A stinging attack
How can we protect our honeydew beech forests from wasps (12 minute read)
As I step under the forest canopy, I can feel my body respond. I begin to relax as the noise of the modern world fades, the sound waves muffled by millions of leaves. At first, I’m half-blind in the gloom, but then my eyes adjust, and begin to discern the details of ferns on the forest floor and mosses on tree trunks. As I draw breath, I can feel the humidity in the cool air. As I walk, I brush my fingers over tree trunks or over the surfaces of leaves.
Every forest is a different sensory experience. The dominant trees affect the quality of the light – a dense canopy of large-leaved karaka and kohekohe casts a much deeper shade than the tiny leaves of kahikatea or the beeches. The levels of moisture and types of fallen leaves change the sensations and sounds under my boots as I walk. The colours are different – every shrub and fern beneath the canopy is a slightly different shade of green – and each species feels distinct to touch. The different forests even smell different.
There’s one kind of New Zealand’s native forest I can recognise by smell alone: the southern beech forests. These forests smell uniquely sweet. It’s not the ephemeral fragrance of a flower, but something else entirely, something which is present year-round. It’s the work of a tiny insect and it affects every aspect of the forest. The smell is honeydew.
Honeydew is a sugary waste product excreted by certain insects. All these insects belong to the same broad group, the true bugs, a group which includes aphids, planthoppers, scale insects, stink bugs and cicadas. The majority of the true bugs feed exclusively on the sap of plants, and it is these bugs which produce honeydew.
As a food source, sap is not ideal. While packed with energy in the form of sucrose, it contains low levels of protein, and much of the protein present is not the kind animals need. Bugs need to suck up large amounts of sap to get enough protein, but that means that they eat far more sugar than they need. If humans were feeding on sap, we would convert the excess sugar to fat, and it would do us no good at all. But bugs simply excrete the excess sugar in the form of honeydew, providing a valuable resource for many other animals as well as fungi.
New Zealand has hundreds of native bugs, from the cicadas which sing in the summer to tiny, jewel-like scales which live only on the nikau palm. But two tiny species have a disproportionate importance for our natural environment. These are the sooty beech scales, one which lives mostly on beech trunks and the other which lives mostly on the branches1. Scales are bugs which spend most of their lives in one spot. They plug their mouthparts into a plant and then coat themselves in a waxy coating for protection.
Living in just one spot means that scale insects risk their waste, honeydew, building up around them. The sooty beech scales solve the problem by having an anal tube which is many times longer than the rest of the scale insect. The drops of honeydew form on the ends of these tubes. The trunk and branches of a beech tree, particularly black beech, can be covered with scales, and therefore covered with tiny tubes excreting drops of honeydew.
The amount of honeydew produced by sooty beech scales in some South Island beech forests is remarkable – one study estimated that for every hectare of forest, the dry weight of honeydew produced was 3,800-4,600 kg per year. The trunks of one species, the black beech, are black because sooty mould fungi grow on the honeydew. But it isn’t only fungi which live on the honeydew. Birds, bats, reptiles, other insects and soil microbes all use the honeydew. The sooty beech scales are considered to be keystone species because of their importance in supporting other species in the forest.
But all is not well in our beech forests. Possums and rats eat the honeydew, competing with native species for this essential, year-round supply of energy. But there’s something else, too. Introduced wasps have discovered the honeydew. They are doing more than just competing with other animals for this essential forest resource. The wasps have developed the habit of chewing the anal tubes off the scale insects. This temporarily increases the flow of honeydew for the wasps, but it also means that other animals are unable to harvest the honeydew, and it can even kill the scales.
When I was looking into gene drives, I could see that they were being considered as a possible tool for wasp control. To learn more about both the impacts of wasps and about the potential use of gene drives, I contacted Professor Phil Lester, from Victoria University of Wellington. Phil studies invasive insects in New Zealand, particularly social insects such as ants and wasps. He is leading a research programme on gene drives for wasps, but this research isn’t what you might expect.
