Pesticides are an important part of modern farming. The proper use of pesticides helps ensure that Americans have a safe, abundant and inexpensive supply of food and fiber. Pesticides improve crop yields and quality and help to make farming profitable. However, the improper use of pesticides can harm the environment and wildlife.
As stewards of the land and its natural resources, farmers should strive to use pesticides properly in order to minimize environmental risks. There are ways to reduce the amounts of pesticides used, and to use them more efficiently, without compromising agricultural yields or profits.
We study the past, and apply it to the present, so that we may affect the future.
-- An anonymous historian
Pesticides became widely used following World War II. At that time the most common insecticides were the organochlorine compounds, which included DDT, aldrin, dieldrin and others. Eventually, environmental problems associated with their use became apparent.
One problem was that organochlorine compounds, sometimes referred to as chlorinated hydrocarbons, are very persistent in the environment and may take years to dissipate. Most organochlorines are relatively insoluble in water and can be carried from fields into streams by heavy rainfall.
Also, these chemicals bioaccumulate. That is, they accumulate at successively higher concentrations in animals higher up the food chain. For example, concentrations of the insecticide DDT might be as low as 0.02 parts per million (ppm) in plankton, but 900 ppm in small fish, 2,000 ppm in larger fish, and even greater in fish-eating birds such as gulls or bald eagles. Such accumulations caused egg shell thinning in many birds of prey such as bald eagles, brown pelicans and peregrine falcons, so that when contaminated birds attempted to incubate their eggs, the eggs often broke under the birds' weight. The widespread use of organochlorine insecticides contributed to the decline of these birds. Fortunately, most organochlorine compounds have not been used in the U.S. since the early 1970s. Consequently, populations of eagles, falcons and brown pelicans are improving.
Herbicides account for 70 to 80 percent of all pesticides used. Only a few herbicides are acutely toxic to wildlife, but they can have a major effect on wildlife habitats.
Insecticides are more acutely toxic to wildlife. Most of the insecticides in use today are organophosphates (such as malathion, parathion, dimethoate) or carbamates (such as aldicarb and carbofuran). While these chemicals don't tend to bioaccumulate or persist in the environment, most are more acutely toxic than the organochlorine compounds. Pesticides from these two groups that are most often associated with wildlife kills include carbofuran, diazinon, parathion, chlorpyrifos and phorate.
The newest group of insecticides, the synthetic pyrethroid compounds, includes such products as esfenvalerate, cyfluthrin and cyhalothrin. Synthetic pyrethroids usually are only slightly toxic to birds and mammals, but are extremely toxic to fish and other aquatic animals.
Some insecticides used commonly in Texas and their hazards to wildlife are listed in Table 1.
Pesticides will continue to be an integral part of agriculture, at least for the near future. However, broad-spectrum pesticides likely will be replaced by compounds that are more selective and environmentally friendly. The use of microbial insecticides will increase. These contain bacteria, fungi or viruses which attack insects. The most common biological insecticide is Bacillus thuringiensis, or Bt as it is commonly known. Biological insecticides usually are highly selective in the organisms they infect and they are nontoxic to birds, mammals and fish.
Other biological controls being developed involve the use of parasites, predators and pathogens to control insect pests. Common examples include parasitic wasps and beneficial predators such as lady beetles. Biological control is most effective in an integrated pest management (IPM) program. IPM practices include using cultural controls, planting pest resistant varieties, scouting fields to monitor pest populations, and using insecticides only when other control measures fail to keep pest numbers or damage below levels which cause economic crop loss.
Most insecticides kill insects by damaging their central nervous systems and can harm wildlife in the same way. Wildlife may be exposed to insecticides by breathing the chemical, swallowing contaminated food or water, absorbing the chemical through the skin or feathers, or by swallowing the chemical when grooming. Some birds may eat granular insecticides, mistaking them for seeds or grit.
Some animals may become sick or die when exposed to pesticides. This is a lethal effect and it is measured as the particular chemical's toxicity. The toxicity of a pesticide to animals is commonly expressed as either its LD50 (lethal dose) or LC50 (lethal concentration). The LD50 of a particular chemical is the dose that kills 50 percent of the animals exposed to it. The LC50 is the concentration of the chemical in the diet, air or water required to kill 50 percent of the animals exposed. LD50s and LC50s are different for every animal species and are determined by laboratory research. For any species, the lower the LD50 or LC50 the higher the toxicity.
Wildlife also may suffer sublethal effects from pesticides. In such cases they do not die, but their behavior may be altered or their survival or reproductive abilities affected. For example, in one study, bobwhites that received sublethal doses of the insecticide terbufos (Counter) suffered higher mortality from predators. This kind of sublethal effect of pesticides is difficult to measure and may be underestimated.
Exposure to pesticides may pose particular problems for certain endangered species. In fact, the presence of threatened or endangered plants or animals may restrict the use of pesticides in certain areas of the state. Contact your county Extension agent or the Texas Department of Agriculture for restrictions that may apply in your area.
One of the greatest risks associated with pesticides is the movement of the chemical, through drift or runoff, from the target crop to adjacent wetlands or other sensitive habitats. Most pesticides are applied either as liquids (sprays) or granules. Spray should be applied under conditions that will minimize drift into sensitive habitats.
Granular pesticides are much less susceptible to drift, but they pose a special threat to some species of wildlife, especially seed-eating birds. Birds may mistake pesticide granules for grit or seed. It takes only a few granules of some insecticides to kill a sparrow-sized bird. When granules are applied, take special care to cover them with soil and completely disk under any spills.
Most herbicides are only slightly toxic to wildlife. (A notable exception is paraquat.) While herbicides rarely have lethal effects, they can affect wildlife populations indirectly by altering the structure of the habitat. Many species of weeds and brush provide important food or shelter for wildlife. Care should be taken to protect wildlife habitats when applying herbicides.
Every pesticide user has a legal duty to use chemicals according to label directions. Failure to use pesticides according to label directions not only jeopardizes wildlife, but also exposes the pesticide user to criminal liability and may ultimately result in further restrictions on pesticide use. Please do your part to see that pesticides are used responsibly according to label directions.