Rodents – their biology and control

Rats and mice belong to a group of mammals known as rodents and belong to the class Rodentia. Most pest rodent species belong to the family Muridae.  Pest rodent species are commensal, literally meaning ‘eating at the same table’. Pest rodents eat man’s foods, damage his possessions and use his transport to distribute all over the globe.  The top and bottom pairs of rodent incisor teeth grow throughout the animal’s lifespan.  These teeth are used for gnawing or  

cutting and are vital in the animal’s search for food and gaining access to food supplies and shelter. Rodents have to gnaw constantly to keep their incisors at a manageable length. They can potentially chew through materials such as plastic pipes and electric wiring and cause considerable damage in the process. Pest rodents are carriers and transmitters of diseases, e.g. Bubonic Plague is quite well-known and was responsible for the Black Death in Europe during the middle ages.

 

 

 

 General rodent facts

•                      Rodents are primarily nocturnal.

•                      Rodents are choosy and won’t take moldy food.

•                      Rodents have a highly developed sense of smell and taste and will initially reject anything unfamiliar.

•                      They can develop bait shyness – once a negative experience is communicated to other group members, they will avoid the bait.

•                      Rodents know their territory well and know were to find food and water.

•                      Rodents can walk/run against horizontal and vertical lines and pipes, bricks or other rough surfaces.

•                      Rodents multiply fast – a female rat and her offspring can theoretically produce 15 000 rats a year.   

•                      Rodents don’t vomit.

•                      Rats travel along a definite route, leaving well defined trials or runways. 

•                      Food sources are dominated by higher rat members in the rat hierarchy order.

•                      One building can be occupied by a variety of rat groups rather than one large rat community.

•                      Rats often kill and eat house mice.  This behaviour is called Muricide.

•                      The Norway rat is one of the most studied animals in science.

•                      Laboratory mice belong to strains of house mice and are by far the most commonly used laboratory animals.

 

Rodent species

There are three main domestic pest rodent species in South-Africa:

Norway or Brown Rat (Rattus norvegicus)

Norway Rats are stocky, medium to large rodents with tails shorter than the head and body length (body and head: 18-25 cm; tail: 16-21 cm).  Norway Rats are reddish-brown to grayish-brown with white bellies. Their ears are small, close-set and are half buried in fur. Their droppings are about 18 mm long with blunt ends. Under natural conditions, Norway rats can reach 2 years and under protected conditions, 4 years.

Norway rats are vocal and use visual cues such as body postures in their communication efforts. They have relatively good hearing and tactile capabilities and are able to sense very minute vibrations in the ground.  They can feel their way through total darkness with their paws and whiskers.  The Norway rats' sense of smell is excellent and use it to find food as well as distinguishing between individuals of a group. 

Norway rat males can start reproducing at the age of 3 months and females at 4 months.  A female can have 7 litters per year with an average of 8 pups per litter.  The gestation period is 22-24 days. Parental care is provided by the females who nest communally, i.e. the litters of various females stays in one nest.  If a female dies, the other females will take care of her young.  The pups are weaned at 3-4 weeks when they leave the nest.  

Roof or House or Black Rat (Rattus rattus)

Roof Rats are moderately sized, slender and agile rodents with tails longer than the head and body length (body and head: 16-21 cm; tail: 19-25 cm).  They are seldom completely black, but are slaty-grey or different shades of brown with grey bellies.  They have large, thin and hairless ears that stand out from the fur. Their droppings are about 12 mm long with pointy ends.  Under natural conditions, roof rats live up to one year and under protected conditions, 4 years.   Social groups consist of various males and females.  One male will be dominant, followed by two to three dominant females. Groups have territories of up to 100 m².  Roof Rats eat mostly vegetable foods and can consume large quantities in one feeding.  The rats can survive without water for long periods. They are skilled climbers and are at home both on ground level and at higher levels. During the winter months, Roof Rats are mainly found indoors.  They are closely associated with man and can be found in homes, shops, warehouses and grain stores, poultry farms and animal houses.  They tend to invade upper levels of floors, ceilings and roofs of buildings, where they use habitual paths which can be identified by greasy smears.  During the summer months, Roof Rats invade orchards and plantations where they nest in high branches of trees.  Citrus fruits, pineapples, cocoa, avocado, bananas and macadamia nuts have all been known to be attacked by these rats. They are skilled travelers and are well-known on ships.  Roof Rats are less aggressive than Norway Rats

and may be displaced by them if both are present in the same area. 

Roof rats can reproduce when they are 3 to 5 months old and a female rat can have up to 5 litters per year.  The gestation period is between 21 and 29 days.  Mainly the females take care of the pups who are weaned at 3-4 weeks. The pups remain, however, in their mother’s nest until they reached full adult size.

 

The House Mouse is the smallest of the pest rodent species (body and head: 6-10 cm; tail: 7-11 cm). House Mice are dark to medium brownish-grey with a light grey to brown belly.  They tend to have longer tails and darker fur when living closely with humans. In comparison to their body, mice have large ears. Their droppings are 3-4 mm in length with pointy ends.  Under natural conditions, house mice live up to one and a half years and under protected conditions, 3 years.   Mice are territorial.  One dominant male usually lives together with several females and young and dominant males respect each other's territory. They normally only enter another's territory if it is vacant.  Mice are less dependent on buildings than the Roof Rat and can live outdoors and indoors. They are graminivorous (grain and seedfeeders) and eat small portions in one feeding. They are good swimmers, climbers and jumpers.  They can be found in all climates and are able to endure wide variations in temperature and humidity. House Mice are resistant towards dehydration and can survive in haystacks or stored grain without any external source of water for long time periods. They can also survive in chilled compartments used for transporting frozen foods, whilst feeding on frozen meat.  When food is plentiful their numbers build up rapidly, giving rise to plaque conditions where 30 000 mice per hectare may exist.  On a lesser scale, mice can become a serious problem in homes or restaurants where conditions are suitable and steps to control them are not taken in time.

