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Look at this cute, adorable, fuzzy little mongoose. It is a horrendous pest and a
nuisance and anyone who lives in Hawai’i will tell you: Do not be fooled by those beady little eyes.
Mongooses are not indigenous to the islands, but they live there now thanks to a failed
attempt at pest control. And yet, today, despite many catastrophic and well-publicized failures,
some people say that using invasive animals as pest control is cost-effective, self-sustaining,
and preferable to pesticides. Even in Hawai’i. To call this debate controversial would be an
understatement. But at the end of this video, you should understand why anybody would advocate for
doing this. So let’s take a careful look at the successes and failures of this strategy,
called biological control, the arguments for and against, and what we have learned that can help us
do a better job of it. The term biological control covers a broad category of techniques. It can
refer to things like conserving native predators to keep pests down. But we’re going to focus on
what people generally mean when they use the term: an attempt to control a particular pest species,
usually an invasive one, by introducing new predators to the ecosystem. You see, humans
have had this habit of introducing new pests to different areas, on purpose or accidentally. Rats,
for instance. We’ve brought rats to many islands via ships, including Hawai’i. Now in the places
they originally evolved, rats might not have been a big problem. “Pests” is, after all,
a word with a lot of human judgment attached. In those places, rats were part of a whole ecosystem,
complete with predators to keep them in check, like owls or foxes or cats. But in a new place,
like the islands of Hawai'i, those naturally rat-eating predators aren’t present. And without
the pressure those native predators put on the rats’ numbers, their population can explode and
start really messing things up. Humans might dislike rats because they eat the food we’re
growing, but rats also are a danger to many of Hawai'i’s native birds. They’ve caused plants to
disappear by eating all their seeds. They even eat sea turtle eggs. And rats are just one example.
There are plenty of invasive species out there. Now, we can try to control rat numbers with traps,
or hunting, or even poison, but these can be difficult to use. Rats are often pretty good
at avoiding traps, for instance, and putting rat poison all over the forest would be difficult,
expensive, and, you know, there are other animals out there who we definitely don’t want to poison.
So one idea to control rat numbers is to bring in some of those rat-eating predators instead,
since not having them around kind of led to the problem in the first place. This is the basic
idea behind biological control, as we have defined it. The predator, or sometimes a disease-causing
pathogen, puts pressure back on the introduced species. That’s a key point, since predator-prey
interactions in nature basically never wipe out the prey species. Think of it more like a thumb on
the scale, where the predator can manage the prey population enough that it’s no longer a problem.
To be fair, it doesn’t always work out that way. One study, looking at using insects to control
other insects, found that only 11% of attempts achieved complete control. Most of the time,
the control agent couldn’t establish itself well enough in the ecosystem to work and just kind
of died off. But when it does work, biological control can be very effective. And, better yet,
it can be self-sustaining, meaning you only have to do it once. If you were using rat poison,
you might have to hire a bunch of people every year to go out and spread bait. But predators
can just kind of refresh the program themselves every time they have babies, with their numbers
expanding and contracting as prey populations change. Now, that might all sound really nice.
You’re restoring balance by having both prey and predator, right? But as elegant as this may sound,
if you ask Hawai'ians about it, well, they may have some other ideas. In fact,
let’s talk about Hawai'i in depth real quick. Because Hawai'i has some of the biggest history
with this whole scheme, for both good and bad. Now, Hawai'i, of course, is an island chain. Very
far out in the ocean away from everything else, and it consequently has a lot of really unique
species there. Endemic species, or the kind of things that aren’t found anywhere else. You’ve got
birds like the i’iwi and the pueo, mammals like the Hawai'ian monk seal, and a ton of different
plants like the koa tree or the hāpuʻu tree fern. These islands may in fact be home to more than
10,000 unique forms of life. But, notably, none of those species are mongooses, which are
originally found in Africa, Southern Europe, and the southerly parts of Asia. They are never found
in the middle of the Pacific Ocean. The story goes that, back in the 1880’s, there were a bunch
of plantation owners on the Big Island who were all in an association together. They were upset
about rats in their sugar cane fields and got the idea from basically some dude in the Caribbean to
introduce mongooses to their fields. So they did. And the idea spread to other Hawai'ian islands.