“We're not developing a gene drive for release. We have a grant to find out if it would be possible to develop a gene drive. We are considering what that would look like, what effects it would have, and if the general public would accept it as a control for wasps.
“Right now there are no releases for gene drives anywhere. It's a bit of an unknown technology. I think the first will be for malaria, and that'll be really interesting. The technology is a little bit fraught since it's never been done before. Lots of scientists are working on this and trying to figure out what effects it could have, but it's still possible that there's something weird could happen out there that we don't know.
“What we need to do is have people at a point where we can tell them in an honest way: these are the approaches that we think are viable, this is possible using this technology, this is how we would do it, and this is what we believe the effects of it would be. What we're after with this work is to have an informed debate. There's an awful lot of conjecture and theorizing out there with genetic modification. What would be nice is if that debate could be informed by data.”
To understand why gene drives are being considered we need to take a closer look at wasps and their impacts. I’ve mentioned that they feed on honeydew and can even kill the scale insects which produce it. But there’s more to the story.
Wasps belong to a large and important group of insects which also includes bees, ants and sawflies. Some of this group are crucially important pollinators for our crops, while others are destructive pests. While most members of the group are solitary, some are social insects2. These, Phil explains, are not just species which like to gather together. “The true social insects have different castes and overlapping generations within a hive or nest. It isn’t just queens and workers. They might have different sized workers, sometimes referred to as soldiers and workers, with different roles. Or at different ages workers will be doing different tasks. A young worker might be looking after the brood and older workers will be foraging or being guards.”
Until Europeans arrived, New Zealand had some social ants, but all of our 2,000-3,000 bee and wasp species were solitary. Since then, five social wasp species have established. A species of Australian paper wasp made it to New Zealand in the 1880s, an Asian paper wasp arrived in the 1970s and a European paper wasp arrived within the last 20 years. However, the impacts of these pale in comparison with the two yellowjacket species, the German wasp, first found in 1945, and the common wasp, first found in 1978.
Phil tells me that the absence of native social bees and wasps may have left an opportunity open for social wasps. “It's a vacant niche, where invasive wasps could come in and wreak havoc.” One of the biggest impacts they have had is on another social insect, one which is of great importance to us, the honeybee. Wasps attack honeybee hives, killing the bees and their young, as well as stealing honey. This affects honey production, but it has wider impacts too, because bees do more for us than make honey. A number of important plants, such as the clovers in our pasture and many fruit crops, are dependent on honeybees as pollinators. Phil points me to an economic analysis done on wasp impacts in 2015. “The number one economic problem is the impact they have on pollination, such as fertilisation of clover in pasture situations. Farmers have to put on more fertilizer to make up for the reduction in clover.”
But the largest and most intractable problem is in the beech forests where the scale insects are producing honeydew. “The wasp densities in those forests are the highest known anywhere in the world. We're seeing up to 40 nests per hectare and each of those nests might have 5,000 wasps in it. That's a lot of wasps around, and they're hungry and aggressive. If you go there in the autumn, into that typical South Island beech forest, the forest is buzzing. You don't appreciate until you get in there and experience it for yourself just how buzzy it really is.”
Phil explains that there are two different ways that wasps have an impact on our beech forests. Firstly, as I mentioned earlier, they take the honeydew. “If you're in an area where there are lots of wasps around, you don't even smell the honeydew in the air. Since many birds depend on that honeydew, those birds are really hungry. It means the honeydew’s not on the soil as well. When you have lots of honeydew falling onto the soil, it has some interesting effects on the soil microbes. Without the honeydew, the soil community is driven by bacteria, but with the honeydew, it's driven by fungi3.”
The wasps are after the honeydew for energy, but Phil says that it doesn’t give the wasps all they need. “If they want to reproduce, then they need protein. They get that protein from preying on insects and other invertebrates. They're very efficient hunters and they hunt down anything in those forests. The chances of any native spiders or moths or any invertebrates surviving in those ecosystems is really low, because there are just so many wasps.”
