Tag Archives: natural enemies

The Curious Case of the Shark-finned Aphid

The large (giant) willow aphid, Tuberlolachnus salignus, is, in my opinion, one of the world’s greatest unsolved mysteries.  This aphid is sometimes regarded as being the largest aphid in the world.  It can reach a length of 5 mm, can weigh up to 13 mg as an adult and the new-born nymphs weigh about 0.25 mg (Hargreaves & Llewellyn, 1978).  You can get an idea of how big it is from the picture below.

willow aphid on finger

http://www.rothamsted.ac.uk/PressReleases.php?PRID=100

This is pretty big for an aphid, although not quite as big as one of my former PhD students (Tilly Collins) liked to pretend!  The picture below used to appear on her website and was the envy of a number of Texan entomologists.  Tuberolachnus salignus, as you might expect, since it feeds through the bark and not on leaves, has rather a long set of stylets, up to  1.8 mm, more than a third of it’s body length (Mittler, 1957).

tilly on aphid

This picture emphasises the first mystery: what is the function of the dorsal tubercle, which so closely resembles a rose thorn, or to me, a shark’s fin.  Nobody knows.  Is it defensive? Unlikely, since T. salignus being a willow feeder is stuffed full of nasty chemicals and very few predators seem to want, or be able to feed on it.  They feed in large aggregations on the stems of their willow tree hosts and can have serious effects on tree growth (Collins et al., 2001).  As the aphids produce a lot of honeydew, they are often ant-attended  (Collins & Leather, 2002) and these also deter potential predators.  In fact the aphid colonies produce so much honeydew in the summer that they attract huge numbers of vespid wasps that are in search of energy-rich sugar sources at that time of year.  These too are likely to make potential predators and parasitoids think twice about approaching the aphids.

Tuberolachnus

Photograph courtesy Dr Tilly Collins

The wasps also cause a problem for researchers and when Tilly was doing her PhD, she used to have to confine her fieldwork to those times of day when the wasps were not around.   In addition, if you crush one of the aphids you will discover that it stains your fingers bright orange and that this stain will last several days if you don’t try too hard to wash it off.  If you get this aphid ‘blood’ on your clothes they will be permanently marked and Tilly used to say that she ought to be paid an extra clothing allowance.

Tuberolachnus salignus, is as far as we can tell, anholocyclic, no males have been recorded and no matter how hard people have tried to induce the formation of males and sexual females, they have not been successful.  This is however, not the second mystery.  The mystery is that every year, in about February, it does a disappearing act and for about four months its whereabouts remain a mystery (Collins et al., 2001).  So we have an aphid that spends a substantial period of the year feeding on willow trees without leaves and then in the spring when most aphids are hatching from their eggs to take advantage of the spring flush, T. salignus disappears!  Does it go underground?  If so, what plant is it feeding on and why leave the willows when their sap is rising and soluble nitrogen is readily available?

So here is a challenge for all entomological detectives out there.  What is the function of the dorsal tubercle and where does T. salignus go for the spring break?

Truly a remarkable aphid and two mysteries that I would dearly love to know the answers to and yet another reason why I love aphids so much.

Collins, C.M. & Leather, S.R. (2002) Ant-mediated dispersal of the black willow aphid Pterocomma salicis L.; does the ant Lasius niger L. judge aphid-host quality. Ecological Entomology, 27, 238-241. http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2311.2002.00390.x/full

Collins, C. M., Rosado, R. G. & Leather, S. R. (2001). The impact of the aphids Tuberloachnus salignus and Pterocomma salicis on willow trees. Annals of Applied Biology 138, 133-140 http://onlinelibrary.wiley.com/doi/10.1111/j.1744-7348.2001.tb00095.x/abstract.

Hargreaves, C. E. M. & Llewellyn, M. (1978). The ecological energetics of the willow aphid, Tuberolachnus salignus:the influence of aphid Journal of Animal Ecology, 47, 605-613. http://www.jstor.org/discover/10.2307/3804?uid=3738032&uid=2&uid=4&sid=21101920521473

Mittler, T. E. (1957). Studies on the feeding and nutrition of Tuberolachnus salignus (Gmehn) (Homoptera, Aphididae). I. The uptake of phloem sap. Journal of  Experimental Biology, 34, 334-341  http://jeb.biologists.org/content/34/3/334.full.pdf

Other resources

http://influentialpoints.com/Gallery/Tuberolachnus_salignus.htm

http://www.nhm.ac.uk/nature-online/life/insects-spiders/common-bugs/aphid-watch/

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Silk- not just a spider thing

Mention silk and most people will, I guess, immediately think of spiders and cobwebs.

