Monday, October 10, 2011

Two Noctuidae Moths - The Square Spot Rustic and The Setaceous Hebrew Character

I am an amateur naturalist trying to discover everything living in my garden.

Some time ago I blogged (here and here for example) my homebuilt mothtrap. I set the trap out in my garden only a few times during 2009 and 2010. So great was the catch however that I'm still working through a backlog of photos of the species I caught (all critters were released alive after being photographed incidentally).
Two species from August 2009 I'm tolerably confident to have identified correctly (with the help of my copy of the Concise Guide to the Moths of GB and Ireland, by Townsend, Waring and Lewington) are a Square Spot Rustic (Xestia xanthographa) in photo 1 and a Setaceous Hebrew Character (Xestia c-nigrum) in photo 2.

The curious Latin species name 'c-nigrum' makes sense when you know nigrum means 'black in colour', hence literally 'with a black letter ''c''' [on its wings]. (Some of you may remember the 'white letter c' butterfly P. c-album I blogged here.)

I've entirely failed to uncover the meaning of the genus name Xestia. Can anyone comment?

Turning to the English common names of moths, I learnt previously that many were invented in the 1730's by the Aurelain society of naturalists.  I'd guess (but don't know) those of the moths here were amongst them.

I needed to look up setaceous. It means whiskery incidentally.

From the book mentioned above I learn that both my moths are common in the UK. Both overwinter as caterpillars. The caterpillars of the Square Spot Rustic commonly dine on grasses, and those of the Setaceous Hebrew Character on nettles and other herbaceous plants.

My attempt to learn a little more about my moths led me to some interesting papers by Chapman [1] and  Wood [2] and The papers describe the authors' efforts to track the migration of insects using ground-based radar. The papers are full of amazing details: I had not hitherto imagined that ground based radar would be so sensitive as to allow tracking of a single grasshopper in flight at a height of 1.5km. Further, as the authors explain, that around the globe millions of tons of insects are aloft at any instant, or that at least 2.3 billion(!) insects were involved in the migrations to/from the UK between 2002 and 2007.

What it is the insects (many moths, including mine, amongst them) are doing at heights of several hundred metres to a few kilometres is taking advantage of high wind speeds to propel them to places of seasonal migration. The speeds are several times greater than that at which an insect could fly unaided. The authors' studies yielded estimates that by harnessing winds insects may be able to travel as far as ~2000km during only three of four 8-hour flights. Things are not as simple as the insects being mere passive 'leaves in a storm' however. Rather, the authors discovered that they exhibit a clear directional sense, flying at an angle to the main wind direction so as to control where they end up. (I suppose an analogy would be a rowing boat in a strong ocean current. Just because the current may be faster than you could row, by sculling at an angle to the main current it's still possible to steer somewhat). How the insects are able to navigate at altitude and at night the papers don't say. I guess the moon may be involved, but that much about how they do so remains a mystery.

1. Flight orientation behaviors promote optimal migration trajectories in high flying insects, J.W. Chapman, R.L. Nesbit, L.E. Burgin, D.R. Reynolds, A.D. Smith, D. R. Middleton, J.K. Hill, Science 2010, 327, p.682-685

2. Flight periodicity and the vertical distribution of high altitude moth migration over southern Britain C.R. Wood, D.R. Reynolds, P.M. Wells, J.F. Barlow, I.P. Woidwod, J.W. Chapman, Bulletin of Entomological Research 99(05), p.525-535, 2009

Saturday, October 8, 2011

Cat's Ear Hypochaeris radicata

I am an amateur naturalist trying to discover everything living in my garden.

 The pretty flower in photo 1 is Cat’s Ear - a common weed in my garden.  There are a number of superficially similar British yellow-flowered weeds  including  the Hawkbits, Sow thistles and the Hawksbeards I blogged here.  From my copy of The Wild Flower Key (Rose) I’m fairly confident my plant is none of these, though I’m less confident it is definitively Cat’s Ear (Hypochaeris radicata) and not the rather similar Smooth Cat’s Ear (H. glabra). The book tells me that were my plant to be Smooth Cat’s  Ear , then its yellow ‘petals’ would be only twice as long as wide (mine seem longer). Also that the green stems of H.radicata should broaden as they approach the flower head (more correctly ‘the involucre bract’), which appears to be the case for my plant (see photo 2). On this basis I’m going with the identification H.radicata.

