Sunday, 3 July 2016

Butterflies and moths at Castle Hill LNR - early July

Transect walk season, week 14

We have reached the halfway point in the survey season and numbers remain disappointingly low, despite mostly clear skies during this week's walk. A Beaufort Force six breeze - possibly the strongest I have attempted to survey in - was blowing in off the Channel today, which made for challenging conditions. The wind was gusting to 38mph at times, according to the Met Office website! This and a modest temperature of 17°C conspired to keep many of the insects grounded and only three dishevelled, windblown individuals were seen along the exposed clifftop areas, each one sheltering in a lee of the undulant ground.

As expected, Meadow Brown and Marbled White are now the dominant species, but Large Skipper remains active in the scrubby areas and second brood Small Tortoiseshell and Comma butterflies are emerging. Amongst the moths, the first record at the nature reserve of White Plume Moth was made and a stunning Emperor Moth caterpillar was found munching on a bramble leaf.

Amongst the most conspicuous flowers currently in bloom is Dropwort, a patch of which can be found near to the pond, which itself has half-refilled with water following the heavy rainfall last week.

The open glade areas on the lower, north-facing slope are brimming with flowering grasses and the parasitic flower yellow rattle, which is helping to increase the diversity of flora in these areas. This plant takes some of its nourishment from other vigorous plants, mainly grasses, by tapping into the roots and therefore restricting their growth, enabling many less vigorous species, which would otherwise be out-competed, to gain a foothold. Each year the glade areas develop greater diversity because of plants like yellow rattle, which moves around the glades year on year in pursuit of those vigorous species. Only a few years ago these glades were dominated by willowherb species, but the subsequent diversity in plant species allows colonisation by insects, including butterflies, and this was seen last year with the appearance of numerous Small and Essex Skippers in these areas. There is hope that last winter's bizarre weather has not damaged these tentative colonies and that they will emerge in the coming months. It could take decades before we achieve the restoration of top quality low-nutrient grassland here, but the results of the past several years are encouraging. As always, thanks is given to the Council Rangers who manage the reserve and the various work groups who visit the reserve and manage the habitats there.

Other highlights seen during the walk were few, but a first site record of the common Tortoise Shieldbug was made near the caravans along the west clifftop and a diversion was made to observe a spear thistle alive with insects: thousands of black-fly aphids being predated by Seven-spot Ladybirds and a half-dozen of the impressive parasitic ichneumonid wasp Ichneumon sarcitorius. At first sight I thought these wasps were feeding on the black-fly, but closer scrutiny revealed that they were tapping into the thistle stem, presumably to take a drink. Other neighbouring thistles were absent of insects, so why this one? Does the gregarious nature of the aphids tend towards the mass colonisation of single plants? Are some individual plants more attractive than others? Are the aphids a keystone species which subsequently attracts other insects onto the plant? A transect walk properly made in observance of transect-walking rules does not allow for such distractions, so later observation of the wasps' behaviour may reveal further insights. It is both fascinating and delightful to see so much activity on a single plant - and in the midst of a force six wind!

Emperor Moth (Saturnia pavonia) larva on bramble
Tortoise Shieldbug (Eurygaster testudinaria)

Week 14 results: 11 butterfly and moth species, 36 individuals.

Butterflies (7 species)
57.009  Large Skipper (Ochlodes sylvanus)  10
59.003  Speckled Wood (Pararge aegeria)  1
59.010  Meadow Brown (Maniola jurtina)  23
59.012  Marbled White (Melanargia galathea)  10
59.023  Red Admiral (Vanessa atalanta)  2
59.027  Small Tortoiseshell (Aglais urticae)  2
59.031  Comma (Polygonia c-album)  1

Moths (5 species)1083  Marbled Orchard Tortrix (Hedya nubiferana)  1
1398  Rush Veneer (Nomophila noctuella)  1
1513  White Plume Moth (Pterophorus pentadactyla)  1 - first record at CHLNR
1643  Emperor Moth (Saturnia pavonia)  1 larva feeding on bramble

2441  Silver Y (Autographa gamma)  1

Other highlights
539  Tortoise Shieldbug (Eurygaster testudinaria)  1

1835  an ichneumon (parasitic) wasp (Ichneumon sarcitorius)  6

Here are a couple of further images of the Emperor Moth larva. It is about midway through its development and will pass through several developmental levels known as instars until mid- to late-August, at which time it will spin up a ginger-coloured cocoon and overwinter on the foodplant above ground level. It will then emerge as an adult next April or May and produce next year's offspring.

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