Since publication of the Lepidoptera Atlases in 2002-2004 for Herefordshire and Worcestershire by Michael Harper and myself there have been a lot of new species recorded and significant changes in the status of a considerable number of previously recorded species.
In Worcestershire there have been 15 new species of the larger moths (Macros) and 28 new species of the smaller moths (Micros), and significant increase in records for 23 Macros and 25 Micros. Some of the new records have been migrants, mostly in the remarkable migrant year of 2006; others have been species which have suddenly started to increase their range and/or become much commoner in the U.K. This I feel must be because of increasing temperatures; especially the continuing increase in average winter temperatures. Some migrants have been able to start and maintain resident populations.
It is necessary to make allowance for the huge increase in recording effort over the past 5-10 years in the county. There has been a big increase in recorders, mostly just recording the larger moths in light-traps but also some recorders now becoming increasingly interested and expert in the smaller moths. This has resulted in a much better overall coverage of our fauna, and no doubt accounts for some of the new species recorded. However a lot of changes have been seen by this recording group over the past five years, especially in terms of increase in status and range , and therefore I am sure most of these changes are genuine and not just recording bias.
The increase in recorders may be one reason however for the increase in migrant records of such species as Scarce Bordered Straw Helicoverpa armigera, Bordered Straw Helicoverpa peltigera, and Small Mottled Willow Spodoptera exigua, but also the wonderful migration in the hot summer of 2006 certainly must have skewed the figures (there were over 100 records of armigera where before there had been less than 10) and caused some other rarer species to be recorded ( including a Striped Hawkmoth Hyles livornica larva found in the car park of the Fox and Hounds in Lulsley by Cherry Greenway).
The availability of pheromone lures for the Clearwings has resulted in a big increase in records of these previously difficult to record species. It is now apparent that Red-belted Synanthedon tipuliformis, Currant S. tipuliformis, and Yellow-legged S. vespifomis are fairly common and widespread, and Red-tipped Clearwing S. formicaeformis has now been recorded at two sites in south Worcestershire
Macromoth species which have shown the biggest increase in numbers and range. These include the species of the “Footman” family whose larvae feed on Lichens and Algae on trees which have dramatically increased over the past six years. The Orange Footman Eilema sororcula was last recorded in 1904; the Buff Footman Eilema depressa had only a few records in the county; and the Red-necked Footman Atolmis rubricollis was only an occasional migrant; but since 2002 they have become fairly common and widespread.
The Scarlet Tiger Callimorpha dominula, The Pine Hawkmoth Hyloicus pinastri, The Dotted Chestnut Conistra rubiginea, and the Sycamore Acronicta aceris for example have moved into the county and become much more widespread.
There have been some species which have been introduced to the U.K. in foodplants (e.g. some of the Conifer feeding species), or arrived otherwise, probably in human transport such as lorries, and then have spread rapidly from their point of origin (e.g. the Horse Chestnut Miner Cameraria ohridella.) If the continuing warming trend continues then I am sure we will see other species arriving and spreading.
Conversely there have been some 20 species of Macromoth which seem to have declined in numbers and range, or of which there are no recent records. The reasons for this decline are usually uncertain, but some cases are due to loss or degradation of habitat (e.g. some grassland and open habitat specialist species).