Experts say the mouse plague is unlikely to return to inland New South Wales, but its end has come at a hefty price.
- Farmers are being warned to stay vigilant despite the apparent decline in mouse numbers
- The plague devastated crops in the state, damaged equipment and took a heavy mental toll
- A combination of natural phenomena seems to have extinguished the threat, clean harvesting and early baiting are key ways to keep the numbers down
The same La Niña weather patterns that caused widespread, major crop downgrades across the state seem to have prevented the expected mouse surge in spring.
Heavy, persistent rain, prolonged flooding and diligent baiting have helped to finally get on top of the nightmare plague that caused millions of dollars of damage and devastated communities for more than a year — but landholders are being urged to stay vigilant.
“Areas that are really wet will be not conducive to ongoing mouse breeding,” CSIRO researcher Steve Henry said.
“Farmers have also been really proactive about baiting mice early in the spring and as the crop was ripening we weren’t getting too many reports of really high numbers of mice.”
Mr Henry has been on the ground through the full length of the mouse plague, conducting tests and speaking with impacted landholders.
“Recently I’ve travelled through most of central NSW from as far south as Coleambally up through to Burren Junction and we didn’t find too many signs of mice,” he said.
“There were a few patches with mice in numbers that we would describe as moderate but not high — but even they were very patchy in their nature.”
Baiting hard, baiting early
Farmers are being praised for their fast response to mice when they did re-emerge after sheltering in their burrows through winter.
“One of the really good things that happened was the farmers were really switched on to mice in the early spring, when they were just about to start breeding or had only just started breeding,” Mr Henry said.
“When farmers were seeing any sorts of numbers in their developing crops, they’ve gone in and baited them really hard, really early.
A significant concern is what impact the rushed harvest could have, with growers racing to strip crops during rare dry periods between rain events.
The plague originally exploded after the winter crop harvest in the late spring of 2020 because mice had an abundance of feed due to the grain left on the ground after crops were stripped.
“The wet weather has led to a lot of crop being laid over on the ground and that makes it really hard to pick up,” Mr Henry said.
“When farmers have an opportunity to harvest crops that are still standing, they’re working really hard to get that crop off.
“The potential is that they’ll leave a lot of grain behind for mice for food.
“So through the summer, we’re saying while it’s good that there aren’t very many mice there now, we need to keep looking and make sure that they’re not building up again through the summer, and be prepared to bait when we do see numbers of mice.”
‘Be on the lookout’
People are being warned not to be complacent, given how rapidly the species can breed.
Mice reproduced so prolifically from January 2020 that experts said they easily numbered in the “many millions” by autumn this year.
“The message is that mice are one of the most successful species on Earth and they managed to survive these kind of events all the time,” Mr Henry said.
“They won’t survive in very high numbers through this La Niña period and they’ll have to breed up again.
“If people harvest cleanly, hopefully that creates an environment that’s not friendly for mice and we don’t see them breeding up again through the late summer and in autumn.”
The CSIRO is warning that the typical signs of a plague’s end are yet to be observed.
“We haven’t seen that classic crash where they literally disappear altogether,” Mr Henry said.
“For instance, just a couple of weeks ago, from Walbundrie in southern NSW, we had reports of a farmer who was stopping his header every couple of hours to clean mice off the sieves as he was harvesting his canola.”
How many mice make a plague?
Mice are commonly seen by farmers during summer, but how many mice is “normal” for a typical summer and how is a mouse plague declared?
Mr Henry said it was a complicated issue.
“That’s a ‘How long is a piece of string?’ question,” he said.
“Farmers often ask what the threshold is for damage — if you look at the science, the scientists really don’t agree on how many mice are important.
“Some scientists say 50 mice per hectare is the threshold for damage, other scientists say 200.
“The general rule of thumb for declaration of a plague is about 800 mice per hectare.
“If you’re in a situation where you’re seeing a few mice around … with about 100 burrows per hectare, it probably doesn’t seem to be that many.