WELCOME TO DR. BILL'S SITE. PESTICIDES AND THE HUMAN ECOSYSTEM.

Pesticides are chemicals used to reduce the impact of a variety of living organisms that compete with humans for agricultural products or that are considered bothersome or harmful in the human ecosystem. Today, the ideal pesticide would be one that destroys the pest and then quickly degenerates into non-toxic products. At one time, however, it was considered desirable to use environmentally persistent and long acting pesticides. Unfortunately, the implementation of this early philosophy resulted in chemicals contaminating food, water and some even accumulated in human body fat. No one chemical can destroy all unwanted pests. Therefore, pesticides are usually subdivided into a variety of classes as illustrated in Table 1.

Table 1    PESTICIDES

Kills mites algae bacteria fungi plants insects ticks larvae roundworms eggs lice rodents termites

Class acaricide algicide bacteriocide fungicide herbicide insecticide ixodicide larvicide nematicide ovicide pediculocide rodenticide termiticide

Federal regulations require that each "cide" be both effective and safe when used according to the listed directions. All "cides" are capable of harming man if ingested, inhaled, or exposed to skin contact in sufficient quantitites or if the exposure to the chemical is long enough for that chemical to be absorbed. In addition to potential dangers to man, one must also consider the effects of "cides" on wildlife (fish, birds, mammals, and bees). The simplest way of expressing toxicity is by means of the LD50. The LD (Lethal Dose) 50 is a statistical estimate of the dosages of the chemical necessary to kill 50 percent of the test species (usually rats) under specified conditions. LD50 is expressed in mg/kg (milligrams of substance per kilogram of body weight) and the route of administration is generally oral. When LD50 values are interpreted, several points should be considered: l) route of administration - is it oral, via the skin, or intravenously administered: 2) the dangers to a test animal may be significantly different from the dangers of the test chemical to man; 3) cumulative effects of low dosages are no0t considered in LD50 trials; 4)the LD50 values do not address the minimal toxic dose. Even with these limitations, LD50 values are useful in making comparisons of the toxicity of different chemicals which have the same mechanism in different classes of "cides" In addition to LD50 data which is generally obtained using rats as the test species, many "cides" are also tested in fish. In fish, LC50 is used for data presentation in the literature. LC50 represents the lethal concentration in 50 percent of the test fish with constant exposure for 48 or 96 hours LC50 is expressed in mg (milligrams) of test chemical per l (liter) of water in the test tank. An ADI (Acceptable Daily Intake) value expressed in mg/kg is used to express safe amounts of the test chemical for humans. In some cases an additional measure called the ELD (Estimated Lethal Dose) may be presented in grams or mg/kg dependent upon data presented in the literature in cases of accidental poisoning with the test chemical in humans. For the purpose of this discussion, I have chosen to present the LD50), LC50, and ADI values for selected insecticides, herbicides, and fungicides in Table 2 (at the end of this article) with ratings in a number range from 1 to 4. (with 1 being the safest and 4 the most toxic). This number system allows an easier comparison within classes, and between classes of "cides".

INSECTICIDES

Prior to World War II, insecticides generally in use were: arsenic, petroleum products, sulfur, rotenone, and cyanide gas. Since World War II, the numbers of insecticide examples have increased significantly. They are more easily discussed if they are divided into their major chemical classes as follows: I)Inorganic II)Botanical III)Chlorinated Hydrocarbons - a)Dichlorodiphenylethanes - b)Hexachlorocyclohexanes - c)Cyclodienes IV)Organophosphates V)Carbamates