But then the whole idea backfired. Mongooses don’t just eat rats. They eat a lot of things, including
some of those really cool native species. And once the mongoose was out of the bag, so to speak,
there wasn’t any putting them back. They spread and went off into the wild and had little mongoose
babies and they are now just kind of there, even today. And at the time it was totally legal to do
this! No regulations existed until 1890, when King Kalākaua of Hawai'i passed some laws,
but by then it was too late. And this wasn’t the only example. One person, William Hillebrand,
privately introduced the mynah bird to Hawai'i in 1865 to control army worms. Today, they compete
with native birds for food. And of course, Hawai'i isn’t the only place this has played out. Cane
toads in Australia may be the most famous example of biological control gone wrong. Introduced from
South America to control beetle infestations, they instead killed a bunch of native creatures. They
didn’t even eat the beetles. In fact, they may have made it worse by eating the few species that
were eating the beetles already. So yes, there’s a lot of examples of biological control going bad,
often due to the hasty introduction of generalists, species who are able to eat
or affect multiple prey species. Cane toads are often said to eat anything they can fit their
mouth around. And today, we know in hindsight that even without direct predation for a control agent
to mess up an ecosystem. For example, the control agents themselves might end up as unexpected food
sources for nearby predators. There was a case where scientists introduced a type of fly to an
area, only for the nearby deer mice to end up developing a taste for them. The influx of new
food made mouse numbers more than double, much to the dismay of everything else that the mice liked
to eat. It’s also possible that, even though a certain agent might be fine here and now,
that things will change in the future. Critters can spread beyond the original release area,
or climate change may force them to move. Today, some of the worst invasive species anywhere were
introduced as biological control agents. So it makes sense that people would be hesitant. But
in kind of a mid-episode twist, it has worked sometimes. Even in Hawai'i. For instance,
one of the invasive species that’s come to Hawai'i is the prickly pear cactus. Prickly pears are
native to the American tropics. They were likely introduced to Hawai'i sometime around 1809,
and quickly took over large areas in some of the drier parts of the islands. In the
1940’s and 50’s, scientists looked at a couple of different species to try to control the cactus,
including the cactus moth Cactoblastis. This time they actually tested a bunch of candidate species
before picking one to release, which is nice since it turned out some of the candidates also liked
eating pineapple. Which aren’t native to Hawai’i either, but we like them because it’s pineapple,
but I digress. The cactus moths worked and they’ve been keeping prickly pear in check ever since.
They’ve even been used in other places too, like India. Another invasive plant, strawberry guava,
was controlled after a 2005 release of a scale insect. In that case, researchers took a whole 15
years to investigate it before daring to release the insect. Elsewhere, ladybugs and a type of
fly have been credited with saving the California citrus industry from a pest called cottony cushion
scale. Then there’s cassava, a staple food that feeds 200 million people in sub-Saharan Africa.
50% of it was wiped out by the cassava mealybug, until the bug was brought under control by a
biological control agent. So there are examples of biological controls seeming to work fairly well,
and without major negative consequences to the rest of the environment. So the question then
becomes, what made these biological controls work where the others didn’t? For one thing,
unlike those early releases in the 1800’s by just, like, anybody who felt like doing it, biological
control is now usually subject to regulation and enforcement. In the United States, there
were major laws passed in the 1970’s, such as the National Environmental Policy Act. Many countries
require risk assessments now before signing off on any real-life experiments. We’ve gotten
much pickier about what makes a good biological control agent as well. Do not use generalists,
for one thing. Today, scientists need to do some sort of test with relevant native species to see
if the agent will affect them. If you’re looking for something to control a plant, for instance,
you’d need to do tests where you give candidates the option to feed, lay eggs on, or grow on the
target plant. You’d also give them the option to choose other plants that are present in the area,
and hope they don’t. This may mean looking at several dozen candidates. Some scientists have
also suggested expanding this process to include examining things like how the critters behave,
or the chemicals they might give off, to understand as many potential interactions
with the environment as possible. Scientists may also use models to predict what populations
may look like years down the road and follow up with post-release monitoring to confirm. That is
not to say the process is now foolproof. In the 1960’s, a type of weevil was introduced
to Canada to try to control an invasive thistle. Scientists thought these weevils were specialists,
but it turned out they’re not and it began to attack native thistles as well. The scientists
had tested the weevil before, but blind spots in how they set up the tests meant they didn’t
realize there was a problem until it was too late. So mistakes can still be made. Systems must still
be refined. And that leaves us with a question: is biological control a good idea? Well, I mean,
ideally, we’d just stop introducing invasive species, period. But some are already here,
and we cannot go back in time. Proponents of biological control say it’s better than letting
invasive species wreck things. And they do appear to have helped save Hawai'i from a few invasive
species. We have examples that they can point to where things seem to have worked out. What’s more,
modern examples of biological control have a much better track record than the days of doing
whatever popped into your head. But it requires a lot of study to get this right. And if you don’t,
it is usually a mistake that you are never able to take back, as those mongooses know very well.
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