The scale of the problem makes it particularly difficult to manage. “We have such a large area of forest where there is a massive abundance of wasps. It's a hard environment to work in and we don't want to damage it. We've got limited options.”
So far, the most successful approach has been to use poisoned bait. The bait is delivered from fixed bait stations which need to be within a few hundred metres of a nest to be effective. While this can reduce wasp numbers, it has to be done with considerable care, since the active ingredient, fipronil, is also toxic to other insects, fish, mammals and some birds. Although thousands of hectares have been treated at times, there are a million hectares of honeydew beech forest in New Zealand. There is currently no way to treat large areas effectively.
Phil says that biological control using pathogens has been investigated, but doesn’t look promising. “There are various bacteria and viruses associated with wasps in their native range. However, a lot of those bacteria and viruses can also infect honeybees, and nobody wants to introduce a new honeybee disease. A group at Manaaki Whenua - Landcare Research have been working on insects that could be biological control options. We don’t know yet how effective they might be, but they could be a good option.
“In many respects, gene drives could be another form of biological control. If you release them, they naturally propagate themselves. So you don't need to go back year after to do more control. It's like a biological control option. But it's a very unique and distinct biological control.”
The gene drives that Phil is studying would have two parts. “The first part is the gene you want to genetically modify. This could be something that reduces their ability to mate or to smell food, or immune genes to make them more susceptible to disease – something which makes the wasps less able to survive or reproduce. There are lots of potential targets out there. But you also need to insert the molecular machinery for the modified gene to be copied from one chromosome to another.”
I’ll take a step back and explain this, because it took me a while to understand. If one parent, say the male wasp, had the gene drive and the female had the normal gene, then the offspring would inherit one copy of the normal gene and one copy of the gene drive – one from each parent4. But then, the gene drive would copy itself across to the normal gene, meaning that the offspring would have two copies of the gene drive. It would then pass the gene drive to all its offspring.
Phil and his colleagues aren’t actually doing this, as I mentioned earlier. But they are creating mathematical models to understand what impact it could have on the wasp population. “We run models to find out what would happen if we released a certain number of wasps with different kinds of gene drives. We can look at different scenarios for what might happen.”
There’s a further complication with wasps, though. “Up to the 1970s, it was German wasps which were widespread and abundant in the beech forest. Then the common wasp was introduced. They seem to have kicked the German wasps out of the beech forest and have become hyper-abundant. If you were to take out common wasps, then German wasps would likely reinvade. So you'd probably have to tackle both at once. But it’s these sorts of questions that our research is trying to understand.”
A gene drive to control wasps isn’t anywhere close to being developed yet. But it’s positive to see that as it becomes more feasible we are also doing the research needed to help us have a more informed conversation about whether it’s something we should attempt. In the meantime, though, I feel sad for our beautiful beech forest, as wasps eat their way through more and more honeydew. Sweet-smelling forests which were once filled with birdsong and native insects are becoming barren, with nothing filling the air but the buzzing of billions of wasps.
The two scales also live on slightly different tree species. Both are found on most kinds of beech, but one species is found on a couple of other tree species.
Most social insects – ants, bees and wasps – belong to this group (known as the Hymenoptera). The main exception is the termites, which are more closely related to cockroaches.
Phil makes an interesting point here, explaining that the bacterial soil communities acquire much more carbon than fungal soil communities, so we might see something positive in wasps, that they are helping to mitigate climate change. It’s a reminder that even the worst invasive species have complex impacts, some of which we might regard as positive. Decisions about managing invasive species always need to consider this complexity, as well as questions about whether the control methods would do more harm than the invasive species.
It’s actually a little more complicated than that for wasps, for a number of reasons. In terms of males and females, their genes don’t work in the same way as ours, or even fruit flies. And they are a social insect, so there is a breeding queen and lots of non-breeding workers. This complicates the research.
A massive problem with risky solutions - please proceed carefully you wonderful scientists