bees busily nesting, as long as the weather holds and the bees can forage, as long as the triunglins can hang on in there long enough, and don’t get washed out, or eaten whilst they wait.
Consequently, all UK oil beetle species are in trouble and even the ‘common’ ones like proscarabaeus are decidedly scarce and declining. Two specimens I’ve seen, 20 years apart — that’s a fair indication of how rare they are.