Holly Blue (Celastrina argiolus)
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© Eddie John
The Holly Blue is more wide-ranging than other blues and does not form discrete populations. It occurs wherever trees and shrubs grow where its larvae feed on the flowers and berries of Holly (Ilex aquifolium), Ivy (Hedera helix), Dogwood (Cornus sanguineus) and other species. It is currently expanding its range in Britain. (For further details on this species see http://www.butterfly-conservation.org/).
Status details :
Status since 1976 is Stable with a increase of 43%
Status over the last 20 years is Rapid decline with a decrease of -61.7%
Status over the last 10 years is Stable with a decrease of -30.7%
Log collated index plot
This chart shows the index of abundance (LCI = Log Collated Index) over time. It shows fluctuations in
populations from year to year, and is scaled so that the average index over the whole series is equal to 2
(horizontal line). For greater detail about how this index is derived, click on the green question mark above.
Trend description :
This is our second most widespread species of blue butterfly, recorded on over a thousand different transects since 1976. The all-sites collated index shows well the strongly cyclical pattern of periods of abundance followed by periods of scarcity that is very characteristic of this particular butterfly, and thought to be driven by its host-specific parasite the ichneumonid wasp Listrodromus nycthemerus. Though the long-term trend is positive it is not significant, though significance is harder to achieve when fluctuations are so great.
This map shows the distribution between 1995 and 2016. Data is derived from the Butterflies for the New Millenium dataset via the NBN Gateway
This chart shows the average number of butterflies seen on transects between Arpil and October across all sites (fitted values from a Generalised Additive Model). The blue line gives average counts over the full BMS series (1976 to date) and the red line gives the average for the last year.
This map shows symbols for the mean abundance at transect sites, with the colour of the symbol reflecting the level of abundance. Means are over all years. Grey background squares are the occupied cells as shown by the Butterflies for the New Millenium over the previous ten year period.
In total, Holly Blue has been recorded from 738 transects in the Butterfly Monitoring Scheme. Of these, annual indices of abundance have been calculated from 805 sites, with an average index of 5 individuals per site.
For 402 of these sites, Holly Blue has been recorded well enough to calculate annual indices of abundance in more years, allowing trends to be calculated.
In 2016, 2163 individuals were recorded from 371 sites, producing annual indices at 282 of these.
Links to counts
This map shows the trend in abundance at particular transect sites for which data has been received within the last five years. Trends (increasing, declining or stable) are assessed at sites where the species has more than five years of annual index data. Use the option boxes below to view plots for individual sites.
Links to lists of sites with particular statuses
|Colonised sites (29)|
|Extinct sites (7)|
|Stable sites (470)|
|Increasing sites (31)|
|Declining sites (33)|