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RODENTS

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Introduction

Rodents are mammals which have a single pair of incisors in both the upper and lower jaw. These teeth never stop growing, meaning that the rodent needs to gnaw constantly in order to keep the length of the teeth in check. Their name comes from the Latin rodere, which means ‘to gnaw’.

Making up about 40% of all mammals, rodents are found on every continent except Antarctica. Most cause no problem to mankind and spend their lives out of sight. A few rodents have become able to adapt to almost any living condition, using mankind as a way to increase and spread. These include rats and mice.

In Australia there are 64 native species of rodent and none pose any threat to people. In fact, we are the ones who threaten their existence. But three species of rodent have been introduced that are causing significant trouble through their destructive nature and the spread of disease. These are:

  • the House Mouse
  • the Black rat
  • the Brown rat

Problems caused by Rodents

Rodents can cause problems even if they are few in number. The following outlines some of the most common ones, but for many people just the knowledge that a mouse or a rat has come into their home needs addressing.

Damage

Pest rodents can chew through cables and electric wiring, causing fires and power outages. In 2014 a grade 2 listed farmhouse in England caught fire. It took nearly 50 firemen, some of whom worked throughout the night, to put out the blaze. Rats chewing through electric wires were blamed.

Suncorp Group, one of Australian’s largest insurance groups, has paid out more than AU$3.5 million since 2011 due to rodent-related damage. Some of the claims came from owners of cars that had their wiring damaged when rats took refuge under the warm hoods in cold weather and nibbled on anything nearby in the absence of food.

Population swarms

YouTube has disturbing videos of mice plagues, with millions of the vermin swarming on farms in Southern Australia. The direct costs of these plagues to the agricultural industry can be as high as AU$38 million, with indirect costs adding considerably more. The situation has become sufficiently worrying in rural parts of the country that an official online mouse census is available, with plague threat warnings. This is not purely an Australian issue.

In Asia, it is estimated that the rice damaged by rats and mice in one year would feed approximately 200 million people, which is about 8 times the current population of Australia!

Both Sydney and Melbourne have suffered negative publicity due to rapidly increasing rat populations. In Melbourne, rats are brazenly sitting in the open in parks, even stealing food from possums in broad daylight to avoid poison bait left out by the authorities.

Disease

Rodents are, perhaps, most notorious for the diseases they spread. Worldwide they are known to be vectors of 35 diseases. Of the two species of non-native rat in Australia, the black rat is the carrier of the most infamous in history: the Bubonic Plague or Black Death. Rat’s blood can carry many types of infectious bacteria with no harm to itself, which is what makes it so dangerous.

Diseases include:

  • Salmonellosis
  • Leptospirosis
  • Typhoid
  • Tularemia
  • Lymphocytic Choriomeningitis
  • Streptobacillus Moniliformis (Rat bite fever)

 

There are many more, some endemic to Australia. These can be caused by:

  • rodent urine or faeces contaminating food
  • rodents scratching or biting people
  • biting insects transferring disease from rodents to humans
  • bacterial infection.

 

House Mouse

The common house mouse (Mus musculus) is widely distributed around the world and is the most common of the mouse species. It is always found in proximity with human habitation, which defines its distribution. This close link to mankind has been ongoing throughout recorded history as has its notoriety as a pest.

The house mouse is small, typically only about 75mm in length, and weighs between 10 and 30 grams. It is an agile creature, light to dark brown in coloration, able to swim well, climb cables and wires, and run fast to escape its predators. This mouse can dig, jump up to 40cm high and fall more than 2.5 metres without injury. Due to their small bone structure, they can squeeze through openings as little as 6mm wide.

To further assist in avoiding predators, the mouse has a good sense of smell, hearing and taste. As it is primarily nocturnal, its eyesight is poor and it is colour blind. To overcome these limitations, the mouse moves along scent-marked trails and uses its long, sensitive whiskers.

Although the life span of the house mouse can be a year or more, it is often much shorter due to natural enemies. Strangely, rats are a threat to mice, killing and eating them when given the chance. In other parts of the world, mice do not compete well with other rodents, even small ones, but in Australia they seem to be able to co-exist with native rodents without any problems.

Mice are governed to a great extent by the environment as far as social behaviour is concerned. The males are territorial but this is flexible, subject to the availability of food and space. Normally, one male won’t enter the territory of another. This flexibility in its social behaviour gives the mouse great adaptability, enabling it to spread far and wide to inhabit a broad range of habitats.

