Pest control | PPC94 March 2019

With current rodent control challenges facing pest controllers, one of the essential areas of knowledge is an understanding of rodent behaviour. This article by Sharon Hughes, Global Technical Marketing Manager at BASF, provides an overview of aspects of rat behaviour that are important to consider for effective control.

FEAR OF THE NEW

Rats are exploratory; however, at the same time, they are naturally neophobic (the fear and avoidance of new objects). Neophobia is a survival mechanism to protect from the new and unknown and often occurs when rats are presented with freshly placed rodenticide bait or bait box or monitoring devices.

The resulting neophobic response of the rat is to either ignore the bait completely until it feels confident that it poses no danger (which may be days or weeks) or to take a small taste to see if the bait has any negative effects before returning to feed more freely. Levels of rat neophobia will be lower in infestations which are exposed to regular disturbances and higher in infestations where disturbances are rare.

To help reduce a neophobic response, acclimatise the bait boxes in situ before the bait is added (as in the rural study mentioned opposite). The position of each bait point, as determined by a thorough survey, is also important: “it is not about how many bait stations you install, it’s about the number of bait stations installed at the most effective rodent spots” (Bobby Corrigan).

It has been reported that neophobia to bait boxes may be reduced by acclimatising the boxes with scent marked materials such as soil, rat faeces. However, if the scent mark is of a dominant rat this may have an adverse effect.

FEAR FACTOR

There is evidence that in some areas of the UK, eg Hampshire and Berkshire, there are highly neophobic rats -- that is they exhibit ‘enhanced neophobia’. These rats are extremely difficult to control even with burrow baiting. This ‘enhanced neophobia’ can be classed as behavioural resistance and may be genetically based.

The worst case of neophobia in rats I have experienced was on an agricultural site where the rats took 15 weeks to enter bait boxes. Only when rats entered the boxes was bait placed in them, and control was then achieved relatively quickly.

However, if bait had been placed in the boxes at the start of the 15 weeks, not only would the bait have been untouched by the rats, it would have been present in the environment far longer than necessary with a new risk assessment required after 35 days (five weeks).

TASTE TEST

It is important to apply the most palatable bait for that location so that when the rat takes a small taste of bait it finds it palatable and wants to return to eat more. This quick acceptance of baits may also depend on the type of diet that the rat was already feeding on, including its palatability, nutritional value and ingredients. Hence pasta baits are often more accepted in urban areas where the alternative food is fatty, while grain-based baits are more readily accepted in rural areas.

The quicker the rat recognises the bait as a food source, the quicker it will start to feed on the bait and the quicker control is affected. Typically, wax blocks are the least palatable bait type due to their lower palatability and wax being the least recognised as a food source. However, blocks are securable and so offer a stewardship advantage. Linked to this, as rats have such a sensitive sense of smell (olfactory sense), baits that have an overpowering odour may be unaccepted by the rats.

FAVOURITE ROUTES

Effective rat control requires the monitoring of the rats’ foraging behaviour. Rats generally move along the same pathcway forming noticeable runs or leaving smear marks or faecal droppings along the route. Rats typically move along routes previously used and remembered using kinesthesis (muscle awareness/muscle memory) and touch.

When using bait, it is important to intercept the rat along its foraging route, before it encounters the alternative food source. Having such a predetermined travel path is seen as a survival mechanism for both assuring that the route used is safe and providing a quick tried and tested escape route from danger. However, it has been reported that rats may pass several bait points to feed at their favourite feeding location. Therefore, sufficient bait points must be in place to allow for the natural behaviour of rats and to maximise control. It cannot be automatically assumed that a bait point by a burrow means that the rats from that burrow will feed from that bait point.

Hoarding of food by rats is part of their natural behaviour, and this hoarding increases as the rats approach adulthood (Meehan). During one trial, using unsecured bait, the bait take was as expected, but the control achieved was not. On investigation, we found 2.5kg of bait removed from numerous bait boxes and hoarded in one spot.

