Toxicity of Insecticides

R. D. RADELEFF, G. T. WOODARD, AND R. C. BUSHLAND.

MANY insecticides are highly poisonous to animals and must be handled carefully. When they are used according to the recommendations and instructions of the Department of Agriculture, the State agricultural experiment stations, and the manufacturers, however, there is little danger to livestock.

There are many basic insecticides and a variety of commercial preparations of each, so that there are thousands of different brands and mixtures. For our purpose it is best to discuss the toxicity of the basic compounds because the dosage and its effects do not vary a great deal with the formulation.

PETROLEUM OILS that are used as insecticides and in insecticide preparations are harmless when they are applied in small amounts to the skin.

But in large amounts (4 ounces or more per animal) they cause blistering, excessive salivation, difficult breathing, loss of appetite, depression, and perhaps death in cattle, horses, sheep, and goats.

The effect of the oils usually is observed the first few days after they are applied and may continue for some time. Many instances of poisoning of livestock have been attributed to the insecticide dissolved in the oil because of the failure to recognize the dangers associated with the oil itself.

Straight oil solutions of insecticides therefore are never recommended for use on livestock except as mist sprays applied at rates not exceeding 2 ounces per cow and as saturants for devices commonly called backrubbers, used to deposit a thin film of solution. Even this rate of application, if continued for a long time, may produce a scruffy thickening of the skin that is unsightly, if not actually harmful. Used crank-case oils should not be employed, because they may contain lead or additives, which might cause trouble.

Manufacturers of emulsifiable concentrates intended for use on livestock must arrange their formulas so that recommended amounts of the products will not lead to excessive doses of the oils. The stockman should use only the recommended dosages lest he increase the dosage of oil to a toxic level, although he may be using a safe amount of the dissolved insecticide.

SOLVENTS are used in most liquid preparations of insecticides, They may be natural or synthetic products.

The solvent may or may not be toxic, but the possibility that it is toxic should be considered when losses of animals occur following treatment. Although solvents influence the speed of absorption of insecticides into the body, the total absorption is essentially the same, regardless of the solvent.

Certain solvents, notably xylene and toluene, cause itching and burning for a short time after application in the hot sunshine, even though used in small amounts. If the concentration is high (6 percent), the animal may become dizzy or may even be anesthetized. If still higher (25 percent), it may die.

Each manufacturer is responsible for making certain that the solvents he uses are not toxic in the amounts recommended.

INSECTICIDES PRODUCED FROM PLANTS are generally safe for use on livestock. They are neither acutely toxic nor stored within the animal sufficiently to create a hazard to people. Pyrethrum and rotenone are notable examples of safety.

An exception is nicotine, which, in the form of nicotine sulfate, is used chiefly to control scabies. Because nicotine sulfate has been most commonly used by skilled regulatory officials who have a reliable test for the strength of the dip, poisoning as a result of dipping has been uncommon. Animals poisoned by nicotine sulfate show tremors, nausea, and disturbed respiration; finally they become unconscious and may die.

Some plant products are irritants and cause discomfort when they are applied to animals, but rarely is an animal killed by an insecticide derived from plants.

SULFUR and lime-sulfur have been used as insecticides on animals. Sulfur, used externally, is almost completely harmless to livestock.

Lime-sulfur, which is actually a complex of sulfides, may cause itching, a general discomfort, and blistering. Rarely does it kill an animal.

ARSENIC, as used in cattle dips, is extremely poisonous. The many losses in deaths and in injuries to livestock as a result of burning and blistering after dipping amply prove its toxic nature.

Poisoning by arsenical dips is not always the result of excessive concentration in the dip. Even a normally safe dosage may produce burning or death if the animals are treated in wet weather, or are overheated, or are handled too much after treatment.

Because arsenical dips are primarily solutions of arsenic in water and an accurate test is available, losses have been less than if the dipping solutions could not be easily checked.

The many arsenical compounds used in treating field crops may be poisonous to livestock that eat them. Poisoning has resulted from dusts that drift across fields into pastures, from arsenical compounds remaining in containers carelessly left on premises occupied by livestock, and occasionally from dip emptied on unprotected pasture. Some animals seem to crave arsenic and will seek out the spots contaminated with it.

