Worcestershire Record No. 17 November 2004 pp. 21-23

ACULEATE HYMENOPTERA

Geoff Trevis 

There has been a steady accumulation of records for the last few years and patterns begin to emerge concerning species distribution. The following observations are based on my own database and undoubtedly a more statistically significant picture will result when more information is available. There are no real surprises to date except, possibly, the relatively few records of nationally rare and uncommon species. Worcestershire’s westerly, hence mild, position on the boundary between northern and southern species should produce a rich hymenoptera fauna but, perhaps, we are too far north-west for the southern species and too far south for the northern ones! Nonetheless, a wide range of species is present, with many more to be discovered through systematic recording. 

I am uncertain of the exact number if species on the BRC database but I have 158 on mine. Certainly the work by Michael Archer and Colin Plant at Devil’s Spittleful and Rifle Range and at Hartlebury Common and by Steven Falk at Kemerton Lakes has substantially extended the county list but these records are not included here as they have either been published elsewhere or will be the subject of future publications. Clearly, Michael Archer has demonstrated that the heaths in the north of the county are the premier hymenoptera sites. However, I have included the majority of records from my database, i.e. those species identified by myself, in table 1 in the hope that some conclusions may be reached, whilst table 2 shows the species from three different and species richer sites. I must stress that these lists in table 2 are not the complete lists for the sites but represent only the species sent to me for confirmation or identification. 

The county list shows 83 species of bees and 57 of wasps, a ratio of approximately 1.5:1, whilst the specific site records show 32 species of bees and 10 of wasps giving a ratio of 3.2:1. Can any conclusions be drawn from this? It has to be said that Habberley Valley has been subjected to only one visit (reported elsewhere in this volume of the Record) whilst the other two sites have been subjected to sporadic collecting over a few years and thus it is difficult to reach any meaningful conclusions. Habberley Valley and Little Comberton have a mix of bees and wasps whilst no wasps at all have been referred to me from Rough Hill Orchard. Does this mean that orchards are deficient in wasp species? Definitely not. The majority of sphecid wasps on my database have come from malaise traps in the orchards at Bowcastle Farm, suggesting that orchards are, in fact, rich in solitary wasps. The Bowcastle Farm survey, being part of a much larger study, will be published elsewhere later and cannot be analysed here. Also, work that hopefully will be published later, from data at Kemerton Lakes, a site close to the base of Bredon Hill, shows a paucity of wasps compared to bees making it more akin to Habberley Valley or Little Comberton. 

The site differences must arise from two sources. First, the number of records from each individual site is small and hence subject to statistical bias. Second, and I think the major factor, is the method of surveying. The three sites listed have all been surveyed by netting specimens, either by sweeping or by specific capture. The specimens collected depend on the human eye either seeing a flying insect or selecting it from a sweep net. Not unexpectedly, this is likely to yield larger and/or more brightly coloured species i.e. the larger bees and the black and yellow wasps. Very small black sphecids are likely to be ignored or not even seen. malaise traps, however, are totally unbiased, hence the large proportion of small wasps collected. This indicates that we are not obtaining a real picture of Worcestershire hymenoptera because:

a. Apart from the northern heaths, the site recording has been spasmodic and unsystematic. 

b. The method used to capture specimens is biased towards larger and more brightly coloured species.

 c. The number of recorders is small so that species distributions will tend to mirror the wanderings of recorders rather than the true distribution of species.

In addition to affecting the apparent distribution of species, the above factors also introduce bias into the assignment of rarity indices i.e. RDB and nationally notable designations. The county list shows the species currently listed in these categories. However, the wide distribution of Philanthus triangulum or Lasius brunneus suggest that they should be down graded. Similarly, national work on Nomada lathburiana and the frequency with which I have found Sphecodes monilicornis also indicate that these should be down graded. No doubt other species will be in a similar situation. Of course, changes in climate are also affecting many species that until recently were rare and often restricted to the south and east but that are now spreading north and west (Philanthus triangulum being a classic example). 

In summary, significant progress is being made with hymenoptera recording in Worcestershire but the difficulties of collecting a representative suite of species are formidable. When out collecting, particularly by sweeping vegetation, a special watch needs to kept for small specids etc.. 

Acknowledgements: I am indebted to Harry Green, John Meiklejohn and others for referring specimens to me and to Mick Blythe for separating even the smallest hymenoptera from the Bowcastle Farm malaise traps and sending them via Rosemary Winnall. Their efforts have extended the county database in a way that would have been impossible for me individually.