This extreme level of hoarding in such a short time was likely the result of several rats working together. Not all hoarded food is eaten. Hoarding of rodenticide baits is obviously something that as far as is possible needs to be prevented. Hence the use of securable block baits in locations where bait transference is a risk.

UNDERSTANDING A DAY IN THE LIFE OF A RAT

The movement of rats and mice is characterised by a 24-hour pattern (circadian rhythm). Rats leave their nest for food and water, and breeding. The time when rats are away from their nests is generally between sunset and sunrise, ie they are nocturnal, with most of their eating occurring first and last thing during this period. This moving mainly during the hours of darkness is a safety mechanism to help protect from predation.

A rat’s feeding behaviour reflects its social status, and usually the dominant rats feed exclusively in darkness while the subordinate rats feed in the early daylight hours, thus avoiding any competition or threat from the dominants (Meehan). Therefore, the dominant rats will eat the bait first and die first and the second ‘pulse’ of deaths is from the juveniles or sub-dominants.

In field trials undertaken I have often observed that the first rodent bodies found are that of larger (and assumed dominant) rats and the last rodents to be controlled are the juveniles.

On occasions, rats can be seen during daylight hours. There are a few explanations for this:

• The infestation is large and the older/sub-dominant rats cannot compete with dominant rats for food during the safer night time period
• The infestation has been undisturbed and acclimatised to their environment for so long that moving during daylight hours is no longer seen as a danger
• Food is only available during daylight hours
• These daylight foraging rats may now be less neophobic and hence easier to control.

Whatever the reason, sightings of rats during the day usually means the presence of a well-established infestation.

ODOUR SIGNALS

Rats use pheromones (biochemicals) in urine and faeces as a key way to communicate by scent marking. Communication can be for many things including recognition, alarm (fear) signalling, social organisation and reproduction.

The utilisation of these scent markings was suggested above by placing soil, faeces etc, in bait boxes to help overcome neophobia. Urinary pillars (communications signals) consisting of grease, dust, hair and scent marks are common in heavy mouse infestations and have been seen in established rat infestations.

Odour signals of foods preferred by the mother may be passed on from mother to young. Again, it is therefore important to ensure that any bait eaten by the mother is palatable. Locations with faeces and urine are more attractive to other rodents than those without and provide ideal locations for bait points.

LOOK DOWN, LOOK UP

Brown rats are ground-based and frequently found in burrows. But like many aspects of rat behaviour, there is an opposite. This does not mean that they cannot, or do not climb, and enter premises via roof spaces etc. In fact, they are excellent climbers.

Burrows are located near to food and water sources to reduce the time that has to be spent out in the open and vulnerable.

Burrow baiting means the rat does not have to leave the burrow for food and the bait is not competing with other food sources offering a very effective way of controlling rats. Burrow baiting can also reduce the risk of non-target access to the bait. If securable baits are used for burrow baiting then these can be placed on a wire which is then secured outside the burrow. The bait can then be retrieved and the number of blocks counted to assess what has been taken. Furthermore, at the end of the treatment, the bait can be removed.

The brown rat is also known as the sewer rat. The sewers offer an environment protected from predators, relatively stable and with a constant food source (human faeces and food waste).

However, migration from the sewers to above ground occurs after flooding due to heavy rainfall or disturbance, eg damage (which may be caused by the rats in the first place). This migration results in a new infestation above ground. Alternatively, rats may travel to and from the sewer system each day. In urban areas, new rat infestations are often associated with defective drains.

When rodent activity is persistent despite a comprehensive baiting programme then the sewer system should be inspected as an ongoing source of the rats. In central urban areas, it has been reported that 60-70% of infestations are related to sewer/drainage defects. In some cases, rats may migrate seasonally, returning to the sewers during the winter months to avoid the harsher conditions. Rats may also leave sewers due to overpopulation.

In conclusion, rat behaviour is complex and knowledge of this behaviour is an essential tool in the toolkit of pest controllers.

WANT TO LEARN MORE?

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Source: PPC94