Aspects of the ecology of forest invertebrates and the use of Brodifacoum

• Main Author: Craddock, Paul
• Other Authors: Associate Professor Mick Clout, Dr Dianne Brunton, Dr Carol Stewart
• Format: Thesis
• Language: English
• Published: ResearchSpace@Auckland 2003
• Subjects: New Zealand; Rattus rattus
• Online Access: http://hdl.handle.net/2292/316

Brodifacoum is a potent second-generation anti-coagulant toxin routinely used to control problem populations of introduced mammalian pests (such as possums (Trichosurus vulpecula), rats (Rattus rattus) and mice (Mus musculus) in New Zealand forests. The toxin is packaged in the form of cereal bait pellets, which are distributed into managed forest locations in bait stations or by aerial drops. The toxin is persistent in vertebrate tissues, and there has been increasing concern recently about the potential for secondary poisoning of threatened insectivorous bird species via invertebrates that have fed on bait containing brodifacoum. This thesis studied relevant aspects of the ecology of forest invertebrates and the use of brodifacoum bait at Trounson Kauri Park, Katui Scenic Reserve and Wenderholm Regional Park, North Island, New Zealand. Video recording equipment was used to monitor invertebrate activity at bait stations, revealing that weta (Orthoptera) and cockroaches (Blattodea) were significant users of bait stations loaded with brodifacoum bait. These animals were attracted to full bait stations, and could spend considerable time in contact with toxic bait. Weta were even observed grazing poison baits. Pitfall traps, ‘weta boxes’ and rodent tracking tunnels were used to sample the invertebrate populations in the three study areas. Wetas and beetles were present in significantly greater numbers at Trounson and Wenderholm compared to Katui. It was concluded that this difference was due to the reduction in pest mammal populations, resulting from the intensive pest control at the two former sites. Invertebrates captured in pitfall traps were sent for brodifacoum residue analysis to determine if brodifacoum was present in their tissues. Weta, cockroaches, beetles and other miscellaneous invertebrate species recorded significant residues of brodifacoum (up to 7.47 µg/g). Residues were found to be dependent on the loading of toxic bait into stations. Levels recorded after bait was loaded were significantly higher than levels recorded before bait was loaded in bait stations. Invertebrates carrying brodifacoum were found to disperse up to 10 metres from the source of the toxin (loaded bait stations). Residue levels recorded decreased significantly the further away from the full bait stations that samples were taken. After toxic bait had been removed from bait stations, residue levels in invertebrates took in excess of 4 weeks to return to background levels. Trace levels of brodifacoum were still detectable up to 10 weeks after bait had been removed. Physiological experiments on the effect of brodifacoum consumption on captive locusts found no evident harmful effects. Locusts fed readily on toxic bait with no significant increase in mortality. They excreted brodifacoum rapidly, and there was no evidence of long-term bio-accumulation of the toxin. A risk assessment was made exploring the potential for invertebrates to secondarily poison insectivorous bird species. This found that bird species that consume substantial numbers of weta, cockroaches and beetles (eg. kiwi (Apteryx spp), ruru (Ninox novaeseelandiae) and robin (Petroica australis)) are at risk of secondary poisoning from their food supply during pest control operations using brodifacoum. The risk is higher during and immediately after bait containing brodifacoum is loaded in bait stations. Risks decreases progressively after bait is removed, as invertebrates excrete any brodifacoum residues present in their tissues. The risk of secondary poisoning is likely to be greater during warmer months, due to increased invertebrate activity (and residues in invertebrates) during this time. The frequency of consumption of contaminated invertebrates by birds was also determined to affect risk of secondary poisoning. A series of management recommendations are made to reduce risks of secondary poisoning when using brodifacoum for pest control in New Zealand.