Christmas Island Yellow Crazy Ant Control Program - Moving from Chemical Control to a Biological Control Future
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Christmas Island
Yellow Crazy Ant Control Program
Moving from Chemical Control
to a Biological Control Future
Background
The unique fauna and the ecological role of red crabs
on Christmas Island
The terrestrial landmass of Christmas Island is the top
361m of a 5km high seamount. It is a very remote island
located 350km south of the island of Java, Indonesia and
2,600km north‐west of Perth.
Christmas Island is 135km2 in area with 80km of coastline.
It is markedly terraced from the coastal cliffs up to a
central plateau and covered in thick forest in undisturbed
regions. The climate is tropical with distinct wet and dry
seasons. Image: Geoscience Australia.
Christmas Island, along with the neighbouring Cocos (Keeling) Islands, make up the Indian Ocean Territories
of Australia. They are governed by the federal Department of Infrastructure and Regional Development.
Importantly, 63 per cent of Christmas Island is gazetted as a national park and managed by Parks Australia, a
division of the federal Department of the Environment.
Like many oceanic islands around the world, Christmas Island has
evolved a unique flora and fauna during its many millions of years
of undisturbed and remote existence. This special assortment of
organisms extends from plants to birds, mammals, fish, reptiles
and insects and of course to the famous land crabs for which the
island is internationally renowned.
Geographic location of Christmas Island in the north‐
eastern Indian Ocean. Image: Director of National Parks.
The most iconic of these are the red and robber crabs, but they are
just two of more than 20 species of land crabs on the island. Because
of their huge numbers, red crabs especially provide critical ecosystem
services in the rainforest, eating their way through tons of leaf litter
every year and returning vital nutrients to the soil. Their activities as
seed and seedling consumers create a uniquely open understory in
rainforest on the island. Where land crabs are abundant they can also
reduce the impacts of introduced species on the island’s ecology.
Unfortunately, the land crabs of Christmas Island Christmas Island are
under attack from a highly damaging invasive species, the yellow crazy
ant.
On Christmas Island some trees, like this coral
tree, grow much larger than in other parts of
their range because of the phosphate rich soils. 1
Image: Director of National Parks.
What are yellow crazy ants?
Life history
The scientific name of the yellow crazy ant is Anoplolepis gracilipes. Gracile means slender and lightly built,
a reference to the relatively skinny body and long legs of this species. When disturbed they move around in a
frantic motion, hence the name ‘crazy’ ants. They are aggressive and competitive to other ants and insects
and this enables them to out‐compete and displace other species and dominate food resources.
The yellow crazy ant is an extremely successful and resourceful species, and considered to be one of the
worst invasive species on earth. The home range of yellow crazy ants is not known specifically but they have
spread through tropical and sub‐tropical zones of much of the world. This ant is extremely adept at ‘hitching’
of rides with human produce and materials.
Yellow crazy ants have spread far across the
indo‐pacific region often using human
shipping and air traffic to migrate into new
areas. Image: Wetterer (2005).
It is thought that they were accidently
introduced to Christmas Island through
shipping.
Yellow crazy ants are polygynous (multi‐queened) and unicolonial (they don’t attack each other) which
means that multiple nests can support more than one queen, and quite often 100s or even 1000s of queens
and tens of thousands of workers. In the absence of natural control mechanisms, as the situation is on
Christmas Island, these traits enable the ants to form into large populations known as ‘super‐colonies’. The
largest of these super‐colonies was recorded on Christmas Island in 2001 and was 750 hectares in size!
Yellow crazy ants form nests in every possible niche within the forest and in supercolonies, it impossible to
tell where one nest ends and another begins. They feed on a range of animals to access protein but also they
also obtain carbohydrates from plant nectar and honeydew, which is produced by an introduced insect
called the yellow lac scale. The ants forage 24 hours a day and there can be more than 1000 ants every
square metre.
Consider that very large supercolony: 1,000 (ants per m2) × 10,000 (m2 per hectare) × 750 (hectares) =
7,500,000,000 ants. That’s 7.5 billion ants in just one supercolony. And that’s just an estimate for how many
ants were on the ground – there were probably just as many ants in the canopy visiting scale insects!
This species is known as a fomicine ant because they actually don’t sting but spray formic acid from a small
nozzle at the tip of their abdomen as a defence mechanise and also to subdue prey. Formic acid is one of
nature’s most powerful acids. The acid is a big problem for insects and land crabs which share the same
habitat as the ants and it can also cause irritation in humans. These traits and others enable the yellow crazy
ant to have a significant impact on the ecosystems into which it has been introduced. At 1000 ants per
square metre, yellow crazy ants spray enough formic acid to wipe out local populations of land crabs.
2
The rise of the ants
Since Christmas Island was settled more than 120 years ago, its ecology has been influenced by an
assortment of plants and animals introduced either intentionally or accidentally by humans. While most of
these have had little if any detectable impacts, others have been disastrous for the island. The most
prominent and damaging animal introductions include rats, cats, wolf snakes, centipedes and yellow crazy
ants. Much of Christmas Island’s rich ecology was still considered intact prior to the introduction then spread
of yellow crazy ants.
The yellow crazy ant was accidentally introduced some time between 1915 and 1934. The species is
recognised worldwide as a significant ecological pest that can negatively affect intact ecosystems.