Food and water

A mouse is omnivorous, eating grains, seeds and small insects. It is also opportunistic, which is why it lives close to humans. An adult mouse will eat about 2 or 3 grams of food a day. It doesn’t need to drink as in most cases it can obtain sufficient water from its food.

Breeding

If conditions are favourable, a mouse can breed throughout the year but in Australia the peak breeding season is across spring and summer.

The speed and volume of the reproductive cycle can lead to serious over population.  A female mouse can reach sexual maturity at only 6 weeks old. The gestation period is an average of 20 days and the mother can mate again within a day after giving birth. In ideal circumstances, a female mouse can produce 10 litters a year. A litter normally contains only five or six pups but can have as many as 13 young.

One pair of mice can be responsible for over 500 offspring in a single season of five months. It is easy to see how a breeding season extended by unseasonably warm weather can lead to mice plagues.

Rats

The two most common invasive species of rat in the world are the Black rat and the Brown rat. They share many traits, and both can exploit ideal conditions to rapidly expand their populations.

In Australia, most rodents are relatively slow breeders but these two invaders can be prolific. They can give birth to as many as 12 pups in a single litter after only a 21 day gestation, almost immediately after which the female can breed again. The young are weaned at 21 days and become sexually mature at ten to fifteen weeks.

Rats need protein to maintain breeding condition. They get this easily when living in an urban environment but in colonies away from human populations they eat primarily seeds and grain. The breeding season in rural environments is controlled by temperature and the onset of agricultural activity. Human activity in urban areas has helped the invasive rats ignore these limitations.

Rats can live comfortably in our warm homes and as we have become a society that throws away vast quantities of food, finding enough to eat is not a problem. Left in bins or not disposed of correctly, this food becomes a feast for rats. Compost heaps and bird feeders are two other food sources enjoyed by urban rats.

Black rat (Rattus rattus)

Description

Black rats grow to between 16 and 24cm in length, including a tail which is longer than the head and body combined. They can weigh between 150 and 200 grams, although some individuals can weigh over 300 grams. Overall, they are slightly smaller than Brown rats. Despite being called Black rats, these rodents are often brown or grey in colour.

Being excellent climbers they are able to clamber up ropes, brick walls and other rough surfaces. They nest higher than their cousins, often preferring wall cavities and roof spaces. This has given them their other common name: Roof rat. They are good swimmers and very agile. Like many rodents, they can easily gnaw through wood.

Black rats are communal animals, often existing in groups of 20 to 60 individuals. Commonly seen in urban environments, they are also found in undisturbed areas around the coast and forests. In more rural areas, Black rats are nocturnal but in an urban setting it is not uncommon to see them during the daytime.

Several different types of Black rat have been identified through genetic examination. Australia is known to have populations of Asiatic and Indian Black rats.

Food and water

Opportunistic by nature, an adult Black rat will eat about 15 grams of food a day. In an urban setting its preference is for fruits but it will eat whatever is available. An adult Black rat needs to drink around 15 ml a day.

Brown rat (Rattus norvegicus)

Description

The Brown rat is known by many common names, such as Street rat, Sewer rat, Hanover rat, Norway rat, and Wharf rat. Despite is scientific name, it has no link to Norway nor did it originate there. Current research suggests they came from northern China.

Often twice the size of a Black rat, an adult male Brown rat can grow to between 40 and 50 cm, although the latter would be an exceptionally large specimen. Unlike the Black rat the tail is shorter than the combined head and body length. Males can weigh up to 500 grams, although the average for a male is 350 grams and 300 grams for a female.

As their name would suggest, Brown rats are normally brown but they can also be grey. Brown rats have been domesticated and kept as pets for many years, which has led to many colour variations. Some of the most common colours for pet rats are black (although they are still Brown rats), white, hooded, cinnamon, mink but there are many others. Tailless varieties, rats with red or black eyes, and even hairless strains are now available. These varieties are available in Australia and if they escape and join a wild colony identification could become more complicated.

Food and water

Like many rat species, Brown rats prefer cereals but they are omnivorous. They thrive near human habitation due to the ease with which they can obtain food that has been carelessly disposed of.

An adult Brown rat needs to eat around 30 grams of food a day and drink 60 ml.

Signs of rodent presence

Home owners will normally be aware of the presence of rodents in their dwelling but occasionally the signs are less obvious. As a pest controller, you may be asked to do an inspection and will have access to parts of the home the owner rarely inspects.

