Mouse control in predator-fenced sites - advice sought

[Dan Palmer]

Sweetwater is a private covenant situated in the south Chatham’s. It has a predator fence surrounding 4ha of bush and grass and is designed to exclude rats, cats, possums and other large mammals.  The fence is thought to also exclude adult mice. Bait stations are at 25m x 25m intervals and the first knock down of rats using brodifacoum was in March 2007 which initially resulted in zero tracking of mice.  The first mice print turned up in a tracking tunnel in March 2008 and more recently the tracking has been 100% in all tracking tunnels.  It is unsure if this is a result of a resident mouse population, or if they have moved in through the fence or over the fence via a fallen tree in the winter of 2007. Though the fence is not designed to exclude mice the owners of the site are keen to maintain the site at low mouse densities.

I am looking for suitable advice on appropriate methods of maintaining the site at low mouse densities over extended periods of time.
I am interested in any information on juvenile mouse penetration through weld mesh fencing.

[rgriffiths]

I would suggest talking to Raewyn Empson at the Karori Wildlife Sanctuary. They have been controlling mice for a number of years now.

My advice: an intensive grid (i.e. 25x25m) and a pulsed baiting regime.

Cheers
Richard

[Dave Houston]

At Macraes Flat they're using a grid of 25x25m arranged in a quincunx pattern (from above looks like a number five on a dice and it lowers the minimum distance between stations). Nathan McNally from Macraes said that this is being used inside the two main excluder fences at Macraes, and mice are generally non-detectable but they can erupt.

From looking at DOC's Pestlink summaries I see they used Brodifacoum (Pestoff Rodent Bait 20R) for the initial eradication attempt, but I'm not sure what they're using for control currently.  For a recent  post-incursion eradication on Mokoia Island, Protecta Bait Stations with Ditrac rodent blocks were used.  At Boundary stream they've been using Racumin (Coumatetralyl) blocks.

So it seems there are a few pesticide options and the grid sounds OK, but what happens next depends on what the objective is - keeping mice at a very low or zero density, or tolerating a low density of mice.  The former will probably require having toxin out there most of the time, while in the latter case then regular running of tracking tunnels (monthly) and then responding with toxin when a threshold is reached (detection/10%?) might do the trick.

[Dave Houston]

This response from Raewyn Empson from the Karori Wildlife Sanctuary

Our fence has gaps in mesh of 43m x 6mm – if they distort 1mm to 43x7 mm then small mice can get in. Not sure what mesh you have? My understanding is that 6mm x 6mm will keep mice out – we have kept mice out of a 1ha enclosure for 2 years now with a mesh that is 4.8x12mm. I believe design of tophat has a lot to do with keeping mice out – our original one was risky but modifications have made it secure now we believe.

We have bait stations on a 25m x 50m grid throughout our 225 ha. We now aim to bait them once annually (usually in winter) with brodifacoum and remove bait after a month. We don’t want to maintain poison in bait stations year round to minimise risk to nontargets. That knocks them back severely, to the extent that we often cannot detect them for several months (we have 3 index traplines we run every 2 months).Then we see a gradual increase over summer of assumed immigrants and then locally bred animals – we have detected breeding at low densities year round but not at high densities. This means that maintaining them at low densities is problematic in that any remaining mice will breed and population build up rapidly without ongoing poisoning (which we don’t want to do). We keep an eye on mouse densities – our peaks have not been any higher than our troughs pre-control & we have reduced the population on average over the whole year by c 80-90 % so we think the valley is safer overall, but not year-round. It has helped us to monitor for incursions of rats & mustelids using tracking tunnels that was problematic pre-mouse control (because they were so numerous they ate the bait and left footprints on tracking papers making it difficult to detect anything else) but are a potential ongoing issue.

Hope this helps

Raewyn

[Bruce McKinlay]

You might want to consider using mouse motels. Here is a photo of a design suggested for use inside our skinks fences.

[Andy Hutcheon]

I can fill in a bit more detail about the Macraes operation. Initial eradication of rodents was with Pestoff 20R in the 25m quincunx grid (max distance from bait station ~8m). One fence has shown no tracking of mice since. The other, over three years, has shown mouse tracking 3 times (we run a 50m grid of tracking tunnels monthly). Each time, tracking has started in autumn as the weather became colder. First and third occasions we saw localised tracking, removed a single mouse via trapping. The year between those two we saw an erruption. Localised tracking was followed by localised trapping of an apparently breeding population (we began to catch adults of both sexes and juveniles) which was abandoned as tracking approached 100%. A mid-winter Pestoff operation resulted in no further sign (until ten months or so later).

I don't know whether we have an undetectable resident population that sometimes crosses over into detectability (although I don't see how that would be consistent with the interleaving of "catch one" and "erruption" that we have experienced). The alternative is that mice have got in several times, plausible over several years with a combination of staff taking equipment and other materials into the fenced area regularly and occasional potential breaches (despite our best efforts: floods, erosion and general weathering sometimes open gaps that a mouce could slip through in the short time before we find and fix them). Either scenario is plausible: mice are insidious enough that it may only be possible ro render them undetectable until the next time.

I'd advise monitoring year-round, local reaction (e.g. snap trapping if desirable fauna in the area is compatible) to localised tracking, treat the whole area with an appropriate pesticide in mid winter if the tracking spreads.

[Steptoe]

We breed capitive kakariki, and a few other exotic to cover feeding costs
Yr round vermin maintaince is a must to maintain a nil or near nil population...But baiting in the avairies is expensive using  kiwicare Natural NO Rats  which is non toxic to the kakariki...well not exactly scientific trials , but we have not lost any or any got sick..

We also use wire crayfish type traps, with limited success...yet the strange part we are the only ones who have limited success (?)
Other bird breeders consistantly get several mice per night.

Maybe because of our poison maintance program, over many yrs, it is the lack of population limitting success

The "motel " above  would that be suitable in the avairies?
how does it work?

[Bruce McKinlay]

The motel is designed to show whether a mouse has passed through the area by leaving sign in the motel.  They are not nearly as effective in an aviary situation as a snap trap grid but are more designed for use in sites where visitation is in frequent (eg islands).

One way to test whether your poisoning is being effective is to stop it foe a period and see whats happens to your traps success rates.   

[Steptoe]

Im no expert vermon just a private kakariki breeder..
Im not interested in how many, but simply if they exist or not.

One way to test whether your poisoning is being effective is to stop it

When bait stations have not been touched , or new locations are not touched, surely this would mean there is no vermin left ??