Unfortunately, the most well known case of this invasive potential is here on Christmas Island where since
the late 1990s yellow crazy ants have killed tens of millions of land crabs, the most notable of these being
the iconic red crab. The yellow crazy ants also directly compete with and prey on native vertebrate and
invertebrate species, indirectly cause the dieback of trees, reduce soil health, alter forest composition and
also facilitate the invasion of other introduced species into super‐colony areas.
Interestingly, yellow crazy ants were present on Christmas Island for decades before having an obvious
ecological impact. The formation of damaging super‐colonies is a recent phenomenon, with the first one
found in 1989 in scrubby forest on a rocky terrace high above The Grotto. That supercolony died out, and
the current phase of expansion started around the mid to late 1990s when more supercolonies were found
near The Dales and Greta Beach. By 2001 yellow crazy ants had formed supercolonies in 2,500 hectares of
the island’s forest. Most super‐colonies at this time were located in the national park with a preference for
the coastal terraces. A control program was initiated in the late 1990s to suppress the spread and damage
caused by yellow crazy ants and this control program is ongoing.
On Christmas Island, a combination of factors has
enabled the species to form large, ecologically‐
damaging super‐colonies. Principal among these is
a mutualistic relationship with another group of
introduced species, scale insects. Many of these
are sap suckers, living on trees where they suck
sap straight out of the stems. Their sugar‐rich
waste product is called honeydew, and is avidly
collected by the yellow crazy ants as a food
source. The ants farm and protect these scale
insects so that the sugary food source is
maintained. Several species of honeydew‐
producing scale insects are common in A yellow crazy ant worker feeding from yellow lac scale insects.
supercolonies, but the yellow lac scale insect Image: Director of National Parks.
Tachardina aurantiaca is thought to be the main
contributor to the yellow crazy ant’s honeydew diet. In super‐colonies, this and other honeydew‐producing
scale insects occur at outbreak densities, and yellow lac scales be so dense as to sheath the twigs they settle
on.
3
Clearly the mutualism between the ants and scales is key to supercolony formation, but like the ants,
honeydew‐producing scale insects have also been on the island for a long time. Why have supercolonies
formed only relatively recently? What may have changed about the mutualism that allowed rapid and
extensive population buildup of both partners? El Nino climate events may have been important. El Nino
causes longer, drier dry seasons on Christmas Island, and drought stresses trees. This can have the effect of
making plant sap relatively more concentrated, promoting population increases of scale insects. More scale
insects, more honey‐dew, and more honeydew means more ants. Because ants tend scale insects, more
ants means more scale insects! In other words, the extent to which both partners facilitate each other in the
mutualism could have been promoted by the severe El Nino that occurred in the late 1990s. It’s a plausible
idea, but unfortunately supporting evidence has proved elusive.
The impact of yellow crazy ants on Christmas Island biodiversity
A typical undisturbed patch of Christmas Island forest will experience multiple changes following the
foundation and spread of a super‐colony. Typically, a super‐colony area is devoid of most forms of faunal
life. The ants systematically attack and prey on any animals not fast enough to exit the colony.
Most notable of the impact of yellow crazy ants on Christmas Island is they kill land crabs. When
supercolonies form they take over the burrows of red crabs as nesting sites, and all resident crabs are rapidly
killed. Any crab unfortunate enough to wander into a supercolony will die with 24 hours as a result of
inhaling toxic formic acid. It is estimated that yellow crazy ants have killed tens of millions of red crabs in the
last 15 years and they can even take down the largest of robber crabs, the biggest terrestrial arthropod on
earth. Yellow crazy ants also attack other animals that enter their colony in particular invertebrates and even
small reptiles that are a food source for the ants.
The ants can also alter the entire ecosystem by directly and indirectly altering the vegetation structure and
species composition in areas where they reach high densities. In some places the trees may even die due to
the stresses imposed by the scale insect‐yellow crazy ant relationship. These changes can also facilitate the
invasion into super‐colony areas by secondary invader species such as the giant African land snail that are
controlled by the red crabs in uninvaded areas of the island.
The numbers and diversity of forest birds may also be affected negatively and sometimes positively in super
colony areas.
The ants attack the land crab, spray acid into its eyes and leg joints which renders it immobile. The crab soon dehydrates and dies
where it becomes a food source for the ants. Images: Director of National Parks (left) and Peter Green (right).
4
Large scale changes occur to the forest structure following the removal of red crabs. In healthy forest that
supports a large number of red crabs the forest floor is cleaned of leaves which the crabs eat, and the crabs
also suppress seedling growth and till the soil. In a yellow crazy ant super‐colony there is a large amount of
leaf litter present which has implications for micro climates and nutrient cycling, and there are large
numbers of seedlings that eventually form an understory thickets. Only time will tell how these forest
patches will change over decades compared to forest that has never been invaded by ants.
While the yellow crazy ants themselves do
not affect tree health, the huge numbers of
yellow lac scale they farm can cause die‐
back and even the death of large forest
trees. The stress of hundreds of thousands
of scale insects can stress the tree to the
point where it defoliates. Over multiple
seasons this ongoing and relentless stress
can kill trees. Excess honeydew that falls
from the trees onto the leaves of lower
branches and trees promotes the growth of
sooty mould that suppresses
photosynthesis and can also eventually kill
trees.