Readers of my blog will know I try to do a little research to uncover some point of interest for each lifeform I come across. In the case of Cat's Ear my searches led me to an interesting paper [1] by N. Hartemink The authors asked the question "What does a plant do if you cut the flower buds off?" (these are my words - I'm paraphrasing). A trite answer would be "Grow some more!". Pausing to consider things in more detail however one might begin to imagine more subtle possibilities. Consider a plant growing in a field subject to heavy grazing by animals. If the plant loses a flower one might imagine various responses. For a short lived annual plant, flowering and successfully setting seed in a season is an absolute imperative. One might conjecture that such plants could respond to flower-loss by "stepping up" efforts to produce more and more flowers therefore in the 'hope' that some escape the grazing (of course plants don't 'hope' and this is a poor anthropomorphic description - but, hey, I'm an amateur and it's good enough for me!). By contrast, for long lived perennial plants, flowering does not  have the same urgency. If such a plant has its flowers removed, might it simply 'cut its losses' therefore, put 'on hold' attempts to flower and instead put its energies into growing more leaves and roots?

To add to these considerations, one should remember also that a plant may not have a simple unfettered 'choice' about whether and how many flowers to grow. Resources (food, water and light etc.) are always finite and may impose further constraints on what types of structure (e.g. flowers vs. leaves) the plant is able to produce. A deeper consideration of this resourcing issue has led the experts to formulate such theories as "meristem allocation". (Meristems are specialised regions of a plant where growth 'happens'. Meristem tissue is found at the tips of roots and shoots for example. It is made up of special meristemic cells that are rapidly growing, dividing and transforming into new bits of root, shoot etc.).  I don't understand the 'meristem allocation' theory in detail, but briefly it seems to revolve around the idea that a mersitem at the tip of a plant shoot has a 'choice' to simple carry on creating more and more plant shoot and leaves, or cease making leaves and shoots and instead switch to making a flower. Once the meristem has switched to making a flower however, there is no going back - that meristem is committed and can't later go back to making shoot. In a sense the plant's meristems are like cash in a bank. The plant can keep the 'cash' (mersitems) in the bank growing 'interest' (more shoots and leaves) or 'withdraw' (allocate) some money (meristem) and 'spend it' on a flower. This might all seem rather abstract, but the point is that armed with the idea that meristems are a fundamental unit of 'currency' in the 'economics' of plant survival, botanists can start to construct quantitative and predictive (as opposed to merely descriptive) theories of how plants ought to respond to different environmental pressures (grazing, resource shortages etc.).

So, what does a Cat's Ear do when you cut the flowers off?! Well, the authors above took three plant types: Cat's Ear, Devil's Bit Scabious (Succisa pratensis) and Brown Knapweed (Centaurea jacea). They found marked differences in the reponses of these three to removal of flower buds. Both S. pratensis and C. japea responded to flower bud removal by increasing the number of flower buds. Plants of both species would typically make around 7 flower buds per plant if 'left alone', whilst those plants who had their flower buds removed would 'bounce back' and regrow about 12. Interestingly however, S. pratensis also responded by switching some of its energies away from flower bud growth into increasing growth of plant side 'shoots' (strictly side 'rosettes' of new leaves). Appropriately for this blog posting, Cat's Ear's response was the most dramatic all. Plants 'left alone' produced around 60 buds. The 'decapitated' however, bounced back with a dramatic 240. There is much more in the paper that I could talk about, but I've gone on enough here and will simply refer you to the original if you're interested. What I like about the work is it is true science and yet an experiment that any motivated amateur could repeat and extend: find a patch of weeds and a pair of secateurs and you're all set to become a published scientist.

[1]  Flexible life history responses to flower and rosette bud removal in three perennial herb, Nienke Hartemink, Eelke Jongejans, Hans De Kroon , Oikos 105: 159-167, 2004.