Many growers and merchants may have been surprised and confused by claims by CSIRO and GRDC during the last mouse plague, that currently registered zinc phosphide mouse baits can “kill only 50% of mice” or “are 50% effective”, Dr Linton Staples* writes.
This led to an emergency permit for a 50 g/kg product (at twice the highest international use rate) to solve an apparent crisis, yet most grain producers have used 25 g/kg baits, particularly Mouseoff zinc phosphide (ZP), for 25 years with near total success.
The CSIRO tests pointed to poor mixing of the copy product tested, which had variation in the ZP on each grain, with some grains in the 25 g/kg product having less than 50% of the target dose and less than 45% of the correct average dose in the 50 g/kg product.
Despite this, reports showed both dose rates removed most mice, and this contradicts CSIRO claims. Perhaps it is poorly prepared copy products that contribute to those very few farmers needing to re-bait and spend more on mouse management.
Any other ZP-based product on the market is an attempt to copy Mouseoff but these are likely not formulated or produced the same way, and this may be a problem.
We suspect the so-called ZP resistant mice, claimed to have been discovered in SA by CSIRO (but also not statistically proven), are from a site treated repeatedly with an inferior product made on site.
The products used should be disclosed, along with the manufacturer of the test bait and the basis of why this bait was chosen for testing.
The majority of bait sold in the 2021–22 plague was Mouseoff 25 g/kg product and crops were saved, as usual, as has been the case for 25 years. Thankfully this allowed more crops to be saved with the available chemical supplies during the major plague at a time when global freight issues meant availability of ZP was limited.
Even so, ACTA spent more than $1 million on air freight of chemicals to save crops so we did not want to waste this. While a 50 g/kg product will not be likely to work less, it is irresponsible to use twice as much valuable chemical as is needed for mouse management.
Irrespective of the grain dose of ZP, a treated area might require another application over time in a “mouse hot spot” (i.e. sheer weight of mouse numbers) or if mice reinfiltrate, re-breed or if bait is incorrectly applied, but farmers understand this, and it is well understood – yet CSIRO recommended multiple regular applications of the high dose products.
This seems somewhat inconsistent with an implication that the 50 g/kg dose is somehow 100% effective. If it is as effective as claimed, why would this advice to regularly re-bait be necessary? Infiltration – and not dose rate is now being recognised as the cause of the need to re-bait on rare occasions.
Overuse of a product when it is not needed wastes landowners’ money and could even increase the risk of resistance – though none has been shown in normal usage.
For our part we urge all agronomists to check with their clients to see if any improvement has been noticed with 50 g/kg vs 25 g/kg products, despite the extra cost, and to focus on correct bait application using a quality bait, just as most have done for two and a half decades.
We continue to supply the 50 g/kg Mouseoff version in response to user demand. However, we do not think this approach is correct based on current data.
We recommend a well-made, quality 25 g/kg bait as the most cost-effective first choice, given our own good trial results with it and 25 years of user experience.
*Dr Linton Staples is a vertebrate pest control expert and managing director of Animal Control Technologies Australia (ACTA).
DISCLAIMER: The opinions expressed in this article are those of the author. Rural Business makes no representations about the merits of the comments made. Readers should make their assessment of the products best suited to their own needs and always follow registered label directions and any additional emergency permit use information provided by the APVMA.
The APVMA has now put additional requirements on farmers wishing to access the 50 g/kg product. After reviewing the extensive high quality trail data conducted by ACTA over many years it has advised:
“….the APVMA has not stated, nor concluded, that the currently registered 25 g/kg zinc phosphide products are not efficacious.
“Please note that the current permit, which authorises the supply and use of mouse baits containing 50 g/kg zinc phosphide, is constrained to time-limited emergency use situations in which local pest monitoring has been used to confirm that mice populations have reached a threshold where severe damage is being caused by plague level mouse populations.
“This permit, under no circumstances, represents a substitute for the management of typical mouse populations, beyond the scope of emergency use, with registered rodenticides containing 25 g/kg zinc phosphide. Indeed, the APVMA has not reviewed any data that would indicate the lower rate of 25 g/kg is not effective under normal levels of pest pressure (i.e., non-plague-level). As such, we remain satisfied that the 25 g/kg products are safe and effective against normal mouse populations”.
