15 January 1999

Second generation bait resistance is big fear with rats

By James Garner

DESPITE recent reports, rat numbers remain at only 6-7m but fears of resistance to second generation anti-coagulant baits could mean rat populations are set to increase and control methods must improve.

Controlling rats seems to be more difficult now then ever before, and with tighter welfare and hygiene regulations livestock producers may have to re-think their policy.

Two main problems exist with rat control says Yorks-based Central Science Laboratory, rodent biologist Roger Quy. "These are rats not eating the bait laid down for them, and rats developing resistance, when they eat poison but dont die.

"Resistance to first generation poisons such as warfarin is becoming widespread throughout the country as genes of rats which developed resistance are maintained because susceptible rats die.

"Resistance to stronger second generation anti-coagulants is also increasing and often goes unnoticed for some time.

"We think this has happened in Berks, where outdoor rats are resistant to all anti-coagulant poison licensed for outdoor use. Indoor licensed poisons can control these rats but are harmful to other animals and use is restricted."

Additionally, producers accept a small number of rats. This means its difficult to know whether those rats left are resistant or not, he says. "It also comes down to cost and how much you are prepared to spend on control.

"Where theres animal feedstuffs, rats are often disturbed so they are more likely to take bait offered because they dont have time to gorge on available feed.

"Its also true that in some pig and poultry feeds there are high levels of vitamin K. Anti-coagulant resistant rats seem to have the ability to turn this into the type of vitamin K which allows them to overcome poisoning more quickly, meaning they are harder to control."

Likewise rats feeding on kale, which contains vitamin K in the leaves, are difficult to control, he warns.

Controlling rats is best done by a concerted effort, says Mr Quy. "An intensive period of poisoning can wipe out rats until re-invasion occurs. I prefer this to permanent baiting when bait stations are constantly replenished and so are a constant threat to non-target animals like field and harvest mice.

"Permanent baiting will kill susceptible rats and retain a large resistant population that will be harder to control."

Mr Quy says that rodent populations are not monitored closely enough, meaning that increases in numbers are not noticed until its too late.

Monitoring bait take is important, he says. "You dont want to underbait and you must be able to tell after two weeks whether you are getting any measure of control.

"When there is no sign of control, you must establish why. Are rats eating bait or not? Or are they eating bait and not dying?

Not dying

"Our worry is that in many cases rats are eating bait and not dying. Then rats with a low resistance mate producing offspring with a variation in genes to poison resistance. Some will be highly resistant, extremely difficult to control and so more likely to breed."

Spot rat activity by looking for tell-tale signs, says Mr Quy. "Polished runs, burrow entrances, runs through straw bales, chewed baler twine, fresh droppings, gnawing and seeing rats themselves.

"Poisoning in bait stations is the most common method now and one preferred by the Health and Safety Executive. But rats take a long time to become used to them and by the time they do, you have a greater problem to overcome.

"Baiting burrows is more effective because you have a better coverage of rats in the area and quicker control. However rats will kick bait out of burrows which can mean some non-target animals become victims."

A short, sharp period of poisoning can gain control, but where a low level of rats still exist after a couple of weeks, producers should consider using traps to clear out the resistant rats, says Mr Quy.

According to Ossett-based Killgerm Chemicals biologist Nigel Binns, often too little bait is used. "Putting down the minimum amount of poison will not get enough control.

"Some more powerful poisons are not available to producers so its more difficult to get efficient control with over-the-counter products."

However producers who poison rats themselves often lack the detailed knowledge of rat behaviour, making control more difficult, says Mr Binns. "Its important to a have bait points in the right place and to have enough of them.

"Poison is expensive and you should have bait points every ten paces as recommended but this can seem expensive," he adds.

RAT CONTROL

&#8226 Monitor bait uptake.

&#8226 Use poison for two weeks.

&#8226 Try trapping remaining rats.

RATCONTROL

&#8226 Monitor bait uptake.

&#8226 Use poison for two weeks.

&#8226 Try trapping remaining rats.

Producers should consider using traps to kill any resistant rats remaining after an intensive period of poisoning. says rodent biologist Roger Quy.