Rats are responsible for more animal extinctions than any other cause. The chief victims have been birds native to small tropical islands. Rats prey on both eggs and baby birds and, in some cases, adult flightless birds as well.
For this reason, rat poison may be the the single most important equipment in the world of bird conservation -- though using the right type in the right manner and in the right location is critical. In the wrong hands, rat poison can not only kill rats and mice -- it can kill birds, small native animals, fish, pets and even humans.
Winter poisoning is more effective than summer baiting, and effective use of rodenticides requires acclimating the rats to bait stations and the carrier food source (boiled eggs, corn meal, canary seed, water, etc.)
Rat eradication efforts benefit greatly by periodic switching of poison types, from warfarin (maintain for several months) to zinc phosphate, for example. Ultimately, effective long term rat eradication requires either removing food sources or maintaining a nearly-permanent poisoning regime.
A quick history of the most common types of rat poisons:
- Red Squill (Urginea or Scilla maritima) is a flowering plant native to the Mediterranean and was used as a rat poison as early as 1500 BC. The bulb of the red squill plant weighs up to four pounds and is sliced and dried before it is set out for rat consumption. Red squill is a safe poison because non-target animals that consume it invariably vomit to rid themselves of the toxin. Rats are unable to vomit, however, and so cannot purge their system of the toxins which eventually paralyze their hearts. Red squill has a very strong bitter taste and works well for a single time baiting situation, but rats quickly learn to stay away from it after that first dosing.
- Strychinine is a very old poison and may have been used by Alexander the Great's wife to poison him after he took a homosexual lover (who was poisoned at the same time). Strychnine originates from a small tree-like plant (Strychnos) once endemic to the Indus valley of India. A similar plant of the same family (but which grows in vine form) is used by the Indians of the Amazon to make curare -- a poison used to kill monkeys with poison-tipped darts. Rats tend to shy away from strychnine, but it is very effective on mice. Because strychnine is easy to abuse, and safer rodenticides are far more effective, access to this poison is now strictly controlled.
- Arsenic is another very old poison. Aristotle made reference to the poison "sandarach" (arsenic trisulfide) in the 4th century B.C. Arsenic was commonly sold as "Ratsbane" by the 1500s, and is mentioned by Shakespeare in Henry V, Part II ("I had as lief they would put ratsbane in my mouth as offer to stop it with security.") Arsenic is less effective and more toxic than other readily available rodenticides and is now almost never used for vermin work.
- Warfarin is a modern slow-kill repeat-bait poison and among the safest and most effective rat poisons in common use. Warfarin was discovered after Canadian cattle ate improperly stored sweet clover and began to hemorhage and die. In 1930 the active ingredient "coumarin" was isolated from this clover. In 1940 the Wisconsin Alumni Research Foundation patented a coumarin compounded called Warfarin (named after the foundation's initials). In 1952 warfarin was first used as an anticoagulant on humans, and today it (or some other coumarin derivative) is used by patients with artificial heart valves or who are in danger of thrombosis (blood clots). When used as a rodenticide, warfarin should be set out in feed-on-demand bait stations for at least two weeks. Other anticoagulants that work about the same as warfarin are brodifacoum, bromadiolone, chlorophacinone, diphacinone, fumarin, pival, and PMP. Some rat and mouse populations have become resistant to warfarin and other anti-coagulants -- a good reason to temporarily discontine warfarin after a few months and switch to zinc phosphate or another quick-killing raticide.
- Zinc Phosphide is an effective quick-kill poison and is readily available. It has an offensive odor (it smells like garlic) and is unattractive in color. Rats and mice seem to be attracted by the odor, however, and all species of rats and mice accept it. Zinc phosphide is not absorbed through the skin while mixing, and is not water soluable. Very occasionally animals die from eating the carcasses of rats or mice that have been killed with zinc phosphide, but this is so rare that zinc phosphide is listed as only a mildly hazardous rodenticide. Cause of death is heart failure. Other type of phosphated rodenticides include Aluminum Phosphide and Calcium Phosphide.
- Norbormide (S-6999) is a new single-dose rat poison that is essentially nontoxic to humans and is odorless. Norbormide appears to be nontoxic to birds and other mammals including mice. It kills by constricting blood vessels and comes as a white powder. It is sold as Raticide, Raticate and Shoxin.
- Vacor 1080 (Sodium fluoroacetate) is an extremely powerful single-dose rodenticide. Death normally occurs 4 to 8 hours after ingestion, and little or no bait shyness develops since death generally follows ingestion. Vacor is available in a formulated ready-to-use bait mixture for licensed professional rat exterminators, but is generally not available to the lay public as it is a poison that is powerful enough to kill almost anything else that ingests the bait.
- Sodium Fluoroacetate or 1081, is one of the most effective rodenticides known. It is virtually tasteless and odorless and kills in 1 to 8 hours. No tolerance or bait shyness develops. The drawbacks are that it is highly toxic to all animals, has no antidote, and has a high degree of secondary poisoning for animals eating rats or mice killed by the 1080 poison. As a result, 1080 is classified as extremely hazardous and is available for use only by licensed professional applicators. Cause of death is heart paralysis.
- Fumigants such as methyl bromide and hydro-cyanide gas are fast and effective controls for rats and mice in burrows or tightly closed buildings but should never be used by anyone other than licensed and trained professionals.
- Other Poisons No Longer Used. These poisons are more dangerous and not as effective as their readily-available alternatives: Barium carbonate, phosphorous paste (pictured in the French rat poison bottle at the top of this page), and Thallium sulfate.