Friday, 29 April 2011

Scale insects

Sooty beech scale insect (Ultracoelostoma spp.)
On a visit to Ashley Gorge, Oxford, New Zealand,we were having a quiet relaxing bush walk through  the native forest but by the time we got back to the car we were covered in sooty mould, especially my 10 year old son!!.


 Figure 1: Beech trees (Nothofagus spp.) at the lookout at Ashley Gorge, New Zealand.

My sons had asked all sorts of questions regarding the black mess covering the trees, some of which I could answer and others not. This I decided would be a good subject for a blog.
The sooty mould fungus (Capnodium spp) lives off the honeydew produced by the sooty beech scale insect (Ultracoelostoma assimile (Maskell) and Ultracoelostoma brittini). There are 11 families in New Zealand, most are host specific. The sooty beech scale (Ultracoelostoma spp.) is in the order Hemiptera. There appears to be confusion over which family they belong; in Duncan et al. (2007) it is in the Coelostomiddiidae family and in Wardhaugh & Didham (2006), it is the Margarodidae family. Perhaps someone can clarify this for me? Scientific names aside it turns out to be a fascinating insect. How many insects do you know that spend the majority of its time with its head stuck in a tree and its rear end protruding out (I’m trying to be polite) with a long tube secreting honeydew, the waste product or poo)(fig.3)?

Figure 2; the beech in the background is black from sooty mould compared to the foreground understory, Ashley Gorge.

There are three distinct types; armoured scales, mealy bugs and soft scales. The soft scales’ are the sooty beech scale and tend to be found at the trunk and lower branches regions of the tree living between the bark crevices. They are a sap sucking insect, feeding on phloem (sap) of the tree  

Figure 3; Sooty beech scale insects on a beech tree showing the filamentous anal tube protruding , the white thread  hanging down from the bark. Glistening in the light at the ends of the threads’ was a drop of honeydew (arrow at the top of picture).NB. I can't get the arrow to show but top right of the picture.
Life history (fig.4).
 Each stage of development of the larval form is called an instar. After the egg has hatched, the first instar crawls out of the test to the outside. From this stage the larvae is called a crawler. The test is a hard waxy case developing over and encompassing the mother forming a barrier from the outside world.  Once the crawler is in place, a sticky substance released by glands under the epidermis layer forms the test. This hardens and additional layers are added with each moult. Scientists are able to determine the age by number of the number of casts against the test. However the anal tube or filament extends out beyond the case and is the only opening. The cast is the old skin that has been shed as the larvae out grows its skin and replaced with a bigger one.
The sooty beech scale female has 4 instars. However the last instar it still resembles the larva. This is known as an incomplete metamorphosis. The male has 5 instars and does complete a full metamorphosis taking on a different shape as an adult. The female also has a non terminal instar developing reproductive features of an adult. This is called Neoteny.
Female; the first instar is approximately 1 mm long. Once it has found a suitable crevice in the bark, it inserts its’ stylet and spends the rest of its life fixed to the tree. Once the stylet is attached, the anal tube elongates out of the tree as along filament. Waste products are passed out the tube well away from the body. The waste product is called honeydew and is sweet tasting (We did the taste test, yum) being high in sugars from undigested tree sap.
 The second instar has grown to approximately 1.6 mm in length. The outer test is hard and a reduction in leg length, antennae are shortened and the eyes are reduced. General shape is changing and becoming more spherical. The end cap has darkened and appears black close to the bark.
 The third instar stages, they are 2-4 mm diameter now, tight against the test. There is further reduction of legs, eyes & antennae. The anal area has become thickened with a raised ring around the anus.
 The fourth instar still resembles the larval stage. This is the adult stage but with non functional mouthparts and no legs and it no longer produces honeydew. Eggs are laid within the test then the female shrivels and dies. The crawlers leave through the anal hole.

Male; the male has the same first and second instar stages but remains active in the third. The fourth stage it pupates and is a winged adult at the fifth and final stage. The adult is approximately 3-4 mm and is a reddish pink colour. The male flies to the females in the trees to fertilise the eggs. It has no mouth parts and will live only to reproduce then die.

Figure 4; Life history of sooty beech scale (Morales et al 1988).

The sooty beech scale is a keystone species, i.e. without the scale insects the ecosystem disappears. Honeydew is an essential nectar source for insects including wasps, bees and ants, for native birds such as tui, and bellbirds and also for the sooty mould fungi.
An unexpected by product of the honeydew is exported honey by ‘Airborne’. This is exported to Europe and Germany as honeydew honey. The nectar has special qualities with the presence of oligiosaccharides (complex sugars), higher levels than are present in flowers. It has higher other sugars (maltose, erlose & melezitose) but lower in sugars (glucose & fructose) that cause honey to crystallize. It is high in polyphenolics (antioxidants) & high in Glucose oxidase (antibacterial properties). Another words medicinally healthy honey.
 What a good life a female sooty beech scale has. As much food as you can eat, warm environment with no frosts, snow or rain and what’s even better the male comes to you. Bliss.
I hope you enjoyed my ‘walk’ through the bush. My boys did. They can’t wait to tell all their friends at school they ate poo in the holidays!!
References & Links
Astwood, K., Lee, B. & Manley-Harris. M.,(1998). Oligosaccharides in New Zealand Honeydew Honey, Journal of Agricultural and  Food Chemistry, 46(12), 4958-4962.
Airborne honey web site (1999)..;
Morales, C.F., Hill, M.G. & Walker, A.K. (1988). Life history of the sooty beech scale (Ultracoelostoma assimile) (Maskell), (Hemiptera: Margarodidae) in New Zealand Nothofagus forests. New Zealand Entomologist, 11; 24-37.
Morales, C.F. (1991). Margarodidae (Insecta: Hemiptera). Fauna of New Zealand 21, 124 pages.
Wardhaugh, C.W. & Didham,  R.K. (2006). Establishment success of sooty beech scale insects, Ultracoelostoma spp., on different host tree species in New Zealand. Journal of insect Science, 6: (29) 1536-2442.


  1. Have you tried the honey-dew? I think there's a place in Woodend where you can buy it

  2. Ben: Did you come on the ecol202 field trip last year? I was sure you were in that class? We tried some at the beach forest (In Craigyburn I think). Its only a wee drop per insect, and tastes like sugary water.

    JENNAAAAAY: Will the sooty mould spread to leaves of the tree and disrupt the photosynthesis or is it restricted to where the honey dew is??

  3. Hi Ben, Yes we have tried it. As I mentioned above I showed my boys what to taste hence they have gone to school saying they had insect poo over the holidays!! lol

  4. Hi Jade, Yes alot of sooty mould can affect the plant however the fungi tends to be in specicfc areas rather than the whole plant so they do adjust. If excess honeydew falls onto an understory bush then the honeydew will spread and cover those patches but it will not grow on the plant itself.

  5. It really sad when you walk around the beech forest in the Craigieburn area in the summer there are so many wasps. I also think that a lot of people don't really care about insects in general. And why on earth would one want to try something when you know what end it comes out of ? ;-)

  6. R these found in the united states?