Butterfly Conservation - Upper Thames Branch - Black Hairstreak Report 2011

BUTTERFLY CONSERVATION UPPER THAMES BRANCH

Black Hairstreak Report 2011

Stuart Hodges

Photo © Dave Wilton

Last winter (2010/11) only one Black Hairstreak egg was found and none were found on the dedicated search at Finemere Wood for the first time in seven years of searching. However, Brown Hairstreak eggs were found here for the first time, which was very pleasing to all as it is firm proof that the large Brown Hairstreak colony centred on Bernwood is expanding. Only one Black Hairstreak pupa was found.

There was some speculation about the flight season being early this year. Clearly David Redhead had this in mind when he was looking at Blackthorn on the 24th of May and surprised us all by finding our first Black Hairstreak to be seen in May. In 2007 it was close, with the first seen on the 1st of June.

Black Hairstreak egg
Photo © Tony Croft

My thoughts as the flight season approached were that with the good weather very similar to that of the flight periods in both 2009 and 2010, we would see a similar increase in numbers. In 2009 we recorded 326 and in 2010 it was 1,095, an increase of 236%. So in 2011 we should have seen 3,679 in a similar time, but this was not the case and we only saw 582. I had previously formed the opinion that the weather in the previous year had a considerable influence on the numbers seen, this year proved that theory wrong.

Questions have arisen again as to how long an individual Black Hairstreak lives, with two freshly emerged adults seen on the 7th and another on the 8th and the last Black Hairstreak seen on 26th June (only single individuals seen after 18/6). This would suggest that they have a life of little more than 14 days.

In 2007 when we had 11 pupae being checked regularly to see when they hatched, the first adult was seen on 1st June and only one was seen after the 29th, the last three pupae hatched 1 on the 15th, 1 on the 16th and the last on the 17th, again indicating an adult life span of only 2 weeks. This brings me to a point that has recently been discussed: with such a short life, is nectar very important to their survival?

We only had 168 hours of suitable weather for butterflies to be active, (in 2010 it was 210 and 2009, 205). We had much more wind this year and only 3 days of ideal weather, at one site over 20 were seen on the 7th, but only 2 on the 8th in a similar time, on the 8th the wind was quite strong.

Yearly Comparison of Records
	1998	1999	2000	2001	2002	2003	2004	2005	2006	2007	2008	2009	2010	2011	Total
All time Total no. of Bucks & Oxon Colonies, some now extinct															98
Colony sites visited by the recording group		31	22	36	28	29	40	38	51	54	55	60	67	57	88
Colonies seen by the recording group 1998-2011	7	13	9	15	9	13	34	34	39	41	40	40	46	38	62
Colonies extant 1990 - 2011															74
Colonies found since 1998	2		1			2	2	1	6	4	3	1	4		26
Colonies re-discovered (seen pre 1990)	1	5	2	4	1	1	2	1		2			2		21
Individual Butterflies seen	19	28	73	90	123	109	525	258	995	655	261	326	1095	582
Hours Spent Looking for the B.H.				90.5	52.75	48.7	76	99	112.2	102	91	118	112	158
No. people helping with the recording	4	9	12	13	15	16	17	24	21	20	27	30	32	38
No. days with suitable weather for B.H. activity			16	18	18	21	22	28	28	25	18	30	26	31
No. hours of suitable weather for B.H. activity	40	90	105	149	151	167	150	176	234	140	115	205	210	168
Average no. seen per hour's observation	1.4	0.6	0.8	1	2.2	2.2	6.9	2.7	8.9	6.4	2.8	2.7	9.7	3.67	3.7
Kilometre Squares where B.H. were seen													60	52
New Kilometre Squares													8	2
Total Kilometre Squares with B.H. records															118

Above are the details of our fourteen years of recording. The M40 Compensation area has returned the highest number of records for the last three years, this year the average seen per hour's observation was 9.56 with an overall average of 3.7. The site was developed in 1990/91 because of the damage to butterfly habitats in the construction of the M40 motorway with the first Black Hairstreak being seen there in 1999. There is now speculation as to whether it will continue to be such a good site, or whether it will start to decline.

We are considering setting up a daily transect at one site in 2012 to try and get a better idea of both the length and the peak of the flight period.

Some of the observations from this year

Two very freshly emerged adults were seen, one close by the empty pupa case and another was observed taking salts from the ground. Several were seen leaning over to catch the sun and others were seen rubbing their wings together.

Another observation from one recorder who has been observing them for many years: "On 2nd June, I saw something which I'd never seen before for pruni - there were two clashing around some young ash trees and every so often they would land together on an Ash leaf, very close together, almost touching antennae and circling each other before taking off and repeating the whole procedure several times. Were these 'a couple' in courtship routine, or two males arguing over territory?"

The majority of our searching effort is in the central area of our region; we still need to put more effort into checking old sites on the extremities.

The set of records we now have, have provided information to prove the existence of the Black Hairstreak on three sites where there are plans for development. Should these planning applications be successful either the areas of blackthorn will be protected or mitigation areas will have to be provided, one area could be quite extensive. We've been able to pass on records to ecologists working on these planning applications and the work for this has benefited Butterfly Conservation funds by £130.

Many thanks to all who spent so much time looking for the Black Hairstreak, this year 158 hours, much longer than any previous year. Also thanks for keeping me up to date with your records, including sites visited.

Stuart Hodges
Black Hairstreak Champion
November 2011