Behavioural resistance in rodents… and how Harmonix Rodent Paste can help

Regular and thorough inspections that identify the encroachment of rodents early is a must. Harmonix Rodent Paste overcomes some behavioural resistance due to its wide application allowance on the label.

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What is behavioural resistance in rodents?

There are two main types of resistance in rodents, these are biological or genetic resistance and behavioural resistance.

Frequently encountered are rodents that are trap, box or bait shy or rodents that are simply not eating the baits that they are offered. These are all forms of behavioural resistance.

Rodenticide resistance is frequently encountered by a growing number of professional pest controllers, resulting in stricter measures being introduced to control its usage. The Rodenticide Resistance Action Committee (RRAC) has produced maps that catalogue areas of resistance to certain anticoagulants. You can see these maps here.

Why does behavioural resistance in rodents occur?

Behavioural resistance is defined as “where an animal alters its behaviour, and in doing so increases the likelihood of its survival when faced with a certain treatment or situation,” according to International Pest Control, 2016.

A rat, for example, is a common prey species for a number of predators. In order to survive it has developed a number of risk aversion strategies.

Firstly, a rat is neophobic, which means it actively avoids new things until it has determined it’s not a threat.

Secondly, if a rat has an unpleasant experience with a food source, a control method or a monitoring point they will avoid it in the future. The rat will be less likely to respond positively to similar control methods if they are used again.

While a rat can change its behaviour, it tends to follow the set routines that have kept it safe from predation and it tends not to adjust its behaviour without good reason. These common behavioural traits can be identified in the rodent’s environment by the presence of smear marks, clusters of droppings, urine stains and rat runs.

When controlling rodents, a professional should understand these behavioural patterns, adjusting and tailoring their control methods to the behaviours being displayed by the population they are observing.

Behavioural resistance could appear due to poor practice by pest controllers. This could be attributed to insufficient follow ups, a failure to apply the bait in the appropriate treatment regime or the over reliance on one control measure.


So how can the pest professional avoid behavioural resistance in their targets?

Regular and thorough inspections that identify the encroachment of rodents early is a must. While the rats are first exploring an environment they will be taking note of the food source and of potential threats. If the pest controller is already providing regularly monitored, non-toxic baits then these will be more readily incorporated into the rat's regular routine, allowing for non-toxic bait to be switched out to a toxic bait of a similar type.

Studies have suggested that 10-20% of food management sites have permanent infestations creating “nurseries for further resistance” (BPCA / Ayres, M. 2019). Such infestations will be progressively more difficult to treat as they develop increased behavioural resistance (as survivors teach offspring avoidance techniques) and genetic resistance (as those most resistant to anticoagulants survive).

In situations as described above, there may only be short windows to encourage rodents to start uptake of a rodenticide before they return to their old behaviours. Using a rodenticide that has a palatable bait, that only requires a small amount of the active ingredient to be ingested would be a priority.

Using the rodents risk aversion strategies against them will also be key. The safest place for a rat is in its lair, so locating and placing bait as close to it as possible (by way of techniques such as burrow baiting), means that the rat has to expose itself to less risk and less opportunity to encounter a competitive food source, so there is less opportunity for risk aversion and behavioural resistance to form.

Harmonix Rodent Paste overcomes some behavioural resistance due to its wide application allowance on the label. It can be used in conjunction with the non-toxic bait (Harmonix Monitoring Paste) so that there can be seamless switches between monitoring and control programmes. The non-toxic bait can be used to either monitor or pre bait as part of overcoming rats natural neophobia - prior to switching to the rodenticide bait.

Smaller amounts of the active ingredient are required to achieve control, so less bait is needed due to a ‘Stop Feeding Effect’ created after the rat has ingested a lethal dose of the product. Because of the build-up of calcium, the rodent is tricked into thinking that it is not hungry. With less rodenticide needed to achieve control, there is less risk of environmental contamination.
Harmonix Rodent Paste is a Cholecalciferol bait, which works differently to alternatives used in rodent control. There is currently no known genetic or biological resistance to it making it highly effective.

You can discover more about the benefits of Harmonix Rodent Paste in dealing with rats and how it can support pest controllers dealing with behavioural resistance here.