Meloe proscarabaeus

This surprising recent addition to the Sherwood Forest NNR species list, was discovered at a site within the NNR by Adrian Dutton and his wife on April 2nd 2011 and the colony seems to be increasing, judging by the total of 61 adults found recently on March 23rd 2012.
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Nationally , the UK's four species of Oil Beetle are in decline, with Meloe proscarabaeus presently described as vulnerable. The decline is thought to be directly linked to a similar decline in many bee species. How long this beetle has been within the NNR is unknown, but it must clearly must have been present for several years at least. Oil Beetles cannot fly, so any population may have been carried in by bees from several miles away. With the beetle's large size (up to 40mm for some females) it is hard to imagine they had previously gone unnoticed. 2011 survey results and notes In 2011 we made several attempts to locate this beetle and were eventually successful on our third visit on April 12th. We found ten beetles in one very small area of the site, followed by another female well away from this, later found to be the same location as the original record.
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The following afternoon, a further survey revealed that the colony covered a slightly larger area than was first realised. A count of 33 was made, consisting of slightly more females than males. Several other adults were found trodden on, but the colony seems to be reasonably well populated, with a later survey on April 20th, revealing 37 adults (21f and 16m) and also showing evidence of colony dispersal east and west, but still remaining within a very small area of the site. It was interesting to note that during the morning, both males and females were found grazing on grass, which on very warm days, was often found to be in areas of shade. Fescue grasses were largely ignored by feeding adults, the beetles preferring slightly coarser grass. By April 29th, a further survey of the area found a total of ten adults, all of which were females. Many of these were still burrowing in sandy/stony sections of the preferred path, but there was again, further evidence of continued spread eastwards.
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An unusual life history Oil Beetles have interesting life histories. The larvae are parasites of ground nesting solitary bees, near to which the female excavates a burrow. The female lays up to 1000 eggs that hatch to coincide with numbers of ground nesting solitary bees. What was also especially significant, was the presence of large numbers of Andrena cineraria, possibly the host species for the Oil Beetle larvae (Triangulins). After hatching some eggs in captivity after about two weeks, it seems possible that female beetles emerge and lay eggs around two weeks before the peak numbers of this solitary bee, as there had been little evidence of solitary bee activity along this path in mid-April when the first females were found.
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The Triangulins are extremely active, climbing flowers and waiting for the host bee to come along, but they will attach themselves to hoverflies, butterflies and the wrong type of bee. If they are successful, they attach themsleves to the host bee and are taken back to the bee's burrow, where the Triangulin changes into a grub-like larva which then feeds on the pollen stores and eggs of its host. Pupation occurs within the burrow, before the adult beetle emerges the following Spring.
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Burrow excavation and egg-laying This sequence of photographs shows the burrowing process observed in April 2011. Burrowing was often a lengthy process which most often occurred during the afternoon, with the excavating, egg laying and back-filling taking around two hours to complete. The burrow is excavated in a suitable location near to the nests of ground nesting soilitary bees. Burrowing seemed to commence from late morning onwards and by mid-afternoon, several females could be present along one small stretch of path, actively burrowing within feet of each other.
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When the burrow is complete, females reverse back out and turn around before reversing back down the burrow again. The depth of the burrows we witnessed being excavated, was found to be about 1cm deeper than the length of each female. Particularly large females could go down as far as 6cm.
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The female lays around 1000 bright yellow eggs at the bottom of the excavated burrow. The eggs shown in the following two photographs were taken after two burrows were found opened, presumably by some other invertebrate or even another female Oil Beetle digging an easy burrow in what whas pretty compacted sandy soil..
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Backfilling was quicker than the actual burrow excavation. The process involved the female turning around in the burrow and dragging the loose soil into the burrow with her front legs.
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On finding several burrows opened with the eggs exposed, we took some back home with the aim of photographing the hatched Triangulins a year later (the length of time given by some internet resources). However, we were surprised to find that they hatched just over two weeks after being laid, being yellow on hatching but turning more orange after a few hours. The following photographs were taken on May 2nd 2011 after which the 1.5mm long Triangulins were released back on site. At the time of release, it was obvious that there had been a large emergence of the bee Andrena cineraria (coinciding perfectly with the two week hatching of the Triangulins) with many males and several females present in the exact same area where several female Oil Beetles were still continuing to burrow and lay eggs. It seems very likely that A. cineraria is indeed the host species for this splendid beetle.
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Meloidae
The Oil Beetle Meloe proscarabaeus at Sherwood Forest NNR
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