Black and Yellow

A colleague of mine (one Jenny Brooks, a.k.a Camp Mum) recently updated her blog with an informative report on scale insects, and their place in New Zealand Ecosystems. This has influenced my blog update this week, in which I present the effects of introduced insect species on Scale insects in New Zealand beech forests.

Firstly, I have a bit of background information for you. Scale insects (Ultracoelostoma assimile) are commonly found in Nothofagus forests across New Zealand. Female individuals bury their heads into tree trunks and branches, and process tree sap. A by-product of this sap consumption is the production of honeydew, which is then excreted by the insect via a long anal tube! (Figure 1.0 below)

Figure 1.0 - This picture shows the side of a Nothofagus beech tree. The white thread like structures are the anal tubes of the scale insects, the tip of which contains a droplet of honeydew. Note the mould on the tree trunk, which feeds of the sugars provided by the honeydew.

The honeydew produced by scale insects has an important place in New Zealand ecosystems. It is an important food source for various fungi and other insects. Also, it is a common source of nectar for many native birds such as Tui and Bellbirds.

Unfortunately all of this is under threat due to the effect of invasive insects. The bad guys in this case are the Common Wasp (Vespula vulgaris) and the German Wasp (Vespula germanica) (Figure 2.0 below). These species (introduced in 1978 and 1945 respectively), have been suspected of altering communties across the country, especially in beech forests.

Figure 2.0 - Spot the Difference! Common Wasp on the left, German Wasp on the right.

The main concern with these wasp species is that they have adapted a way of consuming far more than their fair share of honeydew. There are plenty of native wasps that also consume honeydew from scale insects, without causing such damage to ecosystems. "Why are Common and German Wasps such a problem then Tom?", I hear you asking. Well kids, this is why.
Both Vespula vulgaris and Vespula germanica have discovered that they can get a far more concentrated shot of sugar by chewing off the anal tube that stems the flow of honeydew. By consuming the sugary secretions in this manor, wasps can reduce the amount of honeydew that is available for nectar feeding birds and insects. Any honeydew that is not consumed by the wasp cannot be contained by the scale insect, so a lot of nutrients is lost to the forest floor.

A paper titled "Effect of introduced social wasps on the standing crop of honeydew in New Zealand beech forests" investigated what effect introduced wasps were having on the availability of honeydew for native animals. Their experiments looked at the sugar quality of trees exposed to wasp populations, and compared them with those excluded from wasp populations. Their results show that the presence of introduced wasps reduces the energy level of the honeydew by 99.1%, 98.4%, and 91.3% in January, March, and May, respectively. They concluded that this reduction in the nutrient content of the honeydew may be responsible for the significantly lower number of native birds in areas with heavy wasp populations. They finished by stating that the removal of honeydew by introduced wasp species is threatening the existence of some native animals.

All though the problems introduced wasp species are causing in native beech forests does not demand a lot of media attention, their evident spread across New Zealand makes control options limited. Eradication is an unlikely situation, and control efforts should be aimed at reducing numbers enough to allow bird and insect diversity to copete with the wasps.

This is just one example of how invasive insect species are impacting on New Zealand ecosystems. Although the wasps do not kill the scale insects, their actions create a chain of effects which are potentially damaging to whole communities.

It seems that everyone knows about the seriousness of vertebrate pests in New Zealand (possums vector tb, stoats kill native birds, rabbits degrade farmland etc. etc.). What I want to know is, why do you think invertebrate pests receive significantly less attention and smaller budgets for control options? Personally I think it has to do with the lack of visible economic cost that can be associated with issues surrounding invertebrate pests such as introduced wasp species. What are your thoughts?

Cheers guys, cya next time.

Effect of introduced social wasps on the standing crop of honeydew in New Zealand beech forests. Moller, H., Tilley, JAV., Thomas, BW., Gaze, PD
New Zealand Journal of Zoology, Vol. 18, pp. 171-179. 1991.

The prevalence of introduced Vespula vulgaris wasps in a New Zealand beech forest community

Thomas, C.D., Moller, H., Plunkett, G.M., Harris, R.J.

New Zealand Journal of Ecology, vol. 13. 1990.