Wednesday, 10 September 2014

Large Yellow Underwing - Noctua pronuba

One of the most common visitors to the light trap I have put out in the garden over the past month has been the Large Yellow Under-wing. One morning I counted 25 hiding among the egg boxes, all clinging on determedly, and difficult to displace. 



According to the UK Butterfly Conservation in the report "The State of Britains Larger Moths 2013" the incidence of N. pronuba has increased by 186% during the period of 1968-2007 (1). The distribution map from the NBN gateway shows how widely distributed it is.



This is a fairly large moth with a wing span up to 60mm. At rest it's not the most spectacular of moths, with a base colour of brown, and few distinguishing markings besides the kidney spots on its wings. However when it flies there's a flash of colour as the forewings spread out and expose the bright orange/yellow colours of the rear wings. As soon as it comes to rest the rear wings are folded away and hidden once more. They rest during the day, but will flash their bright hindwings if disturbed. This display may be designed to surprise and scare-off predators by the sudden flash of colour.


Moth migration has thrown up a number of interesting questions relating to the navigation challenges that they face during different climactic conditions. It has long been proposed that moths navigate according to the position of he moon. However, what about periods when there is cloud cover and the moon is not visible, a situation not uncommon in the UK. Research has shown that moths are able to use the earths magnetic field as a navigation guide. 
In one of the studies moths were placed into a cage on an overcast night, and reversed the earth's magnetic field (3). When this happened the moths realigned themselves according to the new position of the magnetic field.  How insects and animals are able to determine magnetic variations amazes me - mainly because I just don't know how it's done.

Another study by Chapman et al (4) showed that moths are able to select favourable high-altitude winds to help them travel the often long distances involved in moth migrations. The study used entomological radar (no idea what this is!)  tans found that moths were able to increase the distance travelled by up to 40%. Not a bad trick some a small thing that livess for less than a year.


The larval stage of the Large Yellow Underwing Moth is one of the pests known as a cutworm. E.A. Bowles mentioned their destructive habits in My Garden in Spring, the first in his trilogy of books about his garden in Middlesex. He suggested hunting for them by lamplight to keep them from damaging "the tender, juicy buds" of his beloved early-flowering Irises. Accidental introduction of N. pronuba into the US may constitute a major pest threat to commercial crops (5). The larvae develop between September and April above ground feeding on the stems and leaves of grasses and other plants, sometimes grazing them off at ground level. When disturbed the caterpillar adopts a 'c'-shape and this one was about 35mm in length, but they can be up to 50mm long.




References:
1: Fox, R. et al. (2013) The State of Britain's Larger Moths 2013. Butterfly Conservation. http://butterfly-conservation.org/files/1.state-of-britains-larger-moths-2013-report.pdf
2: NBN gateway. https://data.nbn.org.uk/Taxa/NBNSYS0000006193/Grid_Map. Accessed 2014-09-10.
3: Baker, R.R. And Maher, J. G. (1982). Magnetic compass sense in the large yellow underwing moth, Noctua pronuba. Animal Behaviour. 30(2): 543-8. 
4: Chapman,J.W. Et al (2010) flight orientation behaviours promote optimal migration trajectories in high-flying insects. Science. 327(5966): 682-5
5: Bechinski,E.J., Smith,L.J. And Merickel,F.W. (2009) Large Yellow Underwing: A new cutworm in Idaho. university of Idaho. http://www.cals.uidaho.edu/edcomm/pdf/CIS/CIS1172.pdf.