Pressed a bit further, some may mention silkworms, and some might even know the word sericulture and that the common silkworm feeds on mulberry bushes.   What they may not know, is that the silk worm is the larvae of the moth Bombyx mori and that there are actually four species of lepidopteran larvae commonly used in silk production.  These are pictured below in the lovely illustration from Meyers Konversations-Lexikon; next to the picture are some B. mori larvae.

Silkworm larvae Silkworms

Meyers Konversations-Lexikon, 4th Auflage, Band 14, Seite 826a (4th ed., Vol. 14, p.826a)

Four of the most important domesticated silk moths. Top to bottom: Bombyx mori, Hyalophora cecropia, Antheraea pernyi, Samia cynthia. From Meyers Konversations-Lexikon (1885-1892

Silk production is of course not just a feature of spiders and lepidoptera.  It is a widespread feature of insect life, being used for pupal cases, as a mode of transport (ballooning) as shown by larvae of the gypsy moth and other species of Lepidoptera,

ballooning gypsy moth            ballooning gypsy moth drawing

protective cases as in larval caddis flies or also, by some caddis fly larvae, as fishing equipment.

 caddisfly_larva  Caddis fly net

But in my opinion, the most dramatic use of silk is that seen in a genus of micro-moths, belonging to the Yponomeutidae, the small ermine moths, Yponomeuta.  They and their relatives, are silk-producers extraordinaire.  Collectively, they are known as small ermine moths; so called because of their adult colouration which resembles the ermine worn by nobility and small, because of the existence of several larger moths with ermine in their names.

Yponomeuta_evonymellus

http://commons.wikimedia.org/wiki/File:Yponomeuta_evonymella-02_(xndr).jpg#file

The larvae are less attractive and are the web/silk producers.

Yponomeuta_evonymella_caterpillars

http://commons.wikimedia.org/wiki/File:Yponomeuta.evonymella.caterpillars.jpg

My particular favourite is the bird cherry ermine moth, and not just because the bird cherry is my favourite tree.  (My eldest son’s middle name is bird cherry, albeit in Finnish). The adult moths lay their eggs in August, in clusters of up to 100 or so on young twigs of the bird cherry Prunus padus, cover them with an egg shield and then die (Leather, 1986).  The eggs hatch shortly afterwards and the larvae spend the winter under the egg shield until the following spring.  When the buds begin to burst in spring, the larvae emerge from beneath the shield and begin to feed gregariously on the newly emerging leaves, spinning a web that protects them from natural enemies  and may also help in thermoregulation and as a trail indicator (Kalkowski, 1958)  http://edepot.wur.nl/201846 .  It is possible to have great fun by selecting a lead larvae to act as a trail blazer and watch the rest of the colony follow them to a destination you have chosen.

Every three to four years or so, populations of the moths get so high that they exhaust their food supplies, defoliating entire trees and covering  them with a tough coating of silky white webbing (Leather, 1986; Leather & Mackenzie, 1994).  In fact, in Finland, I once saw three neighbouring trees totally enveloped in a silken tent caused by the bird cherry ermine moth, Yponomeuta evonymellus, that you could enter and shelter inside from the rain.  Once they really get going as spring progresses, the landscape, particularly if in an area where bird cherry is common, begins to take on a somewhat wintry look, which for May is a little odd.  Those of who you, who have travelled north of Perth in Scotland, on the A9, will be familiar with this phenomenon.  It frequently makes the Scottish newspapers and generates headlines such as “winter wonderland” or “ghostly landscape”. As they run out of trees, the larvae begin to migrate in a desperate search for trees with leaves still on them, and by now, have become less fussy about what they eat.  It is at this wandering stage of their life that the true extent

Yponomeuta webbing  bird cherry emrine moth webbing

of their singlemindedness (I have seen a trail of thousands of larvae marching along a railway line; they didn’t survive the passing of the 0850 from Helsinki) and their ability to produce silk becomes startlingly apparent.

Ermine moths on car    Ermine_moth_larva_on_a_Swedish_army_bike

http://commons.wikimedia.org/wiki/File:Ermine_moth_larva_on_a_Swedish_army_bike.jpg

Truly, silk is not just a spider thing.

Kalkowski, W. (1958). Investigations on territorial orientation during ontogenic development in Hyponomeuta. Folia Biol Krakow 6: 79-102.

Leather, S. R. & Mackenzie, G. A. (1994). Factors affecting the population development of the bird cherry ermine moth, Yponomeuta evonymella L. The Entomologist 113: 86-105.

Leather, S. R. (1986). Insects on bird cherry I The bird cherry ermine moth, Yponomeuta evonymellus(L.). Entomologist’s Gazette 37: 209-213.

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