Images: Peter Green & Dennis O’Dowd.
The removal of red crabs from large areas of the forest has also
enabled the persistence of a species that was controlled
effectively by red crabs prior to their removal from an area by
yellow crazy ants. The introduced giant African land snail is now
common in areas of the island where red crabs have been
removed.
Giant African land snails are renowned as a pest
species through the tropics. The implications for
the presence of the giant African land snail on
native snail assemblages in the forest are still
being examined.
5
Island‐wide survey for yellow crazy ants and red crabs
In 2001, researchers from Monash and La Trobe universities designed an island‐wide survey for Parks
Australia to assess the distribution and density of yellow crazy ants and red crabs at 1000 sites across
Christmas Island’s 135 km2. This survey has taken place every two years since and is the primary source of
data on the spread of yellow crazy ants and the population of red crabs. The data enables a targeted control
program to be implemented and is also used to gauge the success or otherwise of control activities.
Since its inception, the island‐wide survey has
evolved into a multi‐species distribution
survey to assess the status of a variety of
introduced and native flora and fauna that
would not normally be monitored so
thoroughly. More than 40 species now have
their distributions recorded with minimal
additional effort. Surveyors are trained to
detect the presence of many native and
introduced flora and fauna species, both at
survey sites and while in transit.
With its 1000+ sites the IWS is one of, if not the most, intensive island
ecological surveys on earth.
The survey is based on a transect methodology where yellow crazy ant activity is measured every five meters
along a 50 metre transect and red crab burrows are counted and measured 1 metre each side of the same
transect tape (100m2). Past experience shows there is a critical abundance of ants where they begin to kill
red crabs. If the sum of ant counts from along the transect is greater than 37, the site is considered a super
colony, subject to further verification).
Island‐wide surveys were completed in 2001, 2003, 2005, 2007, 2009, 2011 and 2013. Each survey has been
undertaken in slightly different ways and has developed in line with advances in technology, especially for
navigation.
6
Options for yellow crazy ant control
Chemical baiting
Since 2001, the Christmas Island National Park in
collaboration with La Trobe University and an advisory
panel of experts (the Crazy Ant Scientific Advisory Panel
– CASAP) has implemented an annual yellow crazy ant
control program to minimise the ecological impacts of
the yellow crazy ant, especially on red crabs. This
program uses insecticides to wipe out super‐colonies
once they have formed.
Ant control has been maintained annually since the
program’s inception using a highly effective ant bait.
The bait is called AntOff®, and the attractant is a protein
To date, Fipronil‐based ant bait remains the only proven based granular product. Fipronil is the active
option for the control of yellow crazy ants on Christmas ingredient.
Island. Image: Director of National Parks.
Research into additional control options has always been a large component of the control program, and a
variety of other chemical ant baits have been trialled over the years. These have included other toxins such
as hydromethylnon and indoxacarb, but also insect growth regulators such as s‐methoprene and
pyriproxyfen which are considered less harmful (i.e. have less potential for off‐target impacts). The bait
matrix used has also varied during the program with combinations of baits and matrixes tested periodically.
Bait delivery methods and their effectiveness
Fipronil is a general invertebrate neurotoxin, meaning that it targets the nervous systems of any animals that
do not have backbones. Not only does AntOff® kill yellow crazy ants, but it will also kill a variety of other
animals if they consume it. These effects are called non‐target impacts, and the first principal of chemical
control in ecologically sensitive areas is to use the bait in such a way that non‐target impacts are kept to an
absolute minimum. Crazy ants are baited only when their densities are high enough that they will
monopolize most of the bait, denying it to other organisms and reducing non‐target impacts. Further, baiting
is conducted during the dry times of the year when any red crabs living on the margins of supercolonies stay
in their burrows to avoid the low humidity – not even the bait will entice them out of their burrows. Robber
crabs are more active than reds during these periods, and they are attracted to the bait. Before any baiting
occurs, Parks Australia rangers lure robber crabs away from the target or remove them by hand to another
location, a process that makes an already labour‐intensive undertaking even more so. Thousands of crabs
have been saved in this way.
On Christmas Island the bait is applied in two ways, either through broadcast by hand or aerially using a
helicopter. For the most part, broadcast by helicopter is preferred as the machine can access parts of the
island that cannot be baited by hand, it is safer for staff and completed quickly. Although it’s a very
expensive exercise, it’s more cost effective per hectare than ground baiting. A helicopter has been brought
7
to the island three times since the program began. At other times national park staff hand‐bait super
colonies that are accessible on foot. These operations can also only be undertaken during the dry season.
The supercolonies are mapped using GPS prior to bating operations, and regardless of delivery method, the
bait is broadcast at a set rate per hectare.
Whether by hand or by helicopter, the use of ant bait to control super‐colonies within the thick island forest is expensive
and a logistically difficult operation Images: Director of National Parks.
Fipronil‐based ant bait is extremely
effective at knocking out super‐colonies;
ant densities are noticeably reduced
within the first week, and the majority of
ants are eliminated within a month of
treatment. In some cases 100 percent of
ants are wiped out in what was previously
a super‐colony area. Unfortunately
though, baiting will never completely
eliminate yellow crazy ants from
Christmas Island. The ants occur at very
low densities in many locations and
cannot be baited safely, and these
populations, plus the occasional survivors
The chemical baiting of yellow crazy ants has occurred over 5,500 ha of the of baiting operations, serve as founder
Christmas Island land mass (13,500 ha) since 2000 with many areas treated
multiple times as super‐colonies re‐invade.
populations for new super‐colonies.
