An important consideration when faced with rodents resistant to anticoagulants is the use of the non-anticoagulant rodenticides. These are generally unaffected by physiological resistance to anticoagulants and therefore present useful options for resistance management (see also Combatting resistance). However, they are mainly older products and the main drawback of many of them is that they are not as reliably effective as anticoagulants. Several of the compounds are fast-acting and require the procedure of pre-baiting. This is conducted to overcome neophobia (the fear of new objects), in which, particularly, Norway rats are initially reluctant to take novel foods, such as rodenticide baits. In pre-baiting, unpoisoned bait (this is the pre-bait) is applied from bait points for a period of time until rodents are feeding freely. Only when this occurs are unpoisoned baits replaced with poisoned baits. Of course, the pre-bait should be as similar as possible to the one used to carry the poison to allow rodents to become familiar with it. In contrast to the anticoagulants, acute poisons have no antidote, and treatment of accidental poisonings is very difficult. (see also Acute-acting substances)
Zinc phosphide is applied in baits at concentrations ranging from 1 to 5%, although 2% is most widely used. Ready-for-use formulations are available, particularly in the USA. The mode of action of zinc phosphide is by the evolution of phosphine gas in the stomach, the gas entering the bloodstream and causing heart failure and damage to internal organs. There is no specific antidote. It is known to cause bait shyness when reluctant feeders take a sub-lethal quantity of bait, survive and refuse to eat the bait again. In spite of its widespread use, little information is available on zinc phosphide from well-conducted trials. A series of applications on UK farms by skilled operators using pre-baiting achieved 84% control of Norway rats. However, this compound is one of the most effective acute rodenticides currently available and was probably the most widely used rodenticide for all purposes, including commensal rodent control, until the introduction of first generation anticoagulant rodenticides.
Sodium fluoroacetate is often known as compound 1080. It is very toxic to rodents and other mammals. It is applied in baits containing between 0.08 and 0.5% of the active ingredient. Compound 1080 acts by blocking the tricarboxylic acid cycle, leading to convulsions and death. 1080 is non-specific and great care must be use when applying it. Because of the high toxicity of the material, the lack of antidote and its secondary hazard, the use of compound 1080 is carefully regulated in the few countries, such as Australia and New Zealand, where it continues to be used.
Alphachloralose is a narcotic with a rapid effect. It slows brain activity, heart rate and respiration, resulting in hypothermia and death. It is recommended against mice in cool conditions. Alphachloralose baits are used containing 2-4% of the active substance. Recent developments in Europe have led to the introduction of several ready-for-use formulations.
Cholecalciferol (i.e. the naturally occurring compound- vitamin D3) and its close relative ergocalciferol (vitamin D2), have been used for many years in rodent control in baits containing about 0.1% of the active substance. Their mode of action is to promote mobilization of skeletal calcium, resulting in hyper calcaemia and the calcification of soft tissues, particularly the major arteries and kidneys. There is no antidote, however treatment of accidental poisoning is possible. Mixtures of ergocalciferol and anticoagulants were tested and found to have good efficacy against rats and mice. There is evidence that sub-lethal poisoning with calciferol in Norway rats leads to a stop-feeding effect or to bait-shyness. Cholecalciferol products are available in a number of countries, including European countries after the active substance was approved in 2019 by the EU.
Bromethalin is used in baits at either 0.005 or 0.01% and is considered effective against many rodent species, although few independent assessments of efficacy have been published. Anorexia occurs after an effective dose has been consumed. The mode of action is to uncouple oxidative phosphorylation in the central nervous system and symptoms include tremors, convulsions, prostration and hind-limb paralysis. Bromethalin remains in use in the USA, and elsewhere, but is no longer authorized for use in any of the countries of the EU.
Powdered Corn Cob
This active substance comprises complex natural products but mainly cellulose (40-45%). It is formulated into bait pellets containing about 90% of the material for use as a rodenticide. The labels of these products accentuate the need to remove as far as possible all alternative foodstuffs. The mode of action of powdered corn cob is uncertain. The only independent published trial, conducted against Norway rats and house mice in Germany, concluded that cellulose-based rodenticides are unsuitable for the control of Norway rats and house mice. However, the European Commission has approved powdered corn cob for inclusion in Annex I of the Biocidal Products Directive. Powdered corn cob is also available in other countries, including Canada and the USA.
This active substance is used in gassing operations, most often administered to rodent burrows as pellets or tablets, using specially-designed apparatus. Once in the damp environment of the rodent burrow phosphine gas is evolved, which pervades the burrow system. These applications are recommended only for trained professionals who apply all necessary safety precautions and employ appropriate personal protective equipment. Burrow fumigation has the advantage that it is safe to non-target animals provided care is taken to ensure that only target species inhabit treated burrow and there is reportedly no secondary toxicity to scavenging and predatory mammals and birds.
There are several other non-anticoagulant rodenticides occasionally used but none warrants further consideration here because they are either too hazardous for their use to be recommended, they are scarcely available or their efficacy is uncertain.