By posting the following signs of what to look out for on your website or in articles, you may even draw the home owner’s attention to a problem they were unaware of, and hopefully get customers.

Signs to look for:

  • Piles of black droppings, between 5 to 18mm in length.
  • Nibbled or partly eaten fruit and nuts.
  • Small bones left in secluded places, such as the corner of a shed.
  • Holes burrowed in the ground near buildings or compost heaps.
  • Signs of damage to wood, cables or plastic caused by gnawing.
  • Food packets and pet food containers may also show signs of gnawing.
  • Scampering or scratching sounds coming from behind walls, under the floor or from the roof.
  • Nests in the roof.

Controls

Prevention

It can be expensive and time consuming to eradicate rodents. As part of a pest controller’s inspection report or after a pest treatment, the home owner should be given guidelines on how to prevent rodents. While the success of these guidelines cannot always be guaranteed due to unknown factors, they can minimise the risk in most situations. Putting such guidelines on your website demonstrates a community spirit. You can also write articles for newspapers and online services as part of your brand advertising campaign.

Rats need three things to survive:

  • A nesting site
  • Food
  • Water

Mice have the same requirements but can get their water from their food. They will drink water if food is in short supply.

By denying rodents access to these three necessities the home owner will reduce the risk of invasion.

The following are simple ways to make a property less attractive to rodents. You will undoubtedly be able to add to this list based on the area you live in, the type of accommodation and the environment.

  • Dispose of rubbish, such as unwanted foodstuffs, in bins outdoors that have securely fitting lids.
  • Indoors, keep foodstuffs in glass or metal containers with lids.
  • Keep the yard or garden tidy.
  • Remove places that rodents might find desirable to build a nest, which would include piles of logs or old firewood.
  • Pet food should be kept in strong containers – rats can easily gnaw through plastic and are attracted to certain dog or cat food pellets.
  • If a compost heap is in use and organic waste is added, there is a risk of attracting vermin. Can it safely be covered with lime?
  • Repair all roof damage and use wire mesh to plug any gaps.
  • Remove all tree branches that touch the home and that might allow a rat to clamber along to gain access.
  • Bird feeders can be a food source that attract rats. Hang them in such a way as to deny access to any rodent.
  • At the end of the season clear away fallen fruit from the ground. Remove fruit and nuts still hanging from trees or vines.

Poisons

Rodent control is normally achieved through the use of poisons. The type of poison used should be given careful consideration. The environment, the home or business owner’s preference, the urgency and the cost should all be taken into account. As with all poisons, extreme caution should be exercised to ensure that children, pets and native wildlife are not affected. Secondary poisoning, caused when a predator eats a poisoned rodent, can be a serious problem.

Many mammals possess the ability to vomit. This reflex action protects them if they eat a toxic substance by quickly removing it from their body. Rodents cannot vomit due to their physical structure. They can regurgitate but lack the muscles and neural network needed to vomit out any toxins.

The inability to protect themselves by vomiting has caused rats in particular to use a different line of defence. Rats are neophobic, which means they don’t like new or unfamiliar things. So if they come across a new food, they will only nibble a small amount. If there are no ill-effects, they will return to it. Also possessing an excellent sense of smell and taste, they can often detect poisons without first testing the food. Rats are observant, noticing if one of their colony dies through eating a particular food and subsequently avoiding that substance in future.

Rats’ neophobic tendency can make poisoning them difficult. Being wary of new objects, baits and traps may be avoided. If a poison is too strong, the rat may either sense it before it is eaten or recognise the danger after tasting a sample too small to have any effect. Successful rat poisons are odourless and tasteless.

A new problem is emerging with traditional poisons. Rats are becoming resistant. So-called ‘super rats’ have developed an immunity which they are passing onto their offspring. It has been estimated that as many as 70% of rats in the UK now exhibit some form of resistance to poisons. Comparative studies are not currently available in Australia but the same resistance is being seen across the country.

New generation poisons are being developed, including such names as Super Warfarin but rats are now even developing resistance to these due to overuse and incorrect application.

Another problem with poisons is that rodent populations can rebound after the initial die off. The exact mechanism behind this fast repopulation is not fully understood. It is a separate issue to poison-resistance.

A new type of biological control is being developed for mice, which involves limiting their reproduction. Using a process called immunocontraception, it fools the mouse’s body into believing that certain proteins on their egg cells are foreign. The mouse’s immune system produces antibodies against what are assumed to be invaders and the cycle is stopped.