Arsenic is absorbed through the unbroken skin and stored in various tissues of the body.

Acute arsenical poisoning usually causes death in 1 or 2 days from the time of treatment. Less acute poisoning may cause blistering, cracking, and peeling of the skin, profuse diarrhea (possibly with some free blood), rapid emaciation, a poor appetite, and obvious pain. Death may not occur until many days after exposure. In animals that die from arsenical poisoning, the intestinal tract is inflamed, the liver and other organs may be swollen, and the lungs may be dark and heavy with blood.

THE SYNTHETIC ORGANIC INSECTICIDES have some factors in common.

All are absorbed through the unbroken body surfaces regardless of the way in which they are applied to the surfaces. The popular preparations for use on animals are the emulsifiable concentrates and the wettable powders, which require dilution with water, and dusts composed of the insecticides diluted with clay or other inert material.

The toxic effects are exerted after absorption from the skin or digestive system. In most (if not all) cases, normal functioning of the nervous system is impaired. It is possible for some of these compounds to produce irritation of the skin and of the lining of the digestive tract, but only rarely do they do so.

At equal concentrations of insecticide, less will be absorbed from dry powders than from the emulsifiable or wettable formulations, but there is no difference in the amount absorbed from emulsifiable or wettable formulations.

Young animals, generally speaking, are more easily poisoned than the older ones. At all ages there is a wide variation in individual susceptibility.

In some cases the physical condition of the animal determines the toxicity of insecticides.

We have observed no difference in toxicity between emulsions and suspensions given orally. In giving insecticides to laboratory animals, some scientists have observed wide differences in toxicity according to the physical state of the compounds. Scientists of the Department of Agriculture have observed differences in the DDT content of milk from dairy cattle as a result of using various solutions and solids.

When emulsifiable materials are used, the emulsion particles gradually increase in size by uniting with one another (the concentration of insecticide in the emulsion remaining uniform), and the amount of insecticide deposited on animal hair increases. This is a characteristic of emulsions used for many years by entomologists to obtain heavy deposits of chemicals on plants by means of emulsions containing large particles, often called quick-breaking emulsions. The first animals dipped in emulsions containing large particles receive the highest dose. This behavior of emulsions explains some of the losses of animals that have occurred after dipping in emulsions known to be safe when applied as fresh dips or sprays.

The increased deposition will occur from any emulsion, but is of greatest importance in dipping because in dipping the animals pass through hundreds or thousands of gallons of emulsion while traversing a vat and may extract the larger particles of that many gallons.

In spraying there is a limited exposure to the emulsion, generally not exceeding 2 gallons, governed by the operator's judgment of when the animal has been thoroughly wetted. The sprayed animal is not exposed to more than the total insecticide contained in those 2 gallons. When the emulsion is allowed to "cream," or partly separate in the sprayer, thereby increasing the insecticide content of one part of the total volume at the expense of the other part, and insufficient agitation follows this "creaming," some animals receive a reduced dose while others may receive a toxic dose.

Because of the influence of size of particles on toxicity, it is not enough to have a method of determining accurately the concentration of insecticide in a dip or spray. Some knowledge must also be had of the size of the emulsion particles and of the uniformity of the emulsion.

Manufacturers of insecticide formulations are aware of these effects and have taken steps to provide emulsifiable concentrates that form emulsions that remain stable for a long time. Nevertheless, constant care is needed to see that the emulsion is of proper stability and consistency before spraying or dipping is begun.

DDT is a relatively safe insecticide. All livestock can tolerate single applications of 8 percent DDT. Ten applications of 2 percent DDT or 36 applications of 0.5 percent DDT at 2-week intervals failed to produce clinical changes in cattle.

Cattle, horses, sheep, goats, and hogs tolerated 8 treatments with 1.5 percent DDT at 4-day intervals.

DDT is safe for dogs. It must be used sparingly on cats, as they may be poisoned by relatively small amounts.

Chickens may be poisoned by dips containing 1 percent of DDT, but dusts containing 5 percent are safe.