Representative examples of each class are presented in Table 2 (at the end of this article) Insecticides target both pests (those insects that may transmit disease or destroy crops) and beneficial insects (predators that keep harmful insects in balance). In addition, many insecticides may also harm fish, birds, or mammals (including man). A good example of a suspected dangerous chemical is the chlorinated hydrocarbon known as DDT. This insecticides has a half-life of up to 20 years in the environment. Half-life is defined as the amount of time required to destroy one-half of the amount of the applied chemical. The half-life of DDT in an animal is up to eight years. The enzymes necessary to destroy DDT are not present in most animals in sufficient quantities to cause a more rapid destruction of this chemical. Generally, if we cannot destroy a particular chemical, we tend to store it in tissues or organs. In the case of DDT, we and other mammals tend to store it in body fat. While accumulating in body fat in humans, DDT has been shown to produce low toxicities in man. It is toxic, however, to eagles, ospreys, falcons, and other predatory birds. These predatory birds feed on contaminated animals, especially fish, thereby accumulatin g DDT in their bodies. The birds are poisoned with DDT and die or they may produce thin-shelled eggs which shatter during incubation. The egg making process is also significantly altered. The results in eagles and ospreys were nearly disastrous and almost resulted in the extinction of these birds in some parts of the United States. The use of DDT was curtailed in 1973 and the numbers of eagles and ospreys have increased.

It is interesting to note that the chemicals that replaced DDT as insecticides, namely the organophosphates, are actually much more toxic to humans than any of the chlorinated hydrocarbons. Parathion is a good example of the organophosphates. Parathion is very effective against a large number of insects, does not accumulate in the environment or in body fat, but is very poisonous to humans. A brief review of relative dangers of the insecticide classes follows. I) INORGANIC INSECTICIDES

Silicates (sodium hexafluorosilicate) are essentially non-toxic to mammals but may induce silicosis (a respiratory disease) among industrial workers. Arsenicals (arsenic trioxide) is extremely toxic to humans and requires great care in its use and handling. Lead arsenate, although no longer manufactures, was used as a fungicide in Florida (in 1988 to treat the early grapefruit crop). Over 90,000 pounds of this chemical were used in one-third of the early grapefruit crop acreage. Lead residues in grapefruit ranged from 4 to 6 parts per billion in juices and up to 170 parts per billion in unpeeled grapefruit. The Environmental Protection Agency (EPA) considers the safe level of leaqd in drinking water to be at around 50 parts per billion. Lead is a cumulative poison and builds up in the bones and tissues of humans.

II)BOTANICAL INSECTICIDES Toxicity of the botanical insecticides ranges from very toxic (nicotine) to relatively non-toxic (rotenone) Rotenone is derived from the roots of tropical plant species (Derris elliptica and Lonchoncarpus nicou). It is relatively non-toxic to mammals because it is poorly absorbed. However, the compound readily passes into the gills of fish and the breathing tubes of insects and is very toxic to these animals. Rotenone can produce numbness of lips, tongue, and throat in humans and may also lead to abnormal respiratory stimulation. Pyrethrins are extracted from chrysanthemums and can result in a high degree of allergy-like symptoms especially in those persons susceptible to ragweed and other pollen allergens. Pyrethroid insecticides entered the marketplace in 1980. By 1982, they accounted for approximately 30 percent of the worldwide insecticide usage. These synthetics arise from the much older class of botanical insecticides, pyrethrum, and include a mixture of six insecticides (pyrethrins, cinerins, and jasmolins) estracted from dried pyrethrum or chrysanthemum flowers (Chrysanthemum cinerariaefolium, C. coccineum). The major active principles in pyrethrum are pyrethrin I (chrysanthemic acid - pyrethrin I, cinerin I, and jasmolin I - and pyrethrin II - pyrethrin II, cinerin II, and jasmolin II. Pyrethrin I is the most active and most toxic, while pyrethrin II has great knowkdown properties for a wide range of insects. The type I poisoning syndrome or T syndrome is produced by -Pyrethrin I, allethrin, tetramethrin, kadethrin, resmethrin, phenothrin, and permethrin. In rats, one sees whole body tremors, aggressiveness and hyperexcitation. In the cockroach, one sees paralysis, incoordination and restlessness. Type II poisoning or CS syndrome is produced by -cypermethrin, fenpropanthrin,fenvalerate and fluvalinate. In rats, type II poisoning results in clonic seizures, writhing, profuse salivation and burrowing. In the cockroach, symptoms include hyperactivity, incoordination and convulsions. These botanical insecticides are not considered to be highly toxic to mammals, but their use indoors in poorly ventilated spaces has resulted in contact dermatitis, and localized redness and skin eruptions.

DDT AND OTHER INSECTICIDES.