There are two main types of poison used: a fast acting toxin and a slower anticoagulant. There are benefits and problems with both types, which will be discussed below.

Fast Poisons

These are designed to affect the rodent in one of several ways, depending on the poison’s chemical composition. Zinc phosphate, for example, reacts with the rodent’s stomach lining to generate phosphine gas. Other types cause brain swelling and seizures or create a calcium overload that damages internal organs.

Most of these poisons can kill the rodent in a day or two.

Anticoagulants

Warfarin has been used in human medicine since the 1950s to prevent blood clots by thinning the blood. It was first used as a rat poison. This and many other types of anticoagulant work by suppressing the animal’s ability to metabolize Vitamin K. This vitamin is necessary for blood clotting.

The downside of anticoagulants is that they can take several days, or even weeks, to be effective. During this period there is more chance of secondary poisoning to predators.

Newer, so-called second generation anticoagulants employ higher doses which kill faster. Anticoagulants are available in single dose and multiple dose forms.

Common Chemical Rodent Controls

Fast Poisons

  • Alphachloralose
  • Bromethalin
  • Cholecalciferol
  • Sodium fluoroacetate
  • Strychnine
  • Thallium sulphate
  • Zinc phosphide

 

Anticoagulants

  • Bromadiolone
  • Brodifacoum
  • Flocoumafen
  • Warfarin

 

Application

The neophobia exhibited by rats means that poisoning is not always straight forward. Rats are wary of any new object, so a new trap, bait or food source will be viewed with suspicion. This often stops rodent eradication from being a swift exercise. Before putting down poison it may be necessary to leave untainted food so that the rodents become familiar with it.

Pulse poisoning is often used to avoid overuse of poisons. Untainted bait is put down first for a while, then poisoned food is left for a few days, followed by more untainted food and so on. The first batch of poison will kill the initial rodents. The next generation will not be suspicious of the non-poisoned food and eat it freely. It can then be replaced with the poisoned food.

Liquid baits can be particularly effective when there is a lot of potential food for the rats. They need to drink regularly so they will be forced them investigate water sources, even when food baits are ignored.

The most common ways to use poisons are as:

  • Pellets
  • Wax blocks
  • Paste or gels
  • Liquids
  • Fumigants are rarely used and only for large infestations that require urgent eradication.

Trapping

There are situations when the use of poisons is impractical. It may be dangerous to use them due to the presence of pets, children or wildlife. The home or business owner might not want to risk the rodents dying in inaccessible places potentially causing bad smells through decomposition. Equally, it may be necessary to use a different method to control poison-resistant rodents.

Traps may be viewed as a primitive way of control but they can be very effective. The traps themselves are normally cheap and reusable.

To use traps successfully the following should be borne in mind:

  • The behavioural differences between mice and rats.
  • The positioning of the traps.
  • The bait to be used.

As noted above, all rats are neophobic and may not approach a trap for several days. Mice, on the other hand, are curious by nature. It is likely that they will investigate the trap on the first night. With this in mind, if no mice have been caught within the first few nights, then the traps are in the wrong position.

Rat traps should be left in position unset for several days so the rats can overcome their natural caution.

The bait used will vary depending on location and the population of rodent in question. Rodents in different areas and environments are likely to have developed favourite foods. By being aware of what food is being eaten, you can adjust the bait accordingly. Some baits that have proven successful include:

  • Nuts
  • Peanut butter
  • Fruit
  • Bacon

This list is by no means exhaustive and rodents in your area may have their own preferences.

Traps can either be designed to catch the rodent alive, after which is disposed of humanely or they can be ‘snap traps’. The latter are made to snap down onto the neck or back of the rodent causing immediate death. Both types of trap should be inspected daily to remove any rodents caught.

Another type of trap is the glue board or sticky trap. These are boards that are coated on one side with a strong adhesive. They are placed along rodent runways and either scented or baited. Peanut butter and chocolate seem to work best. The glue on these traps can hold a 2 kilogram rat.

Glue traps are best used indoors to prevent the adhesive being covered in dust, dirt or rain. The trap and captured rodents are disposed of in one go.

There has been some resistance to the use of glue traps on humanitarian grounds, so check with the home owner before using them. Their use by the public in the State of Victoria is banned and only accredited pest controllers may use them and then under strict guidelines.

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