Image: Director of National Parks.
8
Sustained baiting operations over more than a decade have slowed the decline of the red crab population,
but not completely stopped it. The reason for this is straightforward – the ants have been forming and
reforming super‐colonies and killing crabs at a greater rate than natural recruitment processes can replace
them. Although red crabs migrate to the sea every year, the return of bay crabs is much less frequent, they
too can be wiped out by super‐colonies, and the survivors grow very slowly. At the heart of the problem is
that super‐colonies are difficult to find, and can only be baited safely once the ants have reached very high
densities – and already killed the resident red crab population. Baiting is not sustainable in the long term – it
is reactive, requires constant vigilance, and significant financial resources to maintain.
How can we control yellow crazy ants into the future? Is direct biological control an option?
Research into alternative control options has always been an important part of the yellow crazy ant program
on Christmas Island. Even before the difficulties of a program based on toxic bait were fully apparent,
university researchers were considering if biological control could be used contain the yellow crazy ant.
Classical biological control works on the principle that in their area of origin, native species are kept in check
by their natural enemies, be they predators, parasites or pathogens. There is a lot of scientific evidence
demonstrating that in many cases, species introduced outside of their native ranges become invasive
because they have effectively left these enemies behind. The principle of classical biological control then is
to re‐establish population control over invasive species by first identifying and then importing a natural
enemy – a biological control agent – from within the native range of the target organism. Unfortunately,
ants are an especially difficult target group for biological control, despite their global ecological and
economic impacts. A program for the biological control of the Red Imported Fire Ant (Solenopsis invicta)
using a parasitic fly and a protozoan disease as agents is currently under development in the southeastern
United States, but no species of ants have yet been controlled in the field using biological control agents.
Classical biological control:
In an unbalanced ecosystem, introduced plants and animals
can cause dramatic problems for native species. Biological
control aims to introduce organisms that have the potential
to bring the ecosystem into a balanced state where species
coexist without significant detrimental impacts. The
diagram above demonstrates classical biological control.
The fluctuating lines represent seasonality. Adapted from:
http://www.csiro.au/Outcomes/Food‐and‐
Agriculture/WeedBiocontrol.aspx.
9Indirect biological control as an option
Early on in the history of the yellow crazy ant control project, researchers noted that super‐colonies of
yellow crazy ants always occurred in association with large numbers of honeydew‐producing scale insects.
This association suggested that a plentiful supply of the carbohydrates – sugars – in the form of honeydew
was critical to the formation of supercolonies, and that maybe a biological control agent could be used not
against the yellow crazy ants themselves, but against scale insects, as a way of reducing their food supply.
There has been long history of successfully using biological control agents against a variety of scale insects in
agricultural situations, and in most cases the agents have been tiny wasps that parasitize the female scale
insects and kill them.
In 2009 a team of researchers from La Trobe University in Melbourne, funded by Parks Australia, began a
research program to investigate the idea of indirect biological control on Christmas Island. The program
quickly focussed on two key areas – investigating the importance of honeydew in yellow crazy ant biology
and supercolony formation, and identifying and investigating the biology and natural enemies of the main
scale insect species in supercolonies.
Not a cane toad
Cane toads were released as a biological control agent in 1935 to control cane beetles. Infamously, the cane toad
ignored the cane beetle and ate or killed a huge number of native species. The damage caused by cane toads continues
to this day, especially in the Kimberley and Arnhem Land regions of northern Australia.
However, the circumstances under which the cane toad was released in Australia are completely different to the highly
regulated environment in which we operate today. The cane toad was introduced without scientific consideration of its
biology in relation to its cane‐beetle target, without peer‐reviewed research, and in the absence of a strict approval
process to ensure it would be an effective biological control agent that would not affect non‐target species. Today the
process of nominating a target species and then importing a biological control agent is highly regulated, rigorous and
subject to multiple approval processes based on peer reviewed science and governmental policy.
The cane toad is a spectacular example of a biological control program gone very badly wrong, but the regulatory
environment of modern times makes such a spectacular failure well and truly and thing of the past. In fact, there are
numerous examples of successful biological control agents being used in Australia and around the world. The most
notable of these success stories was the introduction of the Cactoblastis moth in the 1920s to control prickly pear,
which at the time was smothering more than 24 million hectares of north‐eastern Australia! The moth effectively
controlled the prickly pear in just a decade, removing it from large tracts of arable farmland.
10Research into biological control – Yellow crazy ant dependence
on honeydew
Several years of study in the laboratory and the field supports the idea that sugars in honeydew from scale
insects are critical to supercolony formation by yellow crazy ants, and that reducing the supply of this
resource could indirectly control the ants. The researchers found three lines of evidence.
Examining honey dew use by yellow crazy ants using stable isotope analysis
The concept of the food chain has proven very useful in this part of the research. At the base of the food
chain are plants – they produce food for other organisms, without consuming other organisms themselves.
