Monday, 6 June 2016

An invasion of the Diamond-back Moth

May 2016 in Newhaven felt decidedly average when considered from a butterfly and moth perspective - until after the 29th. The final two days of the month saw an explosion in numbers of a tiny migrant micro-moth species: the Diamond-back Moth Plutella xylostella.

Individual Diamond-backs began to appear in my garden moth trap this year from 6th May and continued in this manner every few days until 23rd, when they disappeared for a week. Then, on 30th, I saw twenty. The most I had ever recorded during one night before was 64, so twenty individuals gained my attention as it was a comparatively noteworthy number. The nights following this produced the following numbers:

31st May: 175 individuals
1st June: 138 - after which I expected the spike in numbers to gradually regress back to the mean
2nd June: 382
3rd June: 437
4th June: 1740
5th June: 411
6th June: 360

To place this in some form of perspective, I had recorded a total of 881 Diamond-backs during the previous ten years. This was by no means a high number compared with many other coastal stations, but it is the number which I had recorded at home in Newhaven, and that figure is therefore a useful datum for comparison. In five nights from 30th May that total was comprehensively eclipsed and I recorded 1152 individuals; this total was itself eclipsed on the night of 4th, bringing the total in six nights to 2892! This total did not include the few hundred Diamond-backs seen resting and possibly nectaring on a flowering Cordyline in my front garden on 2nd June; it did not include the 2000+ individuals (a conservative estimate) which I saw on the evening of 3rd while walking from home over the apex of the Downs along the east of the Ouse Valley to Bishopstone - a distance of just over one mile as the crow flies; it did not include many hundreds more seen on 4th along a well-known spot for migrants on the Downs above Denton and Poverty Bottom. It was at this latter location that the scale of this invasion occurred to me. How does one count a tiny micro-moth flitting about here there and everywhere in such great numbers that, for every one seen, another ten - another thousand? - pass unseen? Add to this a carpet of buttercups which spread before me across 45-odd acres in a lurid nebula and the task appeared even less attainable. Counting the buttercups would have been less of a challenge. I gave up. I simply enjoyed the spectacle before me.

Diamond-back Moth Plutella xylostella
So what is the Diamond-back Moth? It is a member of the Plutellidae family of moths. There are seven species found within the UK, but only three of these are found in the south east and all of these have been recorded at Newhaven: the Diamond-back, the common Grey-streaked Smudge (Plutella porrectella) and less common Bittercress Smudge (Eidophasia messingiella). Each of these species feeds on plants of the cabbage family. The Diamond-back is a small species: about 8mm in length at rest. It is a neat-looking insect, with a long-thin body around which the wings are folded roof-like when at rest. It has an alert upright resting posture and its antennae are forward pointing, much like its migratory attitude. This moth is a visitor from the continent each year and it is seen in variable numbers from one year to the next. In observing the flight of this species, it is a remarkable thought that they make it across any stretch of water, but this they do, presumably at high altitude, before making landfall in the UK.

Being based on the south coast at Newhaven and only about seventy miles from continental Europe (the French coast between Fecamp and Dieppe is nearest), migrant insect species are both a staple of life every year for the Lepidopterist and a fascinating area of study. The Ouse Valley and Downs to the east are a fairly reputable station for migrant sightings. The Willowherb Hawk-moth (Prosperpinus prosperpina) Oleander Hawk-moth (Daphnis nerii) sightings of 1985 and 2014 respectively are probably the best examples, but getting on for forty migratory moth species have been recorded in Newhaven during the past ten years and these have included occasional sightings of Crimson Speckled (Utetheisa pulchella), regular small numbers of Olive-tree Pearl (Palpita vitrealis), Vestal (Rhodometra sacraria), Gem (Nycterosea obstipata), Ni Moth (Trichoplusia ni), and very good numbers annually of Bloxworth Snout (Hypena obsitalis), which, along with several other migrant species, has begun to colonise along the coastline. Silver Y (Autographa gamma), Red Admiral (Vanessa atalanta) and Diamond-back are the most commonly encountered on a yearly basis, but migrant species are well known for their episodic spikes in abundance during specific continental immigration events. One famous recent example was seen in 2008 when many millions of Painted Lady (Vanessa cardui) butterflies migrated northwards from the Atlas Mountains of North Africa as far as the Arctic, the generations leap-frogging each other as they progressed. 

Most of these and the other migratory species are seen during periods of strong winds from the south, but immigration events occasionally occur from other directions and the current Diamond-back invasion appears to be one such example of an immigration from the east or north east. The spike in Diamond-back numbers which began during the final two days of May appears to be the greatest immigration event by that species and it might yet prove to be amongst the most significant of migratory episodes observed during the past 170 years, when formal recording began. Bearing in mind that these moths will breed following their arrival, we could be in store for a second bonanza later in the summer.

Diamond-back Moth at Castle Hill LNR, Newhaven

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