This is the true story of a cactus attack.
The story of a how the prickly pear ran rampant in Australia for decades, taking over millions
of hectares.
But even more interesting than that story is the story of how the cactus was controlled.
One of the most successful biological control stories of all time, that even many Australians
don’t know.
And it all has to do with this understated insect.
Prickly pears were introduced to Australia in the 1700s.
The idea was to start cultivating cochineal insects, which live on the cacti and feed
on them.
These insects when crushed, form the basis of a nice red dye, which you might have seen
on food packaging as carminic acid, cochineal, or 120.
Throughout the next century, into the 1800s, more cactus species were introduced, in several
waves.
This includes lots prickly pears, which are part of the Opuntia genus.
Obviously, introducing new species doesn’t always go as planned.
Beyond the intentional replanting of cacti for cochineal insects and for fencing, Opuntia
started to spread on its own.
It did this in 2 main ways.
The prickly pears of the prickly pear cactus are delicious, so Australia’s wildlife,
in particular birds, started munching on the fruits, and spreading the seeds.
This is a pretty simple mechanism – the bird eats the fruit with its mouth, the seed
travels through as the bird flies, or walks a distance from the original plant, and the
seed is deposited onto the ground via the cloaca.
But the more insidious spread came from the pads of the cacti themselves.
See, Opuntia pads, which are technically called cladodes can regrow into a whole new plant
if broken off.
Once on the ground, they grow roots of their own, and form a new plant.
Farmers trying to slash the cacti soon found that the cladodes left on the ground began
to reanimate and regrow.
Not to mention a particularly nasty flood in 1893 that damaged lots of cacti, and spread
their fallen limbs far wide to regrow where they washed up.
The prickly pear didn’t take over all at once, but once they do take hold, cacti, it
turns out, are somewhat prickly customers.
By the early 1900s, the prickly pear had taken hold over swathes of farmland in Queensland
and New South Wales.
It was completely dominating some parts of the land, leaving it unable to be used.
At its peak – Opuntia took up an area of land in Australia similar to the size of the
entirety of the island of Great Britain.
That’s about 250,000 square kilometres.
So we have a plant that’s super hard to get rid of because of its naturally defensive
cactus spines, and its innate ability to regenerate from its amputated limbs, and has Australian
animals in its thrall to spread its seeds via delicious fruits, that is dominating an
area about the size of Oregon.
So, Australia needed to do something.
They tried slashing, burning, and poisoning, but all these methods were either incredibly
labour intensive, thanks to the dense growth of cactus adorned with thorns), or had little
effect on the cactus, thanks to its ability to regenerate, or were expensive, or were
bad for livestock, or all of the above.
To further investigate other means of cactus control – the prickly pear travelling commission
was formed in 1912, followed by the Commonwealth Prickly Pear Board in 1919.
Researchers travelled to native cactus habitats in the Americas, where they hoped to find
a solution.
The idea was that the cactus had become a pest in Australia because of the lack of natural
adversaries.
In their native habitat, prickly pears have to contend with all kinds of diseases, fungi
and insects that didn’t exist in Australia.
So the researchers brought back many different kinds of insects to experiment on controlling
the cactus.
The cochineals, they found, were pretty good at eating cacti, in particular the tree pear
species – but the real heroes, were drab, brown moths, and their brilliant orange larvae
– Cactoblastis cactorum.
Just as a quick reminder, moths and butterflies start life as caterpillars, which are larvae,
which form pupae, which become moths, with lay eggs, which hatch larvae.
So, before I show you some of the most amazing before and after photos you’ve probably
ever seen – let’s find out how Cactoblastis cactorum was so successful.
Basically, they did this.
The larvae of the Cactoblastis moth are voracious, and they burrow into the cladodes of the prickly
pear and eat it from the inside.
Then, when they’re nice and fat they form pupae, undergo metamorphosis, become moths,
fly off to a new cactus, lay eggs in an egg stick that looks like a cactus spine, the
eggs hatch to release more larvae which burrow into the fresh cactus and the cycle continues.
Once scientists realised how successful the moth could be, they started breeding it en
masse in research stations.
From there, they distributed around 3 billion eggs to cactus infested areas between 1926
and 1930, including supplying farmers with Cactoblastis eggs free of charge.
Within the next decade, even in the next 5 years, the cactus population had been obliterated.
The mechanism by which the moths can find new cacti is actually REALLY cool, and has
to do with the cacti’s own adaption to arid environments.
I’ll link a good video that describes how cacti do photosynthesis in more detail, but
if you want a SUPER simple summary – plants have pores in their leaves called stomata,
through which they suck in carbon dioxide in order to strip the carbon and use it to
build sugar to store energy.
They do this by using much smaller bite-sized packets of energy, which they make using the
energy from light.
This whole process is called photosynthesis – and Kurzgesagt recently made a nice video
on these tiny energy packets (which I’m sure many of you have seen but if you haven’t
you should check it out).
Having your pores open all day causes you to lose some moisture to the atmosphere, and
that is particularly true if you’re in a very dry climate.
So, cacti, and other plants, have evolved a clever way of reducing their water loss.
It’s called CAM photosynthesis.
Basically they open their pores up at night, to suck in carbon dioxide.
Remember, photosynthesis needs light – so when the stomata are open at night when there
is no light, they store the carbon for later use.
During the day the pores are closed to stop evaporation and moisture loss, but that’s
ok because the stored carbon is then used for photosynthesis with the daylight.
That is a very long way of saying that the Cactoblastis moths can sense carbon dioxide
gradients.
So when then cacti are sucking in CO2 at night, they are creating a relative sink, which,
along with other compounds, the moths use to locate them.
So, back to our voracious larvae, the cacti which have been eaten from the inside finally
die.
This was unbelievably successful in terms of controlling the overgrowth of prickly pear
in Australia.
Land that had been completely overrun was cleared.
Farms that were abandoned were usable again.
This turned out to be one of the greatest ever biological control stories in history.
And aside for how industrious the distribution of eggs was, there was one defining reason
which made it so successful – the moths only like eating cactus.
All insect species were tested rigorously before being released, and unlike other introduced
species, they weren’t interested in other sources of food that Australia had to offer,
so they didn’t wreak environmental chaos of their own, but instead they reached an
equilibrium.
That means that once the cactus population had been reduced so much it couldn’t sustain
a large moth population – the excess moths just, died.
They didn’t go off and ruin native species.
There are still areas of wild prickly pear in Australia, and there is still Cactoblastis
feasting on it.
There is one more thing to say before closing out this story – Cactoblastis was a huge
success in Australia, even garnering tributes from several memorials – which you should
visit in the towns of Chinchilla, Boonarga, and Dalby if you’re ever in Queensland.
But, in the US, the Cactoblastis moth is more of a villain.
Since being introduced via the Caribbean, there are fears that Cactoblastis is endangering
native cactus species across the southern US, and could endanger Opuntia in Mexico as
well.
One region’s unprecedentedly successful biological pest control, is another region’s
threat to the delicate Cactaceae family in its native habitat.
Thanks for watching, I hope you at least enjoyed it, and at most learnt something.
I’ll see you next time.