Herbivores feed on plants, and predators feed on herbivores. Plants, herbivores and predators are known as
trophic levels. Some animals, such as the yellow crazy ant, feed on more than one trophic level. Honeydew
is a plant product, so ants act as herbivores when they feed on it. However yellow crazy ants also act as
predators when they kill and consume other animals, mainly other invertebrates. The point of interest is the
balance between those two dietary components – if honeydew is very important in supercolony formation,
it’s reasonable to expect a high percentage of honeydew in ant diets, and therefore, that ants should behave
mostly as herbivores when they occur at extremely high densities. Alternatively, when the ants occur at
much lower densities, their diet will contain a lower of percentage of honeydew, and their foraging patterns
will be less herbivore‐like and more predator‐like.
Ever tried watching and measuring what an ant feeds on? Its next to impossible to observe exactly what
individual ants eat from day to day, so researchers used an indirect method, known as isotope analysis, to
determine where yellow crazy ants sit on the trophic ladder, and how that position shifts depending on their
abundance. The researchers collected samples of ants in several supercolonies over a period of months, as
the supercolonies boomed and then busted naturally. The isotope analyses showed a subtle but very clear
pattern – yellow crazy ants always have a high percentage of plant products in their diet regardless of their
density, but as predicted, the trophic position of the ants is more herbivore‐like when their supercolonies
where booming, and significantly less herbivore‐like as the supercolonies declined. These patterns are
consistent with the idea that ants rely heavily on honeydew to sustain their high densities in supercolonies.
Crazy ants from within a forming colony
consumed more protein based foods than
ants in a super‐colony whose proportion of
carbohydrate based food, supplied from the
honey dew, was much larger Image: La Trobe
University, unpublished data.
11Proof of concept field experiment
In a large field experiment, the researchers attempted to mimic the effect of a successful introduction of a
biological agent on the honeydew supply to ants. Most of the scale insects in super‐colonies live in the
canopies of the largest trees, and so removing the scales manually or with insecticides was impossible.
Instead, the researchers prevented the ants from gaining access to the scale insects by banding all the trees
on an experimental plot. From the ant’s point of view, this achieved the same effect as if a biological control
agent had killed the scale insects – a drastic reduction in their honeydew supply. Another plot was set up as
an unbanded control next to the banded plot.
The density of yellow crazy ants was at supercolony levels on the banded and unbanded plots for several
weeks before the experiment commenced. It remained high on the unbanded plot throughout the
experiment, but declined dramatically on the banded plot once the bands were put on the trees; density fell
by around 80% within a matter of weeks. The rainforest canopy harbours other resources for ants besides
scale insects and their honeydew, and ants were blocked from these resources too. However, the
researchers were confident the decline in ant density on the banded plot was because the ants no longer
had access to all that honeydew, because only a tiny fraction of those ants that got caught in the log jam
above the plastic bands were carrying prey items in the mouthparts, but the abdomens of a very high
percentage of ants were obviously swollen with translucent honeydew.
Plastic film sprayed with furniture polish
was wrapped around trees is an effective
barrier to ant traffic on tree trunks. All
tree trunks and stems were banded and
this even extended to large multiple
stemmed fig trees. Yellow crazy ant
heading up the trunk could not get past
the bands, and coming down trunks
formed logjams. These ants were gently
brushed off the trunks each day until the
canopy was effectively empty of ants.
Image: Sara Wittman.
12The effect of carbohydrate supply on yellow crazy ant growth and behaviour
The stable isotope study and field experiment both show that carbohydrates in honeydew are important to
build and sustain high densities of ants in supercolonies. But what does the honeydew actually do to help
the yellow crazy ants? The sugary honey dew is not just a food source for yellow crazy ants. It actually
changes the way in which the ants behave in their daily lives. It’s very much like giving limitless sugar to a
group of small children... chaotic! By designing an experiment where the amount of sugar available to
laboratory colonies of yellow crazy ants was controlled, the researchers were able to measure the effect of
sugar on rates reproduction by queen ants, rates of survival of worker ants, and on ant behaviour.
Compared to laboratory colonies were the supply of carbohydrate was limited, colonies with access to
abundant sugar resources had more productive queen ants, and lower rates of death among worker ants.
The workers themselves showed different behaviour too – when fed a high sugar diet there was a higher
percentage of workers foraging for food, they were more aggressive towards other species of ants, and were
more exploratory in their environment. This experiment explains why there was a significant drop in ant
densities on the banded plot in the field experiment, and confirms that if the honeydew economy is
restricted by a biological control agent, the ant’s capacity to form supercolonies will be very much reduced.
Yellow crazy ant activity steadily declined once access to the forest canopy was restricted (blue
line). After eight weeks the number of ants was reduced to almost zero as opposed to the
control (red line) that continued to increase. Image: La Trobe University, unpublished data.
13Research into biological control ‐ Scale insects and enemies
The other part of the research program funded by Parks Australia was about the scale insects; their overall
diversity and status on Christmas Island, the honeydew‐producing species most important in supercolonies,
and their biology and natural enemies on Christmas Island and in Malaysia, part of their home range in south
east Asia. The study produced vast amounts of information about these fascinating organisms.
