Characteristics of common rodenticides

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_UNITE: 6TATES DEPARTMENT OF THE INTERIOR
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Vie ayat § Leaflet WL - 337
‘ = ae) January 1952
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a Bocuse OF COMMON RODENTICIDES

Research and experience, over the years, have thus far failed to produce a rat
poison that is universally effective, one that meets all requirements under all conditions.
A number are quite useful; most have one shortcoming or another. A rodent-control
investigator or operator finds it necessary to be familiar with the characteristics of all
useful poisons in order to select the one - or series - that best fits each particular
circumstance. Such factors as toxicity, dosage levels, and relative effectiveness are
obviously important. Less often considered, but of equal importance, are degrees of
acceptance and reacceptance and the development of tolerances. Odor and taste may be
considerations in some instances. Solubility of the material has a definite bearing on
bait mixing techniques. Safety precautions are an essential part of any procedure,
requiring attention to hazards to the user as well as to humans and animals that might
come into contact with exposed baits. The appended table is an attempt to classify the
common rodenticides so that they can easily be compared for these purposes.

It is obvious that full justice cannot be given to some of the categories in the
abbreviated space allowed in the tabulation; many do not lend themselves readily to one
word descriptions. Although an effort was made to qualify noteworthy variations with
footnotes, minute and technical differences had to be sacrificed for the sake of brevity.
For example, the degree of hazard existing when a poison is used is purely empirical
unless it can be assumed that normal precautions will be followed. The term "antidote"
actually means the counteracting of an effect, whereas most so-called antidotes of
economic poisons are in reality first-aid treatments, followed by palliatives and sedatives.
The statements listed in that column are simply abbreviated descriptions and are not
intended as complete directions. The differences between solutions and suspensions and
between different types of oils have led to a simple listing of whichever material serves a
useful purpose in bait mixing; even then some qualifications are necessary and occasional
variations in technique have to be ignored.

Not all animals react alike. Even within the same species some individuals are
considerably more resistant to toxic effects than the average. Some effects vary with
seasons, age classes, diet, and even with sexes. Dosage levels are usually calculated
to include the bulk of the above-average resistant animals. It is essential to understand
that no value accrues from increasing those levels. Such practice is doubly objectionable,
for acceptance is usually decreased while the hazard to other larger animals is greatly
increased. All that is accomplished is doubling the cost of the bait. Continued re-use of
the same poison in the same location, except as noted below, generally results in a decline
in acceptance, bait shyness, and poor control. Poisons highly effective in one place are
sometimes less effective in adjacent properties. A thorough knowledge of the materials
available will assist in overcoming these problems.

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Warfarin, red squill, zinc phosphide, and ANTU are generally recommended for
use by the public. They are usually available commercially and are the ones with which
the untrained individual is the least likely to experience difficulties. Commercial
preparations of prepared baits containing arsenic, phosphorus, and strychnine are
commonly sold on the retail market; their use is best restricted to indoor protected
stations where accidental poisoning hazards are minimized. Barium carbonate, once
rather widely recommended, is rarely used today as it is too weak to be effective.
Thallium sulfate is very dangerous and quite costly; except for some grain-treated
baits used for mouse control, it is used almost exclusively by professional operators.
Sodium fluoroacetate, or Compound 1080, is not available to the general public because ©

of its extreme toxicity.

Of the first four materials mentioned above, red squill is considered among the least
hazardous poisons for the untrained individual to handle. Its emetic factor offers
protection to animals capable of vomiting. However, since some farm animals do not
vomit, care must be taken to prevent their contact with the material. Red squillisa
relatively distasteful mild poison, hence its shortcoming; so much must be incorporated
into the bait that some rats object to the taste and soon learn to refuse it. ANTU, if used
properly, will give good results against Norway rats, but is ineffective against roof rats
and, like squill, is of no value against house mice. The strong reluctance of rats to
accept a second dose as well as the marked tolerance which is developed have combined
with the accidental hazards to reduce sharply the use of ANTU throughout the country.

It should not be used more often than at four month intervals to obtain best results and
baits containing the material should not be left in place for more than three or four days.

Zinc phosphide is not widely used, partly because it is not readily available and
partly because of its high toxicity and strong odor. However, this offensive odor and
unattractive color provide a safety factor. Most domestic animals will not touch baits
prepared with it, but rodents seem to like the pungent odor of phosphorus compounds.
Furthermore, all species of rats and mice, both domestic and native, readily accept
zinc phosphide treated baits. The net result is that of the three poisons thus far
mentioned, it is the most likely to produce universally satisfactory results.

A greater degree of protection to other animals, when either zinc phosphide or
ANTU is used, may be obtained by incorporating tartar emetic (antimony and potasium
tartrate) in the bait mixture. Approximately equal amounts with ANTU or three parts
of tartar emetic to eight parts of zinc phosphide provide about the same relative degree
of protection to other animals as may be expected in the case of red squill.

Warfarin, outstanding in its value to the general public, is the most recent material
to be added to the list. It incorporates a radically different principle of control by poisons.
An anti-coagulant, this chemical must be taken daily over a period of several days as
opposed to the single dose compounds, The animals, apparently not associating the
cumulative effect of internal hemorrhaging with their food supply, return to feed on
warfarin treated baits again and again. Thus the problem of bait shyness is largely
overcome. At the same time, hazards to other animals from single accidental feedings
is greatly reduced. Since repeated feedings are required, permanent covered bait
stations provide an excellent technique for exposure; relatively large supplies of bait
obviate the necessity for daily care. Warfarin possesses the added advantage of being
effective against all species of rats and mice.

Much of the research in rodenticides today is directed along the lines of the
anti-coagulants. Used dry, in a cereal mixture such as cornmeal, or with a little
oil added to prevent dusting, insects and mold may attack the finished product. Most
insecticides and mold inhibitors leave objectionable tastes that cause marked reduction
in acceptance, so investigations are being made of candidate materials to overcome
this problem. Although warfarin is exposed in such dilute mixtures that primary
poisoning by accident would require the ingestion of a huge quantity of bait, that
occasionally has happened. Also, a few cases of secondary poisoning have been
reported, due to pets feeding daily on dead or dying rats or mice poisoned with
warfarin. Efforts should be made, then, to recover rodent carcasses whenever
possible. As with other poisons, the majority of complaints of warfarin failure can
be ascribed to faulty technique in exposure or failure to maintain a fresh, acceptable
bait supply until the entire rat colony has been eliminated. Since some rats may not
start feeding on the bait until it has been exposed for some time, or feeding may be
intermittant, two or three weeks may be necessary to produce results; a mouse colony
may require as long as thirty days to complete their removal.

No mention has been made of desirable bait materials, since local availability
and cost are determining factors. Then too, acceptance is so widely varied, even on
adjacent premises, that selection must be left to the judgment of the operator. Often
only trial and error will provide the correct answer, pre-baiting being the most useful
technique. Here, again, knowledge of the characteristics of the various poisons is
important, as many of the bait formulations are based on the specific chemical that is
used. Weather conditions should be considered before exposure. Above all, hazards
to other animals must receive top attention; using materials and employing techniques
best calculated to destroy rodents without attracting or being easily available to others.

The destruction of rats and mice is one of the important steps in attaining
permanent rodent control. Poisons represent the most efficient and useful tools in
accomplishing this end. Full advantage should be taken of the particular characteristics
of each of the rodenticides, selecting those most likely to produce the desired results
under the specific conditions where they are being used.

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