Poisoned rat carcass found - assessing the risk of invertebrate and bird losses due to rat poison

We see a bait station has recently been placed in the canopy margin below Gahnia Grove's Arena or CHF Bank.

We found a dead rat

It was astonishingly blue, so we turned it over with a stick to see if the corpse contained a large exposed piece of bait. (We could not de termine this, but nothing fell out when we turned it).

As the blue was so intense and covered such a large area, we wondered whether it might be a viable mass of bait, and thought of the invertebrates and insect-eating birds like like wax-eye, or the tui we regularly see feeding in ground recently disturbed by hand-weeding.

We have now learned that Brodifacoum is not used in urban reserves due to the secondary poisoning issue. The baits being used currently are Bromadiolone or Diphacinone.

Brodifacoume
However, the information about Brodifacoume secondary poisoning of fauna was of interest, as was information about the toxicity of Bromadiolone.

We have our own bait stations (currently using Bromadiolone) at our home on a nearby margin of Eskdale Reserve. In the last few years we have not seen the previously-regular winter visits of wax-eyes, or huhu bugs, or wetas, and in the last year we have seen no praying mantis.

We note other possible explanations for the reduction in invertebrates at our home - neighbouring cats, rats, and outdoor insecticide use by neighbours, though this is unlikely since none of the neighbours maintains a garden. The absence of huhu mothand praying mantis sightings in the house could be due solely to the natural pyrethrum atomizer we use to keep the Gisborne cockroaches mostly outside (since they became distressingly numerous, and it is unpleasant to wake with one clinging to one's face, which happened more than once).

Regarding Brodifacoume, we found these studies, suggesting tree weta and ground-foraging birds are at risk. This is concerning due to the number of observations we have made of both blackbirds and tui, both adults and juveniles, foraging on the ground within 10 m of the dead rat found.

This 2006 study concludes that "Ectothermic vertebrates, though at low risk of toxicosis themselves, may act as vectors of brodifacoum and create a risk of secondary poisoning to native birds. The effectiveness of using poison bait to protect mammal-free ecosystems is uncertain, due to the abundance of alternative food supplies available to an invading rodent. However, where sustained brodifacoum use is deemed appropriate, the role of reptiles as consumers and vectors of anticoagulant poison should be a research priority." Is there subsequent research n th e issue?

Another of the studies at the above link states "... the researchers report on brodifacoum residues detected in dead Stewart Island robin nestlings. Thirteen dead nestlings were collected 3-4 months after the brodifacoum application on Ulva Island. Twelve of the nestlings were found to contain brodifacoum at levels known to kill adult birds of other species, suggesting exposure to the poison was lethal. The anticoagulant is highly toxic to birds as well as rats.

The researchers were unable to confirm that the residue was the definitely cause of death in these cases as obtaining the 13 dead nestlings was opportunistic and they were unable to test residue levels in live nestlings for comparison. They do, however, describe their work as “the first apparent case of secondary brodifacoum exposure and subsequent poisoning in nestlings of an insectivorous passerine…”

The source of the residue in the nestlings was likely to be secondary poisoning, as death occurred 52-92 days after the brodifacoum application and the researchers note: “Our results highlight the potential role of invertebrates as vectors of anticoagulant rodenticides in the environment, as well as the need for further research on this exposure pathway.

The full article is published in the New Zealand Journal of Ecology and is freely available".

Posted on September 16, 2019 12:11 AM by kaipatiki_naturewatch