Scale insect survey on Christmas Island
One of the first things the researchers did was to take an inventory of all the different scale insects that
occur on Christmas Island. This was to identify which species produce honeydew and therefore could play a
role in supercolony formation by crazy ants, but it also enabled researchers to determine if there were any
species that were endemic to the island, and possibly at risk as non‐target species in a biological control
program. Scale insects are very small, and Christmas Island has a lot of trees, many of which are extremely
tall. In order to locate and identify all the scale insects present on Christmas Island researchers spent three
years travelling all over the island, searching for scale insects. The hunt focussed on native and endemic
trees which would be most likely to support native and endemic scale insects.
The number of scale insects found was quite large, with 28 species from six families identified. No endemic
species were found and the majority of scale insects were recent introductions.
There are many and varied species of scale insects on Christmas Island including bishops hats, orange fuzzies and pink Santas.
Some species are mostly harmless to the ecology but a few are highly damaging and of great concern ecologically.
Images: Director of National Parks.
14Identifying the main culprits
The researchers identified four scale insect species that contribute most to the honeydew economy of
supercolonies. These are the soft scales Coccus celatus (coffee green scale), Coccus hesperidum (brown soft
scale) and Saisettia coffeae (hemispherical scale) and the yellow lac scale Tachardina aurantiaca. None of
them are considered as native to the island. All of these scales live on a variety of native rain forest trees,
but these have definite favourites. The soft scales tend to be most abundant on trees with relatively low
wood density, while the yellow lac scale tends to occur on species with denser wood, especially the Tahitian
chestnut Inocarpus fagifer. This is the tree with deeply fluted trunks that dominates wet areas in The Dales,
but it also occurs widely throughout the forest on the island. Researchers consider the yellow lac scale to be
the single biggest contributor to the honeydew economy in supercolonies on Christmas Island.
Coccus hesperidum Saisettia coffeae Coccus celatus
Controlling the culprits
The abundance of these species in yellow crazy ant supercolonies indicates that they are not under effective
control by their natural enemies. Why not? Do the natural enemies of these scale insects occur not occur on
Christmas Island, and if they do, are they not widely distributed? Researchers found that at least two species
of parasitic micro‐wasps do occur on the island, and are well‐known from elsewhere to attack the three
species of soft scale scales. These tiny wasps, known as parasitoids, lay their eggs into a scale insect’s body.
The eggs then hatch, killing the scale insect. The presence of these parasitoids on Christmas Island is great
news, and a program is already underway to propagate and disperse them more widely through the forest.
The story of the yellow lac scale is different. This scale insect appears to have reached Christmas Island
without effective predators. Surveys have shown that birds and moth larvae occasionally attack and eat the
large yellow females, and there is a parasitoid (Marietta leopardina) that parasitizes the much smaller male
lac scales. Although Marietta is widespread, its impact on male lac scales is clearly not great enough to
prevent this species from attaining huge densities and fuelling crazy ant supercolonies. Without an effective
parasitoid of female scales insects, the researchers turned their attention to this species in Malaysia, part of
it home range in south east Asia.
15An early biocontrol success
The survey also turned up a well known problematic scale insect named Pulvinaria urbicola. On Christmas
Island this scale insect has already defoliated large sections of Pisonia grandis forest. This is rare forest type
on the island, and in other parts of the tropics. Over multiple seasons severe defoliation could kill the trees,
which are important nesting habitat for booby and frigate bird species. Luckily there is a parasitoid named
Coccophagus ceroplastae present on the island that probably arrived with this scale insect internally. It is
capable of effectively controlling this species which is known to have almost destroyed forests on Pacific
islands. La Trobe university researches and Parks Australia staff reared and released this wasp in 2011 at a
number of locations, and the indications are that it is successfully controlling the Pulvinaria outbreak, and
dispersing of its accord.
Pulvinaria urbicola is an introduced scale insect capable of killing large trees. In the Pacific Ocean region, whole stands
of Pisonia grandis forest have been destroyed. Luckily, we have a biological control agent already present on Christmas
Island capable of keeping this species under control.
Natural enemies survey, parasitisation rates of lac scale insect and parasitoid behaviour in area of
origin (south‐east Asia)
The most noticeable thing about the yellow lac scale in Malaysia is that its not noticeable at all – in fact, it’s
very rare. The reason for this soon became very obvious – wherever yellow females where found, up to 90
per cent of them showed evidence of parasitization. This is easy to detect – there are large, circular holes in
the sides of the female scales, where the emerging wasp has cut its way free of the dead or dying scale
insect. This type of damage has never been seen in female lac scales on Christmas Island, and is just further
evidence of the lack of effective parasitoids on the island. All of the observations from Malaysia indicate that
the lac scale is very effectively controlled by its natural enemies in Malaysia, despite the presence of many
different species of ants that could disrupt the behaviour of the parasitoids.
Within its home range, the yellow lac scale
insect is quite rare and difficult to locate due to
the control exhibited on the population by
parasitoid micro‐wasps and other predators as
shown here by parasitised yellow lac scale.
The circular holes in the scale insect show where
a parasitoid micro‐wasp has emerged. Image:
Gabor Neumann.
