Australia’s Deputy Prime Minister this week declared that “the only good mouse is a dead mouse.”
Few can have missed the reports in the media of the plague of mice currently afflicting New South Wales, having already caused devastation in Victoria and Queensland.
800 – 1,000 mice/hectare is considered a plague, and it should not be forgotten that a pair of mice can produce 500 offspring in a season, a new litter being born every three weeks. Normal numbers <50/hectare.
Individual farmers have estimated their losses as between 50,000 and 150,000 Australian dollars.
The plague is not a new phenomenon. There is one every few years, but the frequency seems to be increasing.
There is a photograph of a group of farmers in 1911, standing in front of a pile of 500,000 dead mice. The pile comes up to their chests.
Previous plagues have been treated with strychnine and zinc phosphide. Used properly, the maximum concentration of zinc phosphide should be 1kg/hectare, or three small grains/m².
It is, however, safe in the dead mouse’s body, unlike second generation rodenticides.
As the mice run out of food supplies, they turn on each other and start to eat the baby mice. It has not reached that state yet.
There have been several effects on poultry. Reports of depression of egg production by over 30%, mortality increasing to over 15%, egg quality being severely affected, as well as actual egg eating. Feed consumption has also been reported to have gone up by 50%.
The mice have also played havoc with house insulation, as well as electrical cables and computer equipment. They also chewed chick’s feet.
The Australian Pesticides and Veterinary Medicines Authority has not approved the use of bromadiolone, the NSW state has bought 5,000 litres of the second generation rodenticides, with which they plan to treat 95 tonnes of wheat, to distribute to affected farmers, as part of their A$50 million relief package.
The last plague, in 2011, cost over A$200 million in crop damage.
There are worries about the use of bromadiolone affecting predators further up the food chain.
The rodenticides will persist for up to 135 days in carcases.
A 2013 study detected bromadiolone in chicken eggs between 5 and 14 days after the chicken ingested the rodenticides. It is not clear how many of such eggs a human has to eat to get sick.