They have excellent hearing and can perceive ultrasounds. They are known to utter a faint but audible twittering sound when they are in the safety of their nests and when they communicate long distance warnings.  They squeak to each other in the ultra sound range for short distance communication only.

A female house mouse can have 5-10 litters per year with an average of 5-6 pups per litter. The gestation period is 19-21 days.  Pups are weaned 3 weeks after birth but stay with their mothers until they are approximately 21 days old.  House mice can start to reproduce when they are only 5 weeks old, although maturity is reached at 6 weeks for females and 8 weeks for males.  

Economic importance

Domestic rodents are considered pests for the following reasons:

Loss of food supplies 

•                      House Mice and Roof Rats damage growing crops and all three species damage food in storage.

•                      Rice, maize and sugar cane fields, and coconut and cocoa plantations are known to be attacked by mice and Roof Rats.  

•                      Rodents feed on and contaminate stored foods with their hair, urine and droppings (three times the amount of stored products eaten by rodents is contaminated). 

 Damage to structures and materials

•                      Rodents cause damage by gnawing and burrowing. 

•                      Gnawing can cause severe losses to electricity and water supply equipment. 

•                      Buildings and structures can be undermined by burrowing. 

Disease transmission

•                      All three domestic rodents can potentially spread Salmonella spp. (food poisoning) through their droppings.

•                      The Bubonic plaque and Murine typhus fever are transmitted from infected rats to humans by fleas.

•                      Leptospirosis is spread through infected rat urine to food and water sources.

•                      Rat bites can also be infectious.

 

Indications of rat activity

Rodents are nocturnal and are seldom seen.  The followings signs of rodent infestations can be used to verify their presence:

Droppings

•                      The species’ droppings differ in size and shape.

•                      The number and ages of the rodents present can be determined through their droppings.

 

Urine stains, body smears and odour

•                      Rodents frequently urinate and leave body smears as they move around.

•                      The odour of even small populations of rodents is easily identified.

•                      Mice have a different odour than rats.

 

Runways and tracks

• Well-worn paths in vegetation and footprints and tail marks in dust, soft earth or sand, identify a rodent population.

Gnawing damage

•                      The double gouge marks of rodent twin incisors are easily identified.

•                      The relative size of the teeth gouges can give a clue to the species.

 

Rodent control

People who are experienced in pest control will tell you that there are five golden rules when it comes to controlling rat and mouse problems effectively and these are often overlooked.

1. Inspection

Have a careful look and try to determine exactly where the infestation is coming from (typical signs are the smudge marks left by a rodent’s oily hair, its droppings and any damage to goods or structures) so that the problem can be tackled at the source.

2. Identification

By observing the rodent’s identifying characteristics and by inspecting its droppings, you will know which of the three typical species you are dealing with and will then be able to focus on the correct tactics to eliminate the problem. Mice and rats have significantly different behaviour patterns and need to be controlled differently.

Removing the rodent’s food sources is a key factor in successful rodent control.  Garbage, food or feed should be stored in sealed containers and outside debris and vegetation particularly where it is in contact with buildings should be removed.  In addition try to reduce the number of extraneous water sources where rodents are able to drink such as ditches, stagnant pools, fountains and ponds, etc.

4. Rodent proofing

Keeping rodents out of buildings by closing off the typical places where they can gain access is a key factor in successful rodent control.  For example, close all holes in exterior walls, screen downpipes in gutter top and bottom and install self-closing devices on doors that are frequently used.

5. Apply an effective rodenticide

The use of poison baits has proved to be the most cost effective option so far developed in eliminating rodents.  Bayer Environmental science provides the bait solutions to control rats and mice. 

Finale^® contains difethialone an anti-coagulant which is also known as a single-feed rodenticide. The rodent only needs to eat the bait once and it dies within four to ten days.

Anti-coagulants interfere with prothrombin synthesis, i.e. fibrinogen can no longer be converted into fibrin. The blood has therefore lost its ability to clot and the rodent tends to bleed.  In addition, blood capillary vessels become fragile and the rodent dies from anaemia.

The feeding habits of rats and mice differ. Rats tend to eat at familiar places and preferably need 15­30 ml water daily. They tend to feed for longer at a given place than mice.  Mice nibble at food for a while, move on to the next place, nibble again and move on again.  They get most of their water from food. This difference in feeding habits, require different methods of placing the bait.  For rats, the bait must be placed next to walls, boxes, etc. next to their pathways. Less bait areas are needed, but more bait per station is needed than when mice are controlled. For mice, various bait stations must be placed with small amounts of bait at various places.  Always ensure that the bait is placed out of reach of children and animals other than rats and mice. The use of bait dispensers are strongly recommended to keep bait fresh and prevent accidental ingestion by non-target animals. 

Registration holder: Bayer (Pty.) Ltd., Reg. No. 1968/011192/07, P.O. Box 143, Isando, 1600, SOUTH AFRICA

Distributor: Coopers Environmental Science, tel. no. +27 11 979 4246 / 7, fax no. +27 11 979 4248.

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