Science and sensibility

I’ve been trying, between studying cell bio, to write up a post about how eusocial behaviour like that shown in the bumble bee series can evolve. During writing up that post I came across the idea that the bees (family Apidae) may have evolved from wasps that supplemented their diet (which largely consisted of other insects) with nectar or honeydew. In fact there is some evidence for this to found in New Zealand’s beech forests. The density at which a beech forest can support a population of the introduced, invasive wasp Vespula vulgaris is strongly related to the amount of honeydew that forest is producing – suggesting that the honeydew can for an important food source for wasps. The more I read about what amounts to a fairly complex ecosystem the more intrigued I became and as so often happens to me I got entirely caught up in a side track. Rather than wasting all that discovery in an aside in a post about the evolution of sociality I thought in the I’d write another post just about this complex system an how the invasive wasps are threatening it.

Some typical New Zealand beech forest

New Zealand’s sub alpine forests are dominated by various species of Northafagus beech trees. These beech forests play host to a number of species of ‘Sooty beech scale insects.’ These insects live embedded in the bark of the beech and in some of their larval instars they insert their long mouths into the phloem of the tree. The phloem is the tissue in plants that carries nutrients including sugar through the tree. The scale insects suck up the phloem and feed on the sugar, excreting excess as they go. The excreted sap forms a sticky cap at the end of the scale insect’s anus that can stop the flow of phloem through the gut and as such prevent the scale insect from feeding. As ever evolution has furnished a solution to this problem, the scale insects project a long fibrous anal tube through the bark of the beech and present their excreta as a food source. An animal comes across the tiny drop of energy rich honeydew and eats it, removing the sticky cap and allowing the free flow of phloem through the scale insect’s gut. If you’re upset by the idea of an animal’s excrement being a food source consider that food produced by honeybees vomiting this honeydew is highly prized amongst humans.

Honeydew is a very important food source in beech forests. A many as 2 000 scale insects can live on one large tree producing energy rich food in an environment that is light in fleshy fruits and nectar rich flowers. In northern hemisphere ecosystems honeydew produced by insects like aphids is fed on by insects, especially ants. As I’ve mentioned before New Zealand has a uniquely bird based biota and although a number of beetles are known to feed on the honeydew it's largely birds that directly benefit from the sweet, shimmering sheets that beech forests produce. Bellbirds, Kaka and the iconic Tui have all been seen feeding on the energy rich food. Fallen drops of honeydew also provide a source of energy to a species of fungus commonly called ‘sooty mold’ which provides a food source for many species of insect. Again it’s the birds that form the top of this food chain as the insect populations become food sources for insectivorous birds like fantails. Presumably before their extinction and marginalisation the native raptors enjoyed the large population of other birds this “food web” provided.

All of the species involved in this tangled web had arrived over millions of years to a point were their mutual self interest had struck an equilibrium and significant populations of all of the protagonsists could be supported. That is, until someone introduced the common British Wasp to our naïve shores. Although New Zealand supports many species of native wasps most of these are parasitic ichneumon wasps that don’t feed on honeydew at any stage of their lifecycle. The introduced species are whole different kettle of fish, these wasps are highly social and nests can contain thousands of wasps all of which feed on honeydew. In fact, New Zealand beech forest has been shown to hold 10 000 workers wasps per hectare (1 hectare is 2.5 acres) over ten times the upper density their native forests in Europe could support. There are so many wasps in some areas that they make up more of the biomass of a given region than all the other animals in that region combined. As well as depleting the honeydew available for native birds and the sooty mold the wasps are expert hunters, depleting the available insects for the insectivorous birds to eat.

In fact the disastrous effects of the invasive wasps go very deep indeed. It has been estimated that the high rates of insect predation in some forests mean that the probability of some caterpillars developing into adults in the wasp season is 10^-40 while an orb web spider’s probability of surviving a season is 10^-18. Overall introduced wasps kill between 1 and 8 kilograms of insects per hectare per year. As if it wasn’t bad enough to lose all these native insects, our native insectivorous birds also find their food source depleted. Circumstantial evidence suggests a decline in numbers of avian insectivores since the wasps took hold.

Wasps have been known to eat a staggering 99% of all of the honeydew produced in some forests. Studies have shown that when the levels of honeydew available drop below a critical threshold birds like the Kaka (a native parrot) give up trying to eat honeydew and find something else. This is a problem in New Zealand’s pest ridden forests where introduced rodents and possums have seriously depleted other food sources. Given the lack of available food the Kaka needs to conserve energy and spends less time in mating and nesting leading to reduced reproductive success and further decline in the Kaka population. Specialist honey eating birds like the Tui and Bellbird give up feeding on honeydew when wasps visit beech trees at a rate of once every three hours, again looking for alternative sources and limiting their other activities and by extension their reproductive success. And it gets worse, the Tui and Bellbird are the primary pollinators of a number of New Zealand’s native trees. The native mistletoe actually requires an adroit flick from a Tui for its flowers to open. Like a doctor learning how the body usually works by studying the effect of a disease the introduction of wasps to New Zealand can teach us how a food web that starts with energy converted from the sun through photosynthesis and diverted by the scale insect can affect fungi, birds, insects and mistletoe.

Witi Ihimaera’s novel Sky Dancer starts with the line

"In the beginning in the days that have gone before us, Aotearoa [the Maori name for New Zealand] was the Kingdom of the Birds."

And in fact when European explorers first laid anchor in New Zealand’s harbours they couldn’t sleep in the mornings because of the boisterous dawn chorus. New Zealanders are justifiably proud of our forests and the amazing bird life they hold but if we want to keep it and perhaps even build it back to something like it once was then we need to focus not only on the big problems of deforestation and introduced possums and rodents but also the smaller, insidious wasps whose influence can be as significant as their larger counterparts