A to Z of Pests

Seasonal Pest

House Mouse

Mus domesticus

Our featured pest this season is the House mouse. This is one of the most widespread of pests, present in every continent apart from Antarctica. Mice live in dark, undisturbed places, e.g. under floorboards, in wall voids, behind panelling and in roof spaces. They emerge in the late evening to forage for food.

Mice are extremely agile, able to climb up virtually any rough surface. They are also very good jumpers, and readily swim if necessary. They can squeeze through gaps of 6mm, little more than twice the thickness of the base of their tails.

This is why infestations can spread to all parts of a building, as mice explore and exploit the available resources. Proofing is therefore crucial for preventative pest control, and it is important that this is carried out carefully and systematically.

Appearance

The House mouse is 7 – 9.5cm in length, with a tail around the same length and weighs between 12 – 30g. Their relatively small feet & head and large eyes & ears distinguish them from a young Brown rat (Rattus norvegicus).

Life Cycle

Females are pregnant for 20 - 23 days. There are usually 5 or 6 litters a year, with typically 5 - 6 young in each litter. At birth the young are blind and hairless, but in 21 days they can be weaned and capable of living on their own. Eight to twelve weeks after birth, the young are sexually mature, and able to breed themselves.

In addition to this phenomenal reproduction potential, females also have the ability to become pregnant with a new litter while still suckling the previous litter, a phenomenon known as ‘Post-Partum Oestrus’. Thus, if conditions are favourable, a female can potentially be producing a new litter every three to four weeks!

Biology

A mouse’s eyes are specialised for nocturnal vision. Because of this adaptation mice are almost certainly colour blind. Touch is a far more important sense to mice, using their whiskers (vibrissae) to sense their immediate environment. Mice prefer to move in close contact with objects, as they feel ‘safer’ - this kind of movement is called thigmotaxis. Rodents also have a ‘memory of muscle movements’, called kinaesthesis, following habitual pathways by remembering the sequence of muscle movements involved.

A sense of smell is also important, both for detecting food or the presence of predators, and also for communication. In a large infestation, odours are built up in ‘mouse pillars’ marking the runs used by members of that mouse colony. These odours identify this territory to intruder mice from other colonies, and contain other odours (pheromones) that can influence the behaviour of mice in that colony.

A rodent’s sense of taste is apparently similar to humans, able to respond to sweet, sour, bitter and salty tastes. However, they are not particularly sensitive to a substance known as ‘Bitrex’, which is the most bitter taste known to man. This enables it to be formulated into Rentokil rodenticides to reduce the risk of accidental ingestion by humans (especially children), whilst keeping these baits attractive to rodents.

Although sensitive to sounds of a frequency that humans can hear (up to 20 kHz), mice are also able to hear sounds of a much higher frequency (up to 90 kHz). Mice make many ultrasonic vocalisations, as a form of communication between individuals.

All rodents have continually growing incisor teeth, which they wear down and keep sharp by gnawing. Anything can be gnawed, even some metals. The incisor teeth keep growing throughout their life, in order to replace wear caused by gnawing on these hard materials. At up to six bites per second, these teeth can enable rodents to rapidly penetrate barriers to gain access to new buildings or areas in a building.

Food Preferences

Mice are omnivorous – they will eat anything – a major requirement for a pest species to thrive. Adults can usually survive without drinking water if the food they eat contains at least 15% water. Only three grams of food is required each day. Mice tend to acquire this food by feeding in small amounts from many different locations. Because they need so little food, an infestation can often go unnoticed for quite a long period of time.

Signs of Presence

Signs of the presence of mice include holes gnawed in boxes, and droppings where they have been feeding. Each mouse produces about 80 droppings per day. These droppings are generally spindle shaped, up to 6mm long. Structures such as ‘mouse pillars’ are signs of a large infestation, as are smear marks, where the grease from their fur deposits on habitual mouse runs.