The danger of using rat poison was brought home forcefully to a local Richmond egg farmer recently when he discovered one of his resident Barn Owls dead.
According to Graham Willson of Woodlands farm, he had been obliged to set the baited rat traps to comply with the South African Food Safety and Inspection Services Regulations (SAFSIS), which stipulate that these measures must be taken if he wishes to sell eggs to supermarkets. He said he had been assured by the pest control company that had laid the bait that the poison used (Finale) was relatively safe.
An autopsy conducted by the Allerton Laboratory of the KwaZulu-Natal Agriculture, Environmental Affairs and Rural Development subsequently confirmed Willson’s initial suspicion that the bird had indeed died as a result of ingesting rat poison. It listed, among other symptoms, multiple ruptures of the liver, severe segmental intestinal haemorrhage and severely congested muscle tissue and kidneys. The rat itself was found, still in a partially digested state, in the owl’s gizzard.
When Willson confronted the company that had placed the poison bait with these results, it conceded that perhaps it should have used Racumin, a product that greatly reduces the risk of secondary poisoning of no target animals.
Since owls can play a very important role in rodent management, Willson’s feelings are that the measures stipulated in terms of the regulations could be counterproductive, especially since he did not have a rat problem in the first place (he also keeps cats for this purpose).
He pointed out that other relatively harmless species, such as ordinary field mice, could be lured out of their natural habitat by the poison bait and in turn also be fed upon by owls. He felt that the regulations are too rigid and that natural forms of control, which do not impact negatively on the environment, should be both encouraged and allowed.
Pam Stuckenberg of the Merrivale-based Centre for the Rehabilitation of Wildlife (Crow) said she was not surprised to hear of this incident as this sort of poisoning has become increasingly common with results that “are horrible to see”.
What concerns her are that many of the more dangerous rat poisons are freely available over the counter despite the fact that they do pose a risk of secondary and primary poisoning to nontarget animals such as dogs, cats, wild animals and humans (especially children). If a rat poison has to be used she strongly recommends the use of one of the less harmful types such as Racumin (see sidebar).
Ben Hoffman of the Africa Bird of Prey Sanctuary near Camperdown confirmed her views, saying that the poisoning of owls has become a huge problem and that in the vast majority of cases the affected bird would have died long before it made it to a vet or a rehabilitation centre such as his.
He said that in the case of Warfarin-based products, the poisons work by thinning the animal’s blood and causing internal bleeding. This in turn makes them thirsty and extremely slow moving, making them easy prey not only for owls but other raptors such as Crowned Eagles and African Goshawks.
Because of the dangers their products posed, he felt the pesticide companies need to become far more environmentally aware and advise their clients to use the most bio- friendly products.
Like Stuckenberg he was concerned about the easy availability of rat poisons such as Finale and Rattex. People needed to become more responsible and think about the long-term consequences of using poison bait. If traps have to be used he said the old-fashioned rat trap, utilising a mixture of brown bread and peanut butter as bait, is the most effective in that it is target specific and environmentally friendly.
A strong advocate of natural forms of control, Hoffman said that owls play a very important role in rodent management. Research in the United Kingdom has shown that a pair of Barn Owls can kill and consume between 1 000 to 2 000 rats a year.
In Malaya, a government-backed programme encouraging the use of Barn Owls had led to the virtual elimination of the rodent problem in the country’s palm oil industry. Closer to home Sappi has stopped using poison on their seedlings and started erecting posts so that owls have vantage points to hunt from.
An Urban Owl Project has also been initiated to encourage owls and stop the use of poisons. Owls can be lured into gardens and onto industrial premises and farms by erecting owl nesting boxes. Rehabilitated owls are also being released on user-friendly farms. He encouraged anybody interested in either buying or building their own owl box to go onto the Internet or make use of his website.
Dr Gerhard H. Verdoorn of Griffon Agrochemical-Enviromental Consultants said rodenticides should be used with great circumspection as they do pose a risk of secondary and primary poisoning to non-target animals such as domestic dogs and cats, and wild animals. Here are some important aspects of rodenticide application.
Products with the lowest risk of secondary poisoning should be applied. In terms of the anti-coagulant rodenticides, only four first-generation anti-coagulants (in order of preference) are available in South Africa.
The least harmful (and least risk of secondary poisoning to birds) according to Verdoon’s information is coumatetralyl (trade name Racumin). The avian toxicity of coumatetralyl is low and thus there is strong reason to believe that the risk of poisoning to avian predators is insignificant. A target animal has to ingest the product at least three days in succession before the toxin will manifest in toxic symptoms. It means that an owl that eats a dead rat has a very limited risk of suffering secondary poisoning.
Second-generation rodenticides pose a great risk of secondary poisoning if owls or other animals eat dead or dying rodents. Active ingredients such as difethialone and bromadiolone are second-generation rodenticides. His advice is rather to use first-generation products due to the reduced risk of secondary poisoning.
There is no environmentally friendly rodenticide on the market. They all pose a risk but the first generation are less hazardous.
One could also try EcoMouse or EcoRat that is distributed by Efekto. It contains alpha-cellulose and blocks an animal’s intestines, resulting in internal infections and mortality.
It is very important to observe all instructions on labels of all rodenticides.