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Red crabs migrate across Christmas Island in their thousands each year. Ian Usher/Wikimedia, CC BY-SA

Have you heard the one about the wasp that kills the bug that feeds the ants that kill the crabs that keep the forests healthy on Christmas Island?

If not, that’s because it hasn’t happened yet, but it is a tale worth telling.

In the coming weeks, Parks Australia will release a 2mm wasp on Christmas Island to control the island’s yellow crazy ant infestation. Crazy ants are a major threat to the island’s wildlife, including its famous red crabs.

Biological control – when we use one species to control another – is infamous for giving Australia its cane toad invasion. So, how do we know this one will work?

Christmas Island and its crabs

Christmas Island is a unique natural habitat with many endemic species. The national park covers two-thirds of the island, which has been referred to as the Galapagos of the Indian Ocean.

Many people are aware of the red crabs whose mass migration to the sea has been described as one of the wonders of the natural world.

Christmas Island has many other species of crabs, including the impressive robber crabs. These may be the largest land-dwelling arthropod (the group that insects and crustaceans belong to) on earth.

Together these abundant land crabs clear the forests of leaf litter and maintain burrows that prevent soil becoming compacted, creating an open and diverse forest.

But this thriving natural system was disrupted when an invasive ant species became abundant on the island.

The ants

In the early 20th century, yellow crazy ants (Anoplolepis gracilipes) found their way to Christmas Island. These ants now form super-colonies, with billions of individuals across hundreds of hectares.

The crazy ants spray formic acid in the eyes and leg joints of the crabs, which immobilises them. The crabs soon die and become food for the ants.

In some cases, crabs that live in areas free of crazy ants are killed during their annual migration and so never return to their original forest. This creates crab-free zones even where the ants do not live.

With fewer crabs, the forest has become less diverse, with a dense understory and compacted soils due to the collapse of crab burrows. Other invasive species such as the giant African land snail have become common where crabs declined.

Parks Australia has been trying lots of different methods from aerial to hand-baiting to reverse the impact of yellow crazy ants on red crabs.

The impact was so severe that a chemical control program targeting the super-colonies began in 2001. This program has slowed the decline of crab populations but is expensive and time-consuming, so researchers began to look into other options, including using other species.

The bug: a scale insect

Super-colonies of yellow crazy ants require a reliable food source and this is provided by yet another invasive species: the yellow lac scale insect (Tachardina aurantiaca).

Scale insects (a type of true bug) suck the sap of trees and produce a sweet secretion from their anal pore called honeydew, which ants then harvest.

It seems that the super-colonies of these crazy ants could not survive without the carbohydrate-rich honeydew provided by abundant scale insects in a patch of forest.

There is evidence that the scale insects increase ant reproduction and make them more likely to attack other species. One large field experiment demonstrated that if we stopped the ants getting access to the scale insects, ant activity on the ground fell by 95% in just four weeks.

The scale insects may need the ants as much as the ants need the scale insects. Some ants protect the scale insects in the same way that humans protect their livestock, by chasing away other predators.

The interaction between these two invasive species has allowed them to build their populations to extremely high densities, something known as invasional meltdown.

The good news is that scale insects, unlike ants, are amenable to biological control. For instance, Australian lady bugs were spectacularly successful in controlling the cottony cushion scale in North America.

The wasp

The search began to find a species that could control the scale insect on Christmas Island. And we found it: a tiny wasp known as Tachardiaephagus somervillei, which attacks the yellow lac scale insect in its native Southeast Asia.

This wasp lays its eggs in mature female scale insects and kills them from the inside, producing more wasps that then lay eggs in more females. This wasp (and other predators) are so effective that the yellow lac scale insect is rare in its native habitat.

Obviously, we had to test that the wasp wouldn’t attack other species. Researchers did this in the field in Malaysia, an unusual approach that yielded excellent results. The scientists exposed eight closely related scale insects to the wasp, and none were harmed.

This proves that no other scale insect population on Christmas Island is at risk if the wasp is introduced, with the possible exception of another introduced scale insect that is a pest in its own right.

Researchers also checked that the wasps would still work when the scale insects are being tended by yellow crazy ants – and they still attacked. After years of research it is exciting to be on the verge of releasing this wasp on Christmas Island.

Postscript: the toads

We all know the biological control stories that went wrong. The introduction of cane toads to control cane beetles in Australia backfired spectacularly. In Hawaii, the introduction of mongooses to control rats failed because mongooses are active during the day and the rats were active at night. In both those cases, those species were introduced without sufficient research.

But these examples changed the rules and laws around introducing species. Today governments are much more aware of the risks of invasive species. Rigorous experiments and risk assessments are required before any introduction can occur.

In this case, researchers from La Trobe University have worked closely with Parks Australia and the Forest Research Institute of Malaysia to collect enough data to satisfy the Australian government.

We believe that this is the most closely scrutinised biological control project in Australia. When the wasps arrive on Christmas Island in a few weeks, we are confident that this will set an example for best-practice conservation.

Fewer ants means more crabs, healthier trees, fewer African snails and better soil. And it will save money being spent on expensive conservation efforts for years to come.

