The Oil Beetle Meloe proscarabaeus at Sherwood Forest NNR
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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 our own survey results since. Nationally , the UK's four species of Oil Beetle are in decline, with Meloe proscarabaeus presently described as vulnerable.
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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 that they had previously gone unnoticed.

2014 survey results and notes

With the mild Winter continuing and a gradual warming of the daily temperatures, the first Oil Beetles were found to have already emerged at their Budby South Forest site by 03/03/14, over a month earlier than in 2013.

 
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We were actually surprised to find any, and in fact were just checking the state of the site in readiness for the beetles possibly emerging in a couple of weeks time. So we were amazed to record our first Oil Beetle within a few minutes of searching, eventually recording a total of 19 Oil Beetles, all of which proved to be males. Just under a week later (09/03/14), a further survey resulted in the first females being recorded, but only seven females out of an excellent total of 54 adults.

2013 survey results and notes

Delayed by the very cold weather during March, when we did make one site visit around mid-month, the first adults were not out until 06/04/13, when 25 (17 males and eight females) were recorded during a thorough survey of the western end of the site. A single female found in a new area, was well away from the main colony and the first real sign of range expansion across the site.

The following day (07/04/13) we conducted a second thorough site survey, this time recording approximately 70 adults. The count consisted of 34 males and 26 females, but we were unable to locate Oil Beetles in any additional areas of the site. Most of the beetles recorded were grazing and included several mating pairs. A later site visit was made on 20/04/13, resulting in a count of 41 adults. Burrowing females, completed burrows containing eggs and mating pairs were observed.

On 22/04/13 we received news of the discovery of Meloe proscarabeus by James Glendenning, at a second Nottinghamshire site. Following a short survey of the site the following day, we located a total of five males and four females. Unfortunately, these were the only live beetles we found and the dead numbered over 25. At first we were not going to publish the location of the find, but we now feel that it is OK to mention that the site is located in a very public area of Newstead Abbey grounds. It therefore seems logical to believe that Oil Beetles may also be present at additional Nottinghamshire sites.

2012 survey results and notes

We made just two site visits during 2012, recording totals of 61 adults on 23/03/12, followed by 47 adults on 05/04/12. There was evidence (through records) of a slight increase in range, but this continues to be extremely localised on site. Heavy machinery used in managing the site, or for the movement of Longhorn Cattle, was believed by one observer to have had an extremely detrimental affect on the burrows made by females.

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.

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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