Research suggests that resistance is spreading as a result of the use of rodenticide actives bromadiolone and difenacoum. This is especially prevalent in the south of England
The number of rats and mice has risen sharply during the COVID-19 pandemic, according to the British Pest Control Association (BPCA). A COVID-19 impact study carried out by the BPCA has revealed 51% of pest controllers have reported a rise in rat activity and 41% have reported a rise in mouse activity.
Rats can breed unexpectedly quickly with one breeding pair multiplying to over 1,250 in just one year. Rats and mice represent a significant threat to poultry farms and their control is becoming more difficult as numbers rise, and resistant rodents spread.
Unfortunately, resistance to some anticoagulant rodenticide baits is a growing issue throughout the UK. Research published by the University of Reading Vertebrate Pests Unit (VPU) suggests that resistance is spreading as a result of two rodenticide actives, bromadiolone and difenacoum.
These are the active ingredients in a number of common and widely available rodenticide baits. However, a growing number of rodents are showing signs of resistance to these two actives. Cases are more prevalent in the south of England and a map showing the instances and concentrations of resistant rodent is available on the University of Reading website.
There are three anticoagulant rodenticide actives that rodents have not developed resistance to. These are flocoumafen, brodifacoum and difethialone. “Farmers can help to reduce the further spread of resistance by using products that contain one of these actives. By continuing to use bromadiolone and difenacoum in areas with resistance, farmers may unknowingly be increasing the population size of resistant rodents”, explains Helen Ainsworth, pest control specialist, at BASF.
Triple risk
Rats and mice spread disease and thrive in poultry units where food, water and shelter are plentiful. Rodents are a risk to bird health, housing and equipment. This is because they eat and spoil food with faeces and urine, cause damage by gnawing, and spread diseases such as salmonellosis, pasteurella and leptospirosis. “Controlling rodents is not simply a case of putting some bait down. There are many factors that should be considered before rodenticides are used. If choosing to use rodenticides, farmers should always select a product that reduces the risk to non-target species and that rodents have no known resistance to,” says Ainsworth.
Rodenticide baits are perceived as the go-to solution for rat and mouse infestations. However, growing concern of the risk to non-target species and a desire to reduce the residues in the environment has led to changes in pack sizes to the general public and some farmers. “New 25 parts per million (ppm) rodenticides such as Storm Ultra Secure can be bought by any farmer and can be pulse baited to reduce the amount of rodenticide needed to establish control. Farmers should always follow the directions on the label and only use rodenticides when all other measures have been taken to reduce rodent activity,” advises Ainsworth.
Rodenticides should be a last resort because there are effective non-chemical measures that can be taken first. “A simple site walk on a regular basis can identify common causes of rodents gaining access to poultry sheds. Farmers should remove rubbish and weeds, check for gaps under doors or holes in brickwork and roofs that may allow rodents to enter.”
Understanding the extent of the problem on a farm is the first step to carrying out a suitable treatment method. “Looking for existing signs of rodents and marking areas on a site plan is a good way to understand the magnitude of an infestation and plan for it accordingly. Exposing areas by cutting back vegetation and removing any easily available food and water sources can help to reduce the risk of rodent numbers growing quickly,” says Ainsworth.
Extensive research has helped to create the Campaign for Responsible Rodenticide Use (CRRU) UK Code of Best Practice for rodent control and the safe use of rodenticides. The code advises that rodenticides, such as anticoagulants, should only be used for as long as is necessary to achieve satisfactory levels of rodent control, with a maximum of 35 days without re-evaluation. “Pulse baiting with a highly palatable, single feed anticoagulant bait such as Storm Ultra Secure will reduce the amount of bait placed per bait point compared to multi feed treatments. Using the pulse baiting technique, it is possible to gain control of even a significant infestation in 14 days which will help to reduce the risk to non-target species posed by bait being used for longer periods”, says Ainsworth.
A site plan and checklist that records the frequency and quantity of bait should be kept for audit and legal purposes. Sufficient bait stations should be placed to ensure baits are thoroughly distributed in accordance with the label directions for use. “Bait stations should be checked regularly, and unused bait should be collected at the end of a treatment to minimise the risk to birds and other non-target species. It is also important to remove and dispose of dead rodents as rodenticide residues can also cause harm to non-target species,” she adds.
Steps to effective rodent control
Walk the site to remove rubbish and look for signs of rodent activity.
Proof buildings by filling in gaps in brickwork and roofs or holes under doors.
Remove spilt or available food and water sources.
Create a site plan that details burrows and high rodent activity areas.
Using the label for guidance, place bait stations in areas where activity has been recorded and mark the location of the bait stations on the site plan.
Use bait in accordance with the label directions and monitor bait take at the intervals instructed on the label.
Continue to monitor the site and keep it tidy to reduce areas that rodents can use for harbourage.
Do not use rodenticides for more than 35 days without re-evaluating the site
If rodent activity continues after the bait treatment, contact a registered pest controller
Pulse Baiting in 10 steps
Pulse baiting can only be used with single feed anticoagulants and should always be carried out using the directions on the label.
Walk the site to remove rubbish and look for signs of rodents.
Proof buildings by filling in gaps in brickwork and roofs or holes under doors.
Remove spilt or available food and water sources.
Create a site plan that details burrows and high rodent activity areas.
Using the label for guidance, place bait stations in areas where activity has been recorded and mark the location of the bait stations on the site plan.
Check the bait stations and replace bait after 3 days. Do not tamper with bait stations during this period.
Continue to monitor the bait stations and replace bait every 7 days
Use three to four baiting “pulses”.
Search for and remove any dead rodent bodies.
Do not use rodenticides for more than 35 days without re-evaluating the site