16Tachardiaephagus somervillei, a potential indirect biological control agent for yellow crazy ants
Of the many natural enemies of female yellow lac scales in
Malaysia, one stands out as the best candidate for introduction to
Christmas Island, Tachardiaephagus somervillei. This species can lay
up to eight eggs into one female scale insect, and has the potential
to parasitise over 150 individual scale insects over its life time! This
is the species that was responsible for creating the large exit holes
seen at many locations. Tachardiaephagus occurs in a similar
This highly enlarged drawing shows the
latitude north of the equator to Christmas Island south of the equator potential biological control agent
and has a wide distribution in its range, meaning that it is capable of Tachardiaephagus somervillei. Image:
survival under the range of climatic conditions it would face on Narayanan (1962). Note the scale – in real
Christmas Island. life the insect is only two millimetres long
Tachardiaephagus somervillei is a very small parasitoid micro‐wasp. It is only 2mm long with a 3mm wing
span. Its native range extends throughout Southeast Asia from Thailand to the Philippines. The animal is host
specific, meaning that it parasitises scale insects from the Encyrtidae family only. Amazingly, it also feeds off
the honey dew extruded by the scale insects! Despite being a wasp, this animal is not a threat to humans
and cannot sting us or our pets or any other animals apart from scale insects. In fact, it is so small that seeing
it without a microscope is difficult.
Tachardiaephagus somervillei is a fairly cryptic animal so you may ask; why is
anything known about it? Well in a twist of fate this species is considered a
pest species in the shellac industry of India and Thailand. Shellac is a product
used to make furniture polish, and is produced by a scale insect called Kerria
lacca, in the same family and closely related to the yellow lac scale on
Christmas Island. Context is everything – shellac producers curse
Tachardiaephagus as a pest animal in their industry because it kills their scale
insects, while we value this parasitioid for the potential benefits it will bring to
conservation on Christmas Island.
The potential biocontrol agent,
Tachardiaephagus somervillei,
is so small that it fits within the
zero on a 20 cent piece.
Image: Dennis O’Dowd.
A note on males and females, ♂♀:
It is important that the biological control agent chosen affects the female scale insects. The biological control
of males within a population is ineffective as males are less important than females when it comes to
maintaining a population.
Unfortunately for the boys, any one male can do a man’s job many times but only many females can produce
multiple offspring. In this case, the males are not the target of biological control so they have a reprieve.
17Other benefits of biological control on Christmas Island
While this project is designed specifically to reduce the capacity for yellow crazy ants to form super‐colonies
there is also a potential horticultural benefit from the introduction of Tachardiaephagus somervillei. Many of
the plants grown for horticulture purposes on Christmas Island suffer from yellow lac scale attack (e.g. sour
sop, and many ornamental plants around town). There is also talk of creating a broad‐acre agricultural
industry on the Island. The more we know about plant pests and their biological control by parasitoids, the
better placed the residents on the island will be to grow more of their own food.
An adult Tachardiaephagus somervillei
micro‐wasp under the microscope. The
insect is only 2 mm long.
Image: Gabor Neumann.
18Implementation of biological control
This initial phase of the research by La Trobe University proved the feasibility and safety of indirect biological
control for yellow crazy ants on Christmas Island. In 2013, Christmas Island National Park and La Trobe
University entered into a second three year contract to move to the next phase. Subject to approval, over
the next few years the team will move to implementing a biological control program while still carrying on
some basic research.
Approval processes
The importation of a biological control agent from overseas into Australia and its territories is governed by
strict approvals processes, based on risk management and evidence based approaches (e.g. peer reviewed
scientific research). The approvals process for Christmas Island is determined by two separate but parallel
pieces of legislation, overseen by the Departments of Environment, and the Department of Agriculture. The
steps listed below are those required for completion by the proposer prior to the importation of a biological
agent:
Step 1: Approval of the target species as a candidate for biological control
Step 2: Offshore research on possible agents
Step 3: Host‐specificity test list
Step 4: Permission to undertake specificity testing in contained use in Australia (if/as required)
Step 5: Testing permit for proposed biological control agents that are animals
Step 6: Specificity testing
Step 7: Application to release a biological control agent
Step 8: Assessment of release package
Step 9: Release permit
Step 10: Amending the live import list for biological control agents that are animals
For more information head to:
http://www.daff.gov.au/ba/reviews/biological_control_agents/protocol_for_biological_control_agents/guid
elines‐introduction‐exotic‐bcas‐weed‐and‐plants.
In addition, other assessments and approvals may be required under national environment law (the
Environment Protection and Biodiversity Conservation Act 1999), as is the case for this Christmas Island
biocontrol project.
Evaluating the risks
How do can we be sure that the biological control agent will attack just the target species, and not non‐
target species of concern? The core of the approvals process is ‘host‐specificity testing’, a series of detailed
experiments in which the proposed biological control agent is enclosed with a test species to determine if
the parasitoid will attack it. Researchers conducted these tests in Malaysia to determine the host specificity
of Tachardiaephagus somervillei.
19The test species were all scale insects. It is accepted practise in biological control to first test species that are
closely related to the target. If the parasitoid does not attack them, then the potential for the parasitoid to
attack anything else more distantly related – red land crabs, endemic sea birds, rare reptiles, humans – is
considered impossible. The results of the tests, and additional information gleaned from the scientific
literature, were conclusive. Tachardieaphagus somervillei is highly specialized to attack only scale insects in
the lac family of scale insects, and nothing else. Even if it did, the next mostly likely victims on Christmas
Island would be other species of scale insects, and as the survey showed, known of them are native anyway.
