File: <insecticides

The chemical industry that led to the development of modern insecticides began around the year 1940. However, prior to this an array of substances was deployed to combat insects. Botanicals derived from plants employed nicotine primarily. Metallic compounds centered mainly on lead arsenic. Various oils were used to smother insects on plants and in aquatic habitats. Lime sulfur and sulfurs were used and hydrogen cyanide was used extensively in eradication. There were also many stomach and contact poisons and fumigants used.

Modern chemical insect and mite control in agriculture has strived to restrict applications to periods of greatest target pest vulnerability early in the crop season and before extensive pest reproduction can occur. Various techniques are deployed to determine when pests become active, such as acquiring data from light-traps, using pheromone traps and by making periodic field observations. Placement of a pesticide in a area where insects and mite will come in contact with it is of the utmost importance and frequently requires considerable field research for determination. An example is the control of Australian bush fly in Micronesia using poisoned baits (see Kwajalein). To minimize the exposure of humans to chemical pesticides in the field applicators are required to provide protective clothing to workers and to restrict access to fields that have been treated until harmful residues have become degraded.

Entomology also has traditionally included the study and control of phytophagous mites, which cause extensive damage to food plants (See Arachnida). Details of the various substances used for control are discussed as follows:

Botanical Insecticides

These include some of the oldest of the insecticides. They consist of complex volatile chemicals and their effect is primarily by contact. There were materials used that were highly poisonous to humans and others that were not so poisonous.

Nicotine. -- This had been used since the mid 18th Century, during which time a great number of human suicides were associated with it. Nicotine was formulated primarily as either nicotine sulfate or as a nicotine alkaloid (e.g., Black Leaf 40).

Nicotine was widely used in home gardens for soft-bodied insects, especially aphids. It was considered desirable because it did not harm predatory insects. Because of its high cost there was only limited use of nicotine for commercial insect control. Also, its effectiveness was greatly curtailed during cold weather.

Pyrethrum. -- Several species of chrysanthemum produced this material. It originated in Persia from around 1828. Later Japan developed commercial production for the worldwide market followed by East Africa. Pyrethrum is not poisonous to humans. It is difficult to store as it becomes inactivated in the presence of light. The high cost of production limited its use for commercial insect control.

Rotenone. -- This botanical, which has been used as an insecticide since 1848, is obtained primarily from the roots of over 68 species of leguminous plants in the genus Derris, of the Far East, and the genera Cubé and Timbo of South America. It is poisonous only to cold blood animals and thus may be safely handled by humans. It has been used as a fish poison by indigenous people in the Far East and South America. Although it is very effective against cattle grubs and the Mexican bean beetle, its high cost has limited its widespread usage.

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Metallic Insecticides

Most metallic insecticides, which have been in use since 1860, contained arsenic. They are strictly stomach poisons and therefore effective against chewing insects only. They are also highly poisonous to humans and honeybees.

Standard Lead Arsenate (PbHAsO₄). -- This product had been used as a dust since 1820, being especially effective against codling moth on apples and pears. It had the undesirable effect of burning plant tissues and was not recommended for stone fruits.

Basic Lead Arsenate [ Pb(PbO+1)(AsO₄)₃ ]. -- This material was specially formulated to be effective on stone fruits as it did not burn their foliage. It was used primarily for peach twig borer control.

Calcium arsenate -- This material became important as an insecticide in the mid 1920's. It was used quite extensively on field crops until 1942.

Cryolite (Na₃AlF₆). -- Cryolite is obtained from mineral deposits in Greenland, but was not used extensively with the advent of DDT.

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Oils as Insecticides

The first usage of oils dates to the mid 1800's with kerosene. By the 1900's it was found that one could mix the kerosene with water for better distribution on plants. This mixture is still commonly used in some areas.

Oils are useful against mites, scale insects and the eggs of insects particularly when this is the overwintering stage.

There are two main types of oils: dormant and summer. Dormant oils are used when leaves have dropped off the trees, while summer oils can be used on foliage. For this purpose they must be more refined and contain little unsaturated and aromatic hydrocarbons to prevent burning of plant tissue. Unrefined oils may leave sulfonated residues on the tissue that cause burning.

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Sulfurs & Lime Sulfur

These have been used as a control for mites and as an insect control before the application of fungicides.

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Dinitro Compounds

These compounds were developed in the mid 1930's for the control of weeds, insects and plant diseases. When used in conjunction with oils they also afford better of mite control.

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Chlorinated Hydrocarbons

Chlorinated hydrocarbons act as contact and stomach poisons and have low to medium toxicity to animals and humans. However, they can accumulate in the fat tissue of animals and can produce acute or chronic reactions to their toxicity. For decades in the mid 1900's a small residue was tolerated by regulatory agencies in North America (e.g., the Miller Amendment). But by the late 20th Century many of them were banned from general use as data began to show adverse effects on birds and on human health.

