Pesticide Formulations and Actions – Part II
Insecticides
Insecticides are commonly classified by the chemical group that they belong to. This method classification often reveals similar characteristics such as mode of action. Some of the more common insecticide chemical groups and their modes of action are addressed below.
Petroleum Oils. Supreme oil and superior oil are members of this group. They act as physical toxicants.
Organochlorines. Members of this group act as an axonic poison or as a central nervous system poison (nerve toxin). Examples of this group include DDT, chlordane and toxaphene.
Organophosphates. Organophosphates act by poisoning the central nervous system. Members of this group include malathion and Di-Syston®.
Organosulfurs. Propargite (Omite) is a member of this group and acts as an axonic poison.
Carbamates. Carbamates are poisons of the central nervous system. Members of this group include Sevin® and Ficam®.
Formamidines. These are adrenergic insecticides. Members of this group include Galecron® and Baam®.
Thiocyanates. Thanite is a member of this group and acts by interfering with cellular respiration and metabolism.
Dinitrophenols. Dintrophenols act as metabolic inhibitors. Members of this group include dinoseb and dinocap.
Organotins. Organatins act as metabolic inhibitors. Members of this group include Plictran® and Vendex®.
Pyrethroids. Pyrethroids are axonic poisons. Members of this group include permethrin and resmethrin.
Inorganics. Silica gel and boric acid are physical toxicants. Sulfur is a cellular poison and arsenic inhibits respiration.
Chemical Control of Vertebrate Pests
Chemical control of vertebrate pests consists of five major categories. These are summarized below.
Lethal Toxicants Applied Orally
Lethal toxicants are mixed with baits and when ingested lead to death of the animal. Examples include sodium cyanide and anticoagulants.
Fumigants
This involves the use of products that release toxic gases and result in the death of the pest when inhaled. Examples include aluminum phosphide and zinc phosphide.
Aversive Conditioning Agents
This consists of the use of chemicals that are applied to bait and when ingested make the pest sick or give off a bad taste.
Saponificants (soaps)
This is used predominantly for the control of concentrations of birds. This involves spraying the birds with a soapy solution that break down the feather oils and wets the birds. The birds then die by hypothermia in the cold weather.
Repellants
Repellants act by scaring off or irritating the pest which results in less economic loss. Examples of repellants include mothballs, thiram and capsaicin bone tar oil.
Fungicides
Fungicides are classified as being organic or inorganic. Each of these categories is further subdivided.
Inorganic Fungicides
The earliest fungicides in use were inorganic fungicides. Today, some of these are still used for the control of pathogens.
Sulfur. Elemental sulfur kills some insects, bacteria and fungi by direct contact. Sulfur is still used today for the control of powdery mildew on plants. It acts by interfering with electron transport.
Copper. The use of copper compounds as fungicides goes back to the early 1800s. Copper compounds are generally insoluble in water and not easily washed from leaves. Only a small amount of the actual copper applied will go into solution and this dissolved copper will then be absorbed by living cells. Eventually, enough of the copper is absorbed by the plant pathogen that it is poisoned. Many copper compounds are fairly phytotoxic.
Organic Fungicides
Organic fungicides are synthetic; they are man-made. Organic fungicides are divided into two groups – systemic and nonsystemic.
Nonsystemic
Nonsytemic fungicides are not absorbed and then transported within the plant. Members of this group are addressed in greater detail below.
Dithiocarbamates. This group is made up of some of the oldest organic fungicides. Members of this group include Manzate® and Dithane®.
Dicarboximides. Dicarboximides are widely used as protective sprays of fruit, vegetables, ornamentals and turf and as seed treatments. This group probably acts by inhibiting the synthesis of proteins and enzymes containing sulfhydryl groups. Members of this group include captam, folpet and captafol.
Dinitrophenols. Dinocap was developed in the late 1930’s and used for the control of powdery mildews. By uncoupling oxidative phosphorylation, it upsets the energy systems within affected cells.
Substituted Aromatics. This group is made of fungicides whose basic building block is a benzene ring which has various attached radicals. Two members of this group, PCNB and Hexachlorobenzene were developed many years ago. PCNB was develop in the 1930’s and used for treating seeds and as a foliage fungicide. Hexachlorobenzene was developed in the mid 1960’s and used for controlling turfgrass and cotton dieseases.
Quinones. A member of this group, dichlone, has been used as a foliar fungicide on a variety of crops and in controlling blu-green algae in ponds. By inhibiting the sulfylhydryl groups in enzymes, it affects the oxidative phosphorylation process.
Aliphatic Nitrogenous Compounds. A member of this group, diodine, was introduced in the mid-1950’s for the control of various fungal leaf spot diseases. Diodine affects the cell membrane in a manner that causes affected cells to leak metabolites. It also inhibits the synthesis of DNA.
Antibiotics. There are three antibiotic compounds that have been used to control bacteria on fruits and vegetables. These are:
- Streptomyacin which is commonly used for the control of fireblight on apples and pears. It probably interferes with the synthesis of protein and organic acids.
- Tetracycline which has been used for the control of mycoplasma diseases.
- Cyclohexamide which has been used for the control of various foliage diseases.
Organotins. This group of fungicides was introduced in the 1960’s. They act by blocking oxidative phosphorylation. Du-Ter® is a member of this group. Most uses for this group have been discontinued.
Systemic
Systemic fungicides are absorbed by the plant through the surface of the leaf, stem or root surface and then translocated (moved) within the plant. Sometimes this distance is small – one leaf surface to the other – and at other times, this distance is much farther – the roots to the top of the stem.
The different classes of systemic plant insecticides are addressed below.
Oxathiins. Two of these compounds, Viatvax® and Plant-vax® act by inhibiting succinic dehydrogenase, a respiratory enzyme important in mitochondrial systems. They are toxic to some of the smut and rust fungi and to Rhizoctonia.
Benzimidazoles. These compounds have been used as foliar fungicides, seed treatments, soil drenches and dips for fruit or roots. They act by interfering with the synthesis of DNA, affecting spore germination and growth. Members of this group include benomyl, thiabendazole and thiophenate.
Pyrimidines. This group of fungicides is active against mildews. Members of this group include Milcurb® and Nimrod®.
Acylalanines. These are effective against soil borne diseases as well as downy mildew and some other foliar diseases. Ridomil® is a member of this group.
Organophosphates. This group is probably interferes with phospolipid synthesis. An example of this group is Aliette®.
Triazoles. This is newer group of fungicides that consists of broad-spectrum systemic fungicides that are both curative and protective. Examples of this group include Bayleton®, Tilt® and Baycor®.
Piperazines. A member of this group, triforine (Funginex®), is active against scabs, rusts and mildews. It appears to interfere with the synthesis of sterols.
Imides. Imides are effective against a number of important diseases. Rovral® and Ronilin® are members of this group.
Carbamates. This is a newer group and includes propamocarb hydrochloride (Banol® and Prevex®) which is used as to control pythium blight on turfgrass.
Strobilurons. This is a new category and includes azoxystrobin (Quadris® and Abound®) which is a broad-spectrum fungicide with both systemic and curative properties. Azoxystrobin can be applied on a wide variety of crops.
References
Bohmont, B. L. 2007. The standard pesticide users guide. 7th ed. Pearson Prentice-Hall, Upper Saddle River, N. J., Columbus, Ohio.
Marer, P. J. 2000. The safe and effective use of pesticides. University of California Publ. 3324.
Renchie, D. L. 2009. Texas pesticide applicator general. Texas AgriLife Extension Service Publ. B-5073.