Know your foe: The house mouse
Duncan Bosomworth, NPTA Technical Manager
Mus domesticus
As one of the recognised commensal rodents, and one of four species mentioned on rodenticide labels this delicate little rodent is one of the most common pests in the UK.
Originally thought to originate from the boarders of Russia and Iran and recognised in the UK as far back as the 9th Century BC they continue to be one of the most prolific pests in the world.
IDENTIFICATION AND BIOLOGY
Visually, the majority of mice in a colony aren’t much bigger than your thumb with a tail just shorter than the length of their head and body combined, and like all rodents their teeth are made up of four incisors, separated by a diastema, finishing with a few rows of molars.
House mice can be distinguished from other smaller species of rodents by a small notch being present in their upper incisors.
Colour is not a reliable feature if recognition, however fur is generally a light grey / brown with the underbelly being slightly lighter than the rest of the body, personally I have seen mice that were nearly black in colour though this was only in one particular location in East Anglia and was probably a result of local genetics.
LIFE CYCLE
Breeding rates are high, with breeding taking place potentially every 24-28 days, post-partum oestrus occurs with litter size typically producing 5-6 young, gestation occurring at 17-20 days, weaning at around 21 days and gaining sexual maturity at 8-12 weeks.
HABITS AND BEHAVIOUR
Mainly an internal pest of buildings, eating around a 10th of their body weight a day, they generally live close to their food supply, sometimes even in it!
In large towns and cities they can move freely from building to building given the multitude of small gaps that will be present in the structure of different buildings, whereas in more rural settings they move from building to building via an external route.
As infestations grow dominance structures are formed where breeding occurs with the most dominant males. Territories are marked with urine and are aggressively defended. This can cause several different colonies being found in one infested building, all living separate lives. Their reliance on drinking water isn’t overly crucial to their survival, the complete opposite to brown rats for example. If their food supply has a moisture content of 15-16% (very close in fact to that of stored grain for example) they don’t need a water supply allowing infestations to develop in many situations where other species may fail before they have even started.
Generally living inside (thus the name) and feeding off food or waste left out by us, they can become quite a challenge in inner city environments with quirky feeding habits developing over the generations. They can be neophobic but their inquisitive nature soon overcomes this instinctive behaviour.
SIGNIFICANCE AS A PEST IN THE UK
Weighing around 25g in adulthood they are small by comparison to our UK rat species but the damage they do is probably far greater; if you add in factors like; their small size means they can get into places where rats can’t, their habit of taking small morsels of food each time they feed which can cause more overall damage than that of a rat feeding in a similar location, litter size and frequency of producing young can lead to huge infestations causing widespread damage and contamination. They urinate almost everywhere spreading disease and pathogens as they go.
PROOFING AND EXCLUSION
Internal and external proofing is playing a growing part in the successful control of these rodents mainly due to various resistance mechanisms and the fact that glue-boards have now albeit been removed from our grasp.
TREATMENT
Problems with bait uptake have become very common in the major cites across the UK which makes successful treatment a real challenge; in rural setting this behaviour is far less apparent (poultry sites aside).
Due to widespread resistance to difenacoum and bromadiolone based rodenticides it is now recommended to use single feed products containing brodifacoum or using other none anti-coagulant rodenticides that are now available in the UK.
The use of single feed baits address’s the resistance or tolerance issues found across the UK but the risks of using them can be higher when compared to multi-feed baits in certain situations. That said most of the control is carried out internally so the effects on wildlife for example are reduced significantly.
Trapping can be the most viable alternative to using glue-boards to deal with house mice in everyday situations, but great skill and time is required to get comparative results.
Knowing the behaviour of these rodents is the answer to dealing with them, it all starts and ends with food.