D.D.T. -- This material was synthesized in Germany in 1874, but it was not until 1936 that it was rediscovered in Switzerland. From 1942 to 1946 there was wide scale usage of D.D.T. in agriculture worldwide. Its discovery has been considered as a turning point in the insecticide industry. It was most effective on caterpillars, beetles and adult mosquitoes, but not very good against mites, most aphids and scale insects and the true bugs. It is compatible with most other chemicals used in agriculture, but is not suitable if used with highly alkaline mixtures such as lime sulfur.

D.D.T. has a very low toxicity to humans and its residual action is quite high as it continues to act one month after application. Insects began to develop resistance to D.D.T. after about 10 years of widespread usage. By the mid 20th Century it was found to have a devastating effect on bird populations by causing a thinning of bird eggshells. Nevertheless, it is still widely used in Africa and South America to combat insects associated with dwellings. The incidence of malaria and Chagas Disease has decreased in those areas where it is still applied as a control of mosquitoes and biting bugs, respectively..

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D.D.D. or T.D.E. -- This is a derivative of D.D.T., and although less effective in pest control it does not accumulate in fat tissue. It has considerably lower toxicity in water. However, some insects have been found to be controlled more effectively with this compound.

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Methoxychlor. -- This compound gives faster action than D.D.T., but there is a much shorter residual action. This is the least toxic to humans of all the chlorinated hydrocarbons, and there is no accumulation of residues in fat tissue. It has been used for livestock insect control.

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B.H.C. (Benzenehexachloride). -- The gamma isomer of this compound is most important for insecticidal action. It has a wider and faster range of action than D.D.T. and is very effective against aphids and Hemiptera. It loses toxicity in alkaline mixtures. A great disadvantage is that it imparts off-flavors to products, which is caused by isomers other than the gamma isomer.

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To greatly reduce the flavor problem another related product, Lindane, was developed, which consists entirely of only the gamma isomer.

Toxaphene. -- This is a mixture of several compounds, which once formulated controls almost the same kinds of insects as D.D.T. It has the advantage of being much less poisonous to honeybees. It is also more effective against certain insects, such as grasshoppers, cutworms and armyworms.

The Chlordane Group. -- Chlordane is actually a mixture of chemicals that began to be used in the mid 20th Century. Derivatives of chlordane were developed as Aldrin, Dieldrin and Heptachlor, and Endrin, each with different residual activity and toxicity to humans and animals. All are sensitive to alkalinity and quite poisonous to Hymenoptera and Orthoptera. Their effectiveness in reducing some insect populations in vegetable crops was quite good (see Earwig control)These chemicals ceased to be used in North America during the latter half of the 20th Century as it was found that residues remained in plant tissue for long periods of time, and thus posed a danger to human health.

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Organic Phosphates

All of these compounds are highly toxic to humans and warm-blooded animals except Malathion. They attack the nervous system of insects and play havoc with humans as well. If one has been exposed to them the compound atropine can be administered as an antidote. They are effective against almost all insects and mites through three types of actions as contact poisons, fumigants and stomach poisons. Most of them have a short residual life and safe access by workers to agricultural fields that have been treated is usually only a few days. However, it is especially important for applicators not to let these chemicals in contact with skin, to wear protective clothing and to reduce prolonged exposure. Nevertheless, there continues to be great concern about side effects on humans who have been exposed to them (See 2010 News Release).

TEPP (or HETP). -- This product was originally manufactured as a war gas and thus has a highly acute toxicity. There is little residual life and in agriculture it can be used immediately before harvest. It is highly volatile and functions primarily as a fumigant. It breaks down rapidly with water and is most effective if applied in warm and dry weather.

Parathion. -- This was the most widely used of all the organic phosphates. The residual action is about a week and it is very effective against a large number of insects and mites. It has a slow action on mammals and causes chronic health symptoms.

Malathion. -- This compound has a residual life of about 10 days and is relatively nontoxic to humans and animals. It has been used almost as much as Parathion especially near dwellings. It is very effective against aphids.

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Pathogens

Various pathogens such as viruses, bacteria, fungi, nematodes and protozoa, have been used to control pest insects. Detailed accounts of these can be found at <BC-50>. Bacillus thuringiensis is one pathogen that has been in widespread use for many decades and in 2010 is the source of genes that produce toxins which can be introduced into food an fiber plants to kill insects.

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Systemic Pesticides

These materials were developed so that plants could absorb them into their tissues and thereby provide insect and mite control. They generally belong to the organic phosphate group of compounds.

Systox (Demeton). -- This product killed only piercing-sucking insects, and predators were not appreciably harmed. However, it was highly toxic to humans when contacted directly. Residues remained in plant tissue and the harm caused to public health was not immediately apparent. Systox was used mainly in field crops, apples and pears. There was little contact action reported, but insects built up rapid resistance to it. This was undoubtedly a result of the prolonged exposure to the product that treated plants afforded in their tissues to pest populations.

Miticides or Acaricides

Miticides were developed that specifically did not kill predatory mites. They all had long residual actions. Several materials that were commonly used are as follows:

Aramite. -- This material had 2-3 weeks of residual life and was toxic only under high dosages. It killed only the active forms of mites. It also was one of the first pesticides known to cause cancer in humans.

