Australia’s southern and eastern agricultural regions are deep in the thick of a months-long plague of mice that has destroyed crops and overwhelmed farmers. The infestation comes after years of drought, devastating wildfires and a period of heavy rain that boosted plant growth, creating ideal conditions for the hungry rodents to reproduce exponentially. Now farms and fields are overrun with swarms of mice that have taken up residence in the walls of barns and homes.
Mice first appeared in Australia with the arrival of British colonists in 1788. These days the continent sees populations boom and die off every few years. But many farmers say this year’s infestation is the worst they have ever seen, and there is no sign that it is letting up soon. “The prolonged rain and the bumper crop of food means that the rodents have lots to eat for a very long time,” says Steven Belmain, an ecologist at the Natural Resources Institute at the University of Greenwich in England, who studies rodents’ roles as agricultural pests and disease vectors. Scientific American spoke with Belmain about how climate change has influenced the infestation in Australia, whether other parts of the world could see similar outbreaks and if the present one could lead to the spread of illness.
[An edited transcript of the interview follows.]
Is the current outbreak of mice in Australia surprising?
When the rain started quite severely, a lot of the scientists had an idea we were going to be in for a mouse plague. It wasn’t a great surprise to many of the experts because they have had these outbreaks going back more than 100 years in Australia. They’re periodic, and they are always sort of related to these bumper years of rainfall and wheat production. This one is quite large, but I think that is because the rainfall was particularly widespread. It’s predictable in the very short term—there is no great pattern to them. You can’t say once a decade or once every five years this happens; it really is down to the cycles of the rains.
But some scientists are worried this outbreak might extend longer than usual, right?
This year it’s been a fairly mild and wet summer, so that means vegetation is continuing to grow [into Australia’s winter]. A lot of the mice might survive until next year, and [then] there will be a better starting population. So next year could be even worse, depending on what happens in the next couple of months or so. Or it may just die out—that is very much down to what happens with rain. It’s still kind of early days to figure out which way it’s going to go.
What impact might this infestation have beyond agriculture? Is there risk of disease spread?
In times of pandemic, we become more sensitive to the fact that rodents are great disease transmitters. More than 60 different diseases are transmitted by rodents. But usually, when you have these massive explosions, the population is just expanding so quickly that it’s difficult for a disease to establish because it’s kind of a boom-and-bust situation. A lot of problems with disease tend to be much more chronic.
But in other parts of the world, particularly in parts of Africa and Asia where the disease burden is higher, they are worried about things such as leptospirosis and plague [the disease caused by Yersinia pestis], which follow the population cycles. So bigger population events do lead to increased disease transmission in some cases, but I don’t think that applies really very much to Australia.
What efforts are there to mitigate this infestation?
What they’re continuing to do is baiting with zinc phosphide. It’s a coating, what we call an acute poison. And when the animal eats it, it releases gas into their digestive system and causes them to die. Not a very nice way to die if you’re worried about humaneness. The alternatives are anticoagulant poisons, but those accumulate in the environment. So there are good reasons not to use those, particularly where you’re spreading [them] around by airplane. The advantage of zinc phosphide is that it doesn’t accumulate in the environment, so the danger of other animals having long-term exposure to some poison is minimized. But of course, when you put it out there, everything that eats it is going to die, so this could be a danger to pets.
It’s more about human behavior than about biology—people just have to do something. I don’t think the evidence of its effectiveness is very good. A lot of [what is required is] quite early prevention. And what has happened this year is: people [have been] somewhat taken by surprise, because they forget about the past. Some of the activity to manage [the mouse outbreak] has been delayed to some extent.
How has climate change impacted the current infestation? And how will it impact future population explosions?
Through the past decade or so, there’s been a very severe drought [in Australia]. The mouse population pretty much disappeared, and people thought, “Oh, no more mouse problems!” And that did have a knock-on impact into research and development because a lot of people said, “Oh, we don’t need all these mouse experts anymore.” Funding to that area of research kind of dried up. And so there has been a sort of “Okay, now we’ve got a problem again....” And there is no easy solution. It is driven by rainfall, and trying to manage that process is really quite difficult.
Are these outbreaks something that we just have to live with? Are they going to become more frequent? You could argue [that] with climate change, Australia is going to become much more of a drought-stricken country. Or perhaps these rainfall events are going to just be coming through much more severely, which would then drive the outbreaks to become more frequent. In some other parts of the world, we definitely know that climate change is going to have a bigger impact, particularly where you have extreme weather events such as cyclones and hurricanes.
Should the U.S. expect to experience anything similar to what we are seeing in Australia?
In the Southwest of the U.S., there are gophers, which do go through some outbreak scenarios, and their populations do cycle with El Niño or La Niña. The other rodents [in the U.S.] don’t really seem to go through those sort of boom-and-bust cycles—there’s just not as dramatic a change in the amount of rainfall coming. But a lot of the rodents in the U.S. are still cyclical on a seasonal basis, and they are clearly responding to agricultural production. If you go through the massive interior where there’s maize production, you will get increased numbers of rodents as the season progresses. But then there’s an end of the season. If we were going to have outbreaks [in the U.S.], we would have noticed it by now.
Is there a way to stop these periodic rodent outbreaks?
In Australia, if they stopped producing wheat throughout the wheat belt, there’d be no more outbreaks. But if you’re going to try to convince the Australian government to [do that], everyone [will] just laugh at you. Growing food is much more important. You could easily think of ways of taking the food away from the rodents, but then we need the food, too. Unfortunately, it’s a system we are going to be stuck with, so we might get better at managing it.
We can think about ways of growing crops much more synchronously. Asynchronous planting exacerbates problems. In some parts of Asia, if everyone grows the rice at the same time, then you have clear fallow periods that disrupt the population outbreaks. In Australia, I think, not all the wheat is necessarily grown at the same time. You could try to coordinate that a little bit better. But again, economics comes into this. When everything is synchronous, it may be good biologically, but economically, it means you need all your tractors ready at the same time to do the harvesting. Would it be economically viable? Probably not. Otherwise, we’d probably already be doing it.