Parks Australia has produced a special animation on the program – check it out here at http://www.parksaustralia.gov.au/christmas/news/biocontrol.html.

Susan Lawler has received funding from the Australian Research Council in the past.

Peter Green receives funding from the Department of Environment and Energy

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Batteries may be a good way to store energy in the home. Battery image from www.shutterstoc.com

The Australian government is reviewing our electricity market to make sure it can provide secure and reliable power in a rapidly changing world. Faced with the rise of renewable energy and limits on carbon pollution, The Conversation has asked experts what kind of future awaits the grid.

Storage is the word of the moment in the energy industry. Since Tesla unveiled its Powerwall, politicians, commentators and industry have hyped storage – and particularly batteries – as the solution for getting more renewable energy into electricity grids and reducing our reliance on fossil fuels.

The concept of storage is simple. A storage system takes power off the grid or from a local generation source and puts it back onto the grid or uses it locally later. It seems like a good idea if you have too much energy, or it is cheap at some times of the day and expensive at others.

So could storage be the answer, and how much would it cost?

The costs of storage

Of course storage isn’t free. It comes with both a capital cost (buying it in the first place) and a running cost, which is related to the cost of electricity to charge the battery and the round-trip efficiency – how much power is lost in the charging and discharging cycle.

To be a sensible economic investment, the benefits have to outweigh the costs. In other words, the savings on your energy bill have to be greater than the capital costs plus the running costs.

There are many different kinds of storage technologies, each with different characteristics. Lithium ion batteries are attractive as they operate effectively at small scales, are lightweight and have good round-trip efficiency. But they are currently expensive per unit of storage capacity.

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Flywheels (or rotors) have low round-trip efficiency and don’t store a lot of power, but are able to dispatch lots of power in a short time and can also contribute to frequency stability.

Other storage technologies include compressed air, cryogenic (liquid air) energy storage, flow batteries and hydrogen. Each has its respective pluses and minuses.

Figure on storage characteristics. University of Birmingham Energy Storage Centre Report

Each of these technologies will have an appropriate place in the grid to be installed. Lithium ion batteries are a logical choice for a small-scale distributed application, while pumped hydro will work best at the large scale for grid management.

Flow batteries, liquid air and compressed air are in-between technologies in terms of scale, and flywheels and capacitors are most useful at the substation level for voltage and frequency control.

Batteries versus hydro

Let’s focus on lithium ion batteries and compare them to pumped hydro storage.

Lithium ion batteries are coming down in cost at a significant rate. Bloomberg has plotted the costs of lithium ion alongside solar PV. This shows the two technologies share a similar cost curve gradient, with lithium ion reducing from US$1,200 per kilowatt hour to US$600 per kWh in five years (not including installation costs).

As more batteries are built, the price gets cheaper. Bloomberg New Energy Finance

So where does lithium ion need to get to be cost-effective? Imagine a home with a 4.5kW rooftop PV system and variable electricity rate (for instance off-peak cost of 20c, shoulder of 26c and peak of 40c, similar to this tariff).

In such a home a 7kWh battery needs to cost less than A$7,000 fully installed to actually save the homeowner money. In other words, the cost per kWh of storage should be roughly A$1,000 to break even. Currently, batteries cost A$1,000-3,000 per kWh, so they are on the cusp of being cost-effective.

However, there is an important catch here. Retail electricity rates tend to exaggerate the true range in costs between peak and off-peak. The difference in the wholesale market (where retailers buy their electricity) is around 5-10c per kWh, much less than the 20c range in current variable rates. If retailers begin to lose market share, they may respond by reducing or removing these variable rates. That would make peak rates cheaper and mean that batteries would need to be correspondingly cheaper to be cost-effective.

For instance, a flat electricity rate of 25c per kWh means that batteries would need to cost around A$300 per kWh to be cost-effective. That’s less than a third of their current costs.

You could argue that using batteries also reduces the cost of the network itself. By reducing loads at peak time, we can reduce or even remove the need for infrastructure upgrades (substations and additional power lines, for instance).

But this is only true if electricity demand is growing. If demand is flat or falling, then distribution networks will tend to be under-used. Therefore reducing peak demand will not result in any savings.

Overall demand in the National Electricity Market has declined significantly since 2009, so the benefits of storage on the grid will be negligible other than in high-growth corridors. Demand has rebounded in 2015-16 and it will be interesting to watch and see if this is a resumption of the steady increase or if the demand stays low.

Demand in Australia’s National Electricity Market has been falling.

Pumped hydro, on the other hand, is a relatively inexpensive storage technology (already at around A$100 per kWh) as it can store large amounts of energy using a very inexpensive material.

All you need is some water and the means to pump it uphill. So while it can’t be used everywhere, there are many places in the National Electricity Market where it is possible. There are already 1,500 megawatts of pumped hydro in the market (Shoalhaven, Wivenhoe and Tumut 3).

This would be a more logical solution – cheaper and easier to control by the market operator. But in the same way that rooftop PV has gained more popularity than large-scale solar (even though the latter should be cheaper), distributed storage in the form of lithium ion batteries may be the eventual winner, not because of economics but because of human behaviour.

Roger Dargaville has received funding from the Australian Renewable Energy Agency

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