Intended program outcomes
If permission to import the biological control agent Tachardiaephagus somervillei is granted, micro‐wasps
currently being reared in Malaysia under laboratory conditions would be brought directly to Christmas Island
(i.e. not via mainland Australia). Subsequent importations of the micro‐wasps may occur through time in
order to boost the genetic composition of the population and/or to replace the population if inadvertently
wiped out by a stochastic event (e.g. like a cyclone). Half of the imported micro‐wasps will be released into a
small yellow crazy ant super‐colony upon arrival. The rest will be released into a rearing facility.
Parks Australia has constructed a purpose‐built greenhouse to use as the micro‐wasp rearing facility.
Favoured host plants of the yellow lac scale insect (e.g. Inocarpus fagifer and Milletia pinata) are being
grown in the greenhouse and will be infected with yellow lac scale insects prior to the arrival of the micro‐
wasp. Upon arrival, the wasp will use these yellow lac scales for parasitism and as a food supply. The
greenhouse will, in essence, be a micro‐wasp production facility. Once large numbers of yellow lac scale are
parasitised they will be harvested and released.
A targeted and systematic release of the micro‐wasp into yellow crazy ant super‐colonies or areas with the
potential to become super‐colonies will take place. Novel techniques for positioning parasitised scale insects
into the forest canopy 30‐40 m high require development. Initially, release site in super‐colonies will be
small and readily accessible so that effective monitoring of the scale and micro‐wasp populations can be
undertaken.
In general, field monitoring work will assess the survival, establishment and spread of the micro‐wasp
population and its impact on the yellow lac scale population. Novel techniques to collect data require
development for sampling branches in the canopy. The Island‐Wide Survey will be used as a mechanism for
gathering data on the spread and impact of micro‐wasps at much larger scales.
There will also be a comprehensive laboratory monitoring and experimentation component to the program.
Micro‐wasp fecundity, lifespan and scale parasitism assessments will be performed within the greenhouse
and laboratory. Also, micro‐wasps captured from the forest will periodically be reintroduced into the
greenhouse population to ensure a genetically superior population adapted to Christmas Island conditions is
maintained.
Monitoring of micro‐wasps, scale insects, yellow crazy ants and red crabs will occur on an annual/biannual
basis until such time as super‐colonies are suppressed island‐wide and/or funding for the project is
exhausted.
20Indirect biological control and rainforest recovery
The indirect biological control of yellow crazy ants will significantly reduce the ecological imbalance that has
caused havoc with Christmas Island flora and fauna over the last two decades. In the absence of high
density yellow crazy ants and yellow lac scale, the forest will have an opportunity to recover to a balanced
ecology where red crabs again drive the island’s ecology as the keystone species.
These red crabs along with other land crabs, insects, birds and reptiles will have the opportunity to return to
areas previously dominated by yellow crazy ants. The trees themselves will also have a reprieve with
increased leaf area and longevity, less sooty mould and reduced stress overall. A healthy ecosystem will also
be able to resist and even exclude invasive species such as the giant African land snail.
Eradication of yellow crazy ants and yellow lac scale is not an option for Christmas Island and it is now
considered a part of their global range. However, hopefully Christmas Island will also become home to the
micro‐wasp control agent Tachardiaephagus somervillei, a species with the capability to reduce yellow crazy
ant super‐colony formation and put the distorted ecology of Christmas Island back in balance.
A diagrammatic representation of the historical impacts of yellow crazy ants and scale insects on the red crab
population along with chemical control campaigns and the possible future state following the introduction of
Tachardiaephagus somervillei. The fluctuating lines represent seasonal population increases and decreases
gradually reaching equilibrium.
21Contributors and contacts
Christmas Island National Park
Dion Maple: Invasive Species Project Officer
Phone: +61 8 9164 8700
Fax: +61 8 9164 8755
Email: christmasislandnp@environment.gov.au
La Trobe University
Dr Dennis O’Dowd: Project Management
Dr Peter Green: Project Management
Dr Gabor Neumann: Biological Control Professional
Dr Sara Witt man: Ecologist
Further reading
Christmas Island National Park: http://www.parksaustralia.gov.au/christmas/index.html
Loss of biodiversity and ecosystem integrity following invasion by the Yellow Crazy Ant (Anoplolepis
gracilipes) on Christmas Island, Indian Ocean: http://www.environment.gov.au/node/14577
EPBC Act Referral: Importation, rearing and release of Tachardiaephagus somervillei (Hymenoptera:
Encyrtidae) as a biological control agent for the yellow lac scale Tachardina aurantiaca (Hemiptera:
Kerriidae) on Christmas Island, Indian Ocean. http://www.environment.gov.au/cgi‐
bin/epbc/epbc_ap.pl?name=current_referral_detail&proposal_id=6836
References
Narayanan, E.S. (1962) Pest of lac in India. In: Mukhopadhyay B, Muthana MS (eds) A Monograph
on lac. Indian Lac Research Institute, Ranchi, pp 90‐113 and 330‐333.
Wetterer, J.K. (2005) Worldwide distribution and potential spread of the long‐legged ant,
Anoplolepis gracilipes (Hymenoptera: Formicidae). Sociobiology, 45, 77‐97.
22You can also read