Ovotran (Ovex). -- Only the eggs of mites were killed. It was essentially nontoxic to humans and had a one-month residual action. There was also a high degree of compatibility with other compounds.

Other Materials. -- These include the trade names of Sulphenone, Dimite, Gemite 923 and FW 293.

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Formulation of Pesticides

Various formulations are used to apply insecticides. Each has advantages for different kinds of home or agricultural situation and climates. Considerations involve which pests are targeted, the chemical's safety to humans, its phytotoxicity and the nature of the crop treated. In order to keep ahead of resistance in an insect or mite population, pesticide industry must periodically change the structure of their chemical products.

Dusts. -- Clay powder may be added to such compounds as D.D.T at the rate of 90-95% powder in order to give the insecticide volume and to keep the chemical dispersed. These may then be applied to plant foliage in calm weather. This of course adds to the cost of the material.

Wettable Powders. -- Sometimes an insecticide is mixed with clay and a wetting agent at the rate of only 50 %. Water may then be added just before application. Various stickers and emulsifiers may also be added to improve adhesion to plant foliage.

Attractants. -- An insecticide might be added to an attractant, such as a pheromone or simply a sugary substance to attract insects. Many households use the latter to attract and kill ants, which spread the insecticide throughout their nests.

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Resistance to Pesticides

Insects and mites develop resistance to almost every pesticide that is used against them. For this reason the pesticide industry must continuously modify the formulae of their products in an effort to counteract the resistance. Detailed accounts of resistance to pesticides among pests as well as their predators and parasites may be found at <resist.htm>.

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Transgenic Insecticides

Plant breeders have traditionally sought varieties of plants that have resistance to plant pests. In many cases this resistance was created by selection under experimental conditions. Examples are the grafting of resistant rootstocks to desirable varieties, producing fruits with hardened skins, as in tomatoes, and many fruits and the development of grains resistant to various diseases. However, beginning in the latter decade of the 20th Century researchers have been introducing the genes that produce bacterial toxins directly into the food plant. In this way the plant itself becomes the insecticide. Bacillus thuringiensis, as noted above, produces a crystal, which is toxic to insects when ingested. When the gene for the toxin in the bacterium is incorporated into the genome of the plant, the plant itself produces the toxin and kills the insects when they chew into it. Many nations, especially in Europe, have prohibited the distribution of transgenic food plants, as it is feared that long-term ingestion might be harmful to the human population. They are also very defensive of desirable flavors in food and are wary of the effect such toxins also might have on their desirability. As of August 2010 there are no restrictions being placed on such transgenic plants in the United States.

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Harmful Effects of Pesticides on Humans

There have been many cases of pesticide poisonings among the human population over the long history of pest control. Accidental exposure to lead arsenate, chlorinated hydrocarbons and organophosphates are widespread and serious health problems have been reported. A 2010 report from the University of California on health threats of organophosphate pesticides emphasizes the seriousness of this problem.

Pesticides Linked To Hyperactivity

Prenatal and childhood exposure are associated with

an increase in attention deficit problems.

By Thomas H. Maugh II -- Los Angeles Times, Aug 2010

"A growing body of evidence is suggesting that exposure to organophosphate pesticides is a prime cause of attention deficit hyperactivity disorder. The findings are considered plausible to many experts because the pesticides are designed to attack the nervous systems of insects. It is not surprising, then, that they should also impinge on the nervous systems of humans who are exposed to them.

Forty organophosphate pesticides are registered in the United States; with at least 73 million pounds used each year in agricultural and residential settings. ADHD is thought to affect 3 percent to 7 percent of American children, with boys affected more heavily than girls. Many experts believe its incidence has increased sharply in recent decades, but critics attribute the increased incidence to over-diagnosis. Some attribute the increase to the greater use of pesticides.

The newest study, reported Thursday in the journal Environmental Health Perspectives, examines the effects of both prenatal and childhood exposure to the pesticides, which are widely used in the United States to control insects on food crops. Epidemiologist Brenda Eskenazi of the University of California, Berkeley, and her colleagues have been studying more than 300 Mexican American children living in the heavily agricultural Salinas Valley.

Eskenazi and her team tested for levels of pesticide metabolites in urine in the mothers twice during their pregnancies and several times in the children after birth. They then tested the children at ages 3 1/2 years and 5 years for attention disorders and ADHD, using the mothers' reports, performance on standardized computer tests and behavior ratings from examiners. After correcting the data to account for lead exposure and other confounders, they found that each tenfold increase in pesticide levels in the mothers' urine was associated with a fivefold increase in attention problems as measured by the assays. The effect was more pronounced in boys than in girls.

The study comes only three months after a Harvard study, looking at much lower levels of malathion in urine, found that a tenfold increase in pesticide levels was associated with a 55 percent increase in ADHD.

The researchers believe that most of the children in the study were exposed to the malathion through food.

'It's known that food is a significant source of pesticide exposure among the general population," Eskenazi said in a statement. "I would recommend thoroughly washing fruits and vegetables before eating them, especially if you are pregnant.'"

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References

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