We might think that rats and mice control differ only in size. However, because of their different behaviors, controlling a rat infestation is not the same as controlling a mouse infestation. Unlike rats, rats tend to settle and nest indoors. This, coupled with their tendency to feed on very small amounts of food from various sources, requires a different control strategy than an infestation of rats.
Why is controlling an infestation of rats not the same as controlling an infestation of mice?
If we ask the staff who maintain the metro facilities which pest is more complicated, rats or mice, the answer will be rats. If we ask someone in the food industry or a pest control professional the answer will almost always be mice, and more specifically the house mouse Musculus domesticus.
But, in any case, to get rid of them we will need different strategies.
The rat visits, the mouse settles in
Unlike rats, which tend to be casual invaders indoors, once inside, mice want to stay indoors, enjoying the food and shelter they find there.
This behavior is not just a preference, but an ability that mice possess and rats do not. Rats need a daily source of water without which they will not survive, so they enter, feed and leave. Rats, however, can often survive on the moisture content of the food they eat and therefore don’t need to maintain that escape or movement route in search of water.
Large meals or small bites
Feeding behaviour also differs. Rats are naturally neo-phobic, distrustful of anything new, such as our bait, but once they overcome this fear they will eat large amounts of food from the same source.
Mice, on the other hand, are naturally curious and will investigate baits immediately, but normally they will only ingest very small amounts of the same food source, preferring to get their daily diet from many different places.
Therefore, setting one or two bait points for mice will not solve the problem since they will probably not eat a lethal dose. Against house mice it is vital to place a lot of bait, containing a small amount of rodenticide, with the aim of achieving the lethal dose through small, multi-point shots.
The location of the bait is also crucial. Mice are acrobatic climbers and once inside the building, they will spread quickly everywhere. So the bait strategy should address all levels of space; false ceilings, closets, above beams, ceiling cavities, etc.
The house mouse eats only about 3g of food a day, perhaps that is why it maintains its sporadic feeding behaviour, as they need to feed from several locations if they are to obtain a balanced diet. They like variety in their food and are especially attracted to fats and sugars.
Resistance to rodenticides
As we have seen, understanding and exploiting the characteristics of the house mouse’s behavior is necessary to achieve control. But effective biocidal products are also needed, and here we find another problem.
The house mouse has shown significant levels of tolerance to first generation anticoagulant rodenticides since its inception. It was recognized as early as 1961 that warfarin failed to control some mouse infestations.
The introduction of newer first generation anticoagulants; diphacinone, chlorophacinone and coumatetralyl, did not produce any improvement. The first second-generation anticoagulants, difenacoum and bromadiolone, introduced in the late 1970s, produced good initial results but there were still cases where complete eradication could not be achieved.
Restrictions on the use of anticoagulants
While high levels of control can currently be achieved by anticoagulant rodenticides, excessive or inappropriate use of these biocides has a potential negative impact on health and the environment, with unacceptable contamination in non-target mammals and birds of prey.
For this reason, European legislation has been imposing restrictions on the use of anticoagulant rodenticides. These restrictions mainly affect rat control, where baits are predominantly placed outdoors. Since house mice live almost exclusively indoors and the baits for them are also indoors, neither the rodenticide nor the mouse carcasses present the same level of risk to wildlife as in the case of rats.
In any case, the pest control technician must be able to select the rodenticide product that can actually achieve eradication of the infestation, and know how and where to apply it.
It should also be kept in mind that using rodenticides to control mice and rats is not the only solution. Whether controlling rodents in domestic, commercial, or agricultural settings, the knowledge and skill to prevent their introduction into the facility is necessary. And if we already have them in place, we will look for ways to limit their access to the resources they need to survive and we can apply non-chemical control options such as traps.