The two species of rats considered a nuisance in Zimbabwe are Rattus rattus the roof rat, or black rat; and Rattus norvegicus the ground rat, Norwegian rat or brown rat. It is important to understand the differences between the two as this will determine the best control measures to be used.
Rats prefer a warm sheltered environment, with regular supplies of food and water. Animal houses make an ideal home for them.
Rattus rattus - roof rat has a long thin tail, typically 16-26cm long, a long slender body measuring 16-24cm. It is an agile climber and moves with the tail held in the air. Smaller than the ground rat, it weighs between 150-250g. The droppings are long and thin.
This rat is frequently seen in high places, spending less time in the ground than the ground rat. These habits should be taken into consideration when placing bait stations.
Rattus norvegicus – ground rat is a higher, stockier, larger rat, weighing 275-575g, with a body length of 17-23cm. The tail is shorter (17-23cm) and trails behind the rat. Droppings are shorter and fatter than those of the roof rat. The ground rat typically spends most of its time at ground level, burrowing under skirtings etc. The nose is blunt and the rat is less shy and more aggressive than the roof rat.
Rats have extremely strong teeth and must keep chewing to maintain them. They will gnaw through plastic, wood, rubber and aluminium. They are agile, excellent climbers and able to jump to 1 metre high and more than a metre horizontally.
Although their eyesight is bad, the other senses are extremely well developed. Sensory hair endings ‘remember’ a route, allowing the rats to move and hide fast and efficiently.
Rats are distrustful of new objects in their environment. For this reason, when placing bait stations, they should be left for 5-7 days baited but without poison for the rats to accept their presence.
Rats are extremely prolific breeders and the young become sexually active and breed from5-9 weeks. 2 pairs of rats on a farm under ideal conditions can create a population of 2000 rats after 6 months and after 3 years there may be more than 20 million rats present.
Why Control Rats ?
- Biosecurity – Rats carry about 45 diseases which can be transmitted to livestock and man; including leptospirosis, salmonellosis, pasteurellosis, swine dysentery, toxoplasmosis, trichinosis and rabies.
- Feed consumed – Rats will eat about 10% of their body weight per day. 100 rats will eat over a tonne of feed in a year. This constitutes a loss to the farmer.
- Feed contaminated – Apart from feed lost by being eaten, rats will contaminate about 10 times more than they eat with their urine, droppings and hair. Thus – 100 rats will contaminate an additional 10 tonnes of feed a year.
- Destruction of insulation ( Relevant in poultry brooding houses) leading to costs of replacement, increased energy costs and lower feed conversions.
- Destruction of buildings, electrical wiring etc which may be a fire hazard as well as necessitating costs of replacement.
Signs of Rat Infestation
- Sounds – gnawing, climbing or running sounds, squeaks.
- Droppings – along walls and around food supplies
- Runs – dust-free areas along walls and behind storage materials etc
- Smell – a persistent musky odour
- Visual sightings are rare. Rats are nocturnal and only seen during the day when populations are very high.
- Smudge or grease marks on pipes, rafters, around holes.
Methods of Prevention and Control
- Rodent proof buildings – Rats can fit through a hole of 1 cm diameter, so even very small opening will provide them with access. Foundations and concrete aprons around buildings will prevent burrowing. Any openings or gaps should be blocked with coarse steel wool or sheet metal, as the rats will gnaw through wood, plastic or aluminium.
- Eliminating nesting and hiding places Rats will avoid open and exposed spaces and choose to hide and nest under piles of old equipment, empty feed bags and other such places. Good house-keeping and general tidiness will eliminate many of these places. Equipment etc should not be piled up against a wall, but stacked at least 30 cm high and 24cm away from a wall.
- Remove food and water sources Repair leaking taps and pipes, as far as possible cover drinking troughs. Store stock-feed in rodent-proof bins, covered cans or metal hoppers with closely fitted lids. Reduce feed spillage and clean up any feed remaining on the ground. Dispose of animal carcasses quickly.
- Elimination of existing populations
- Snap-traps or live box traps – may be suitable for small populations. Locate very close to the walls, behind objects and in dark places where there are signs of rodent activity such as droppings or runs.
- Rodenticides (poison baits) – These are poisonous and constitute a hazard to livestock, domestic pets and wild-life. As such, they must be handled with care, following manufacturer’s instructions and unused bait must be collected and safely disposed of. The bait used is usually grain. The bait must be contained in a bait station or bait box which prevents accidental ingestion by other animals, children or birds. The poison comes in the form of liquids, ground bait or blocks. Poisons cause haemmorhage and/or dehydration. If rats eat enough of the poison to become sick but not die, they will become bait-shy. It is important that dosages of poison are high enough to be lethal. Bait stations should be placed about 7 metres apart to ensure good coverage. Poison bait stations are only successful if other food sources are first removed, as the rats will take food from their usual source before taking bait from a station.
A Presentation by Lee Ashford at Fivet Pig and Poultry Expo 2011