'Too Close to the Brink !'

Dark Bordered Beauty clings on at Strensall 2019

Summary Peter Mayhew

A big thank you to everyone involved with the monitoring this year: Richard Baker, Yves Bouvet, Terry Crawford, Alastair Fitter, Kiera Horner, Penny Relf and Hannah Sanders.

Transect counts

2019 was in some respects the worst year for Dark Bordered Beauty at Strensall since systematic monitoring began, which was a big shock after the progress ofthe last three years. We had the lowest ever total moths (just 15, 12% of the 2018 count), the shortest ever flight season (eight days, 35% that of 2018), the fourth worst peak count (nine) and fourth worst mean count. Moths were only seen in the three most reliable sections, but two of those sections (7 and 8) produced just three moths between them. The weather during the monitoring period was not so favourable as in the previous few years but we had sufficient good conditions,quantity of walks, and other time spent in the right areas to know that numbers had seriously dropped since 2018. Dark Bordered Beauty is therefore once again in trouble in the locations where it was only a decade ago doing best at Strensall

Creeping Willow

For the fifth year, Creeping Willow was monitored in and around some of the small grazing exclosures, 26 in total. In the original 9 plots established in 2015, plants are now 15cm taller than they were in 2015 on average whilst control plants around them are 5cm shorter on average. Across all 26 cages in 2019, plants inside were on average 10cm taller than those outside cages. As we find in every year, there were winners and losers in different plots, but there was no significant change in height overall inside cages since 2018, although plants outside cages shrank by 3cm on average, which we interpret to be due to sheep grazing because of the physical presence of sheep and abundant evidence of nipped stems and shoots. Stem density inside exclosures showed no significant change from last year, with again some winners and losers. The very important plants in transect section 3, where most moths are found, had a mixed year, but nearly all of the plants in the larger cages which were pot-grown have died, and have not provided any useful moth habitat. A fire on 5th May on section 6 of the transect did not touch the main Dark Bordered Beauty areas, but did burn all above-ground plants in one of the cages. There has however been remarkable regrowth of Creeping Willow in this cage from surviving rootstock. The burned area was very attractive to sheep as there was strong regrowth of Molinia grass, and this was kept closely cropped, which probably spared some of the willows elsewhere (especially in section 7 where sheep no longer congregated), but nonetheless grazing damage to plants outside the cages was pretty ubiquitous. In 2019 there was very little damage to Creeping Willow from other insects: Small Chocolate-tip moth spinnings, which had been very abundant in 2018, declined by 2/3rds, and no Poplar Leaf Beetles were seen during monitoring, and no other caterpillars. Almost all of the suitable (i.e. tall, lush) habitat for Dark Bordered Beauty is inside grazing exclosures, which covers very little area. Our anecdotal impression, though our monitoring provides no firm data on this, is that stem density outside the grazing exclosures continues to decline, because it is getting harder to find control plants to monitor outside cages

Conclusions and recommendations

Given the relative abundance of adult moths in 2018, the reduction in 2019 seems likely to be due to high juvenile mortality. The cause is unknown, but the habitat (host-plant quality and quantity) changes this year do not seem to reflect the large decreases in moth abundance, and so is likely to have involved other factors beyond human control. Numbers in 2018 seemed to have reached what the small amount of existing quality habitat could support and perhaps a drop (either from other density-dependent or density-independent causes) was likely. Whatever the reasons, this is a timely reminder that the amount of good quality habitat on the northern part of the Common remains very small, and probably insufficient for the long-term persistence of the moth. We note that other insect herbivores (e.g. Small Chocolate-tip moth) were less abundant on Creeping Willow this year, perhaps for the same underlying reasons. Probably the most immediate action that can be taken is to protect more of the Creeping Willows around the most critical areas for the moth. Plants in sections 7 and 8 seem fairly-well protected at present, so a sensible focus would be to redistribute some of the existing large cages in section 3 around established willows that are currently prevented from growing taller by sheep grazing. This author retains the opinion that much reduced sheep grazing on the northern part of the Common is necessary to restore the habitat to the quality of 2009 or before, which seems the only way to restore the Strensall Dark Bordered Beauty population to that necessary to source introductions to other sites successfully. The current strategy of pro- tecting small plots clearly works to some extent but is not a sufficient or long-term solution. Recruitment of new Creeping Willow plants remains an issue. There have been some successes here, as well as failures, and the successes need to be built on if possible, by growing more plant stock and planting out in more favourable times or locations, perhaps with some “weeding” protection whilst they establish. Ultimately of course, “gardening” is not a viable long-term solution either and management activities need to be established that allow sufficient recruitment